The deep XMM-Newton Survey of M 31 ††thanks: Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.††thanks: Tables 5 and 8 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/
Key Words.:Galaxies: individual: M~31 – X-rays: galaxies
Aims:The largest Local Group spiral galaxy, M~31, has been completely imaged for the first time obtaining a luminosity lower limit erg s in the 0.2–4.5 keV band. Our XMM-Newton EPIC survey combines archival observations along the major axis, from June 2000 to July 2004, with observations taken between June 2006 and February 2008 that cover the remainder of the ellipse. The main goal of the paper is a study of the X-ray source population of M~31.
Methods:An X-ray catalogue of 1 897 sources was created; 914 were detected for the first time. Source classification and identification were based on X-ray hardness ratios, spatial extent of the sources, and by cross correlating with catalogues in the X-ray, optical, infrared and radio wavelengths. We also analysed the long-term variability of the X-ray sources; this variability allows us to distinguish between X-ray binaries and active galactic nuclei (AGN). Furthermore, supernova remnant classifications of previous studies that did not use long-term variability as a classification criterion, could be validated. Inclusion of previous Chandra and ROSAT observations in the long-term variability study allowed us to detect additional transient or at least highly variable sources, which are good candidate X-ray binaries.
Results:Fourteen of the 30 supersoft source (SSS) candidates represent supersoft emission of optical novae. Many of the 25 supernova remnants (SNRs) and 31 SNR candidates lie within the 10 kpc dust ring and other star forming regions in M~31; this connection between SNRs and star forming regions implies that most of the remnants originate in type II supernovae. The brightest sources in X-rays in M~31 belong to the class of X-ray binaries (XRBs). Ten low mass XRBs (LMXBs) and 26 LMXB candidates were identified based on their temporal variability. In addition 36 LMXBs and 17 LMXB candidates were identified due to correlations with globular clusters and globular cluster candidates. From optical and X-ray colour-colour diagrams, possible high mass XRB (HMXB) candidates were selected. Two of these candidates have an X-ray spectrum as is expected for an HMXB containing a neutron star primary.
Conclusions:While our survey has greatly improved our understanding of the X-ray source populations in M~31, at this point 65% of the sources can still only be classified as “hard” sources, i. e. it is not possible to decide whether these sources are X-ray binaries or Crab-like supernova remnants in M~31, or X-ray sources in the background. Deeper observations in X-ray and at other wavelengths would help classify these sources.
Our nearest neighbouring large spiral galaxy, the Andromeda galaxy, also known as M~31 or NGC~224, is an ideal target for an X-ray source population study of a galaxy similar to the Milky Way. Its proximity (distance 780 kpc, Holland 1998; Stanek & Garnavich 1998) and the moderate Galactic foreground absorption (= 7 cm, Stark et al. 1992) allow a detailed study of source populations and individual sources.
After early detections of M~31 with X-ray detectors mounted on rockets (e. g. Bowyer et al. 1974) and the Uhuru satellite (Giacconi et al. 1974), the imaging X-ray optics flown on the Einstein X-ray observatory permitted the resolution of individual X-ray sources in M~31 for the first time. In the entire set of Einstein imaging observations of M~31, Trinchieri & Fabbiano (1991, hereafter TF91) found 108 individual X-ray sources brighter than erg s, of which 16 sources showed variability (van Speybroeck et al. 1979; Collura et al. 1990).
In July 1990, the bulge region of M~31 was observed with the ROSAT High Resolution Imager (HRI) for ks. Primini et al. (1993, hereafter PFJ93) reported 86 sources brighter than erg s in this observation. Of the ROSAT HRI sources located within 75 of the nucleus, 18 sources were found to vary when compared to previous Einstein observations and about three of the sources may be “transients”. Two deep PSPC (Position Sensitive Proportional Counter) surveys of M~31 were performed with ROSAT, the first in July 1991 (Supper et al. 1997, hereafter SHP97), the second in July/August 1992 (Supper et al. 2001, hereafter SHL2001). In total 560 X-ray sources were detected in the field of M~31; of these, 491 sources were not detected in previous Einstein observations. In addition, a comparison with the results of the Einstein survey revealed long term variability in 18 sources, including 7 possible transients. Comparing the two ROSAT surveys, 34 long term variable sources and 8 transient candidates were detected. The derived luminosities of the detected M~31 sources ranged from 5 erg s to 5 erg s. Another important result obtained with ROSAT was the establishment of supersoft sources (SSSs) as a new class of M~31 X-ray sources (cf. Kahabka 1999) and the identification of the first SSS with an optical nova in M~31 (Nedialkov et al. 2002).
Garcia et al. (2000) reported on first observations of the nuclear region of M~31 with Chandra. They found that the nuclear source has an unusual X-ray spectrum compared to the other point sources in M~31. Kong et al. (2002b) report on eight Chandra ACIS-I observations taken between 1999 and 2001, which cover the central region of M~31. They detected 204 sources, of which 50% are variable on timescales of months and 13 sources were classified as transients. Kaaret (2002) detected 142 point sources ( to 2 erg s in the 0.1–10 keV band) in a 47 ks Chandra/HRC observation of the central region of M~31. A comparison with a ROSAT observation taken 11 yr earlier, showed that 4626% of the sources with erg s are variable. Three different M~31 disc fields, consisting of different stellar population mixtures, were observed by Chandra. Di Stefano et al. (2002) investigated bright X-ray binaries (XRBs) in these fields, while Di Stefano et al. (2004) examined the populations of supersoft sources (SSSs) and quasisoft sources (QSSs), including observations of the central field. Using Chandra HRC observations, Williams et al. (2004) measured the mean fluxes and long-term time variability of 166 sources detected in these data. Voss & Gilfanov (2007) used Chandra data to examine the low mass X-ray binaries (LMXBs) in the bulge of M~31. Good candidates for LMXBs are the so-called transient sources. Studies of transient sources in M~31 are presented in numerous papers, e. g. Williams et al. (2006b), Trudolyubov et al. (2006, hereafter TPC06), Williams et al. (2005b), Williams et al. (2006a, hereafter WGM06), and Voss et al. (2008).
Using XMM-Newton and Chandra data, Trudolyubov & Priedhorsky (2004) detected 43 X-ray sources coincident with globular cluster candidates from various optical surveys. They studied their spectral properties, time variability and log N-log S relations.
Osborne et al. (2001) used XMM-Newton Performance Verification observations to study the variability of X-ray sources in the central region of M~31. They found 116 sources brighter than a limiting luminosity of 6 erg s and examined the brightest sources for periodic and non-periodic variability. At least 15% of these sources appear to be variable on a time scale of several months. Barnard et al. (2003) used XMM-Newton to study the X-ray binary RX J0042.6+4115 and suggested it as a Z-source. Orio (2006) studied the population of SSSs and QSSs with XMM-Newton. Recently, Trudolyubov & Priedhorsky (2008) reported the discovery of 217s pulsations in the bright persistent SSS XMMU J004252.5+411540. Shaw Greening et al. (2009, hereafter SBK2009) presented the results of a complete spectral survey of the 335 X-ray point sources they detected in five XMM-Newton observations located along the major axis of M~31. They obtained background subtracted spectra and lightcurves for each of the 335 X-ray sources. Sources with more than 50 source counts were individually spectrally fitted. In addition, they selected 18 HMXB candidates, based on a power law photon index of .
Pietsch et al. (2005b, hereafter PFH2005) prepared a catalogue of M~31 point-like X-ray sources analysing all observations available at that time in the XMM-Newton archive which overlap at least in part with the optical extent of the galaxy. In total, they detected 856 sources. The central part of the galaxy was covered four times with a separation of the observations of about half a year starting in June 2000. PFH2005 only gave source properties derived from an analysis of the combined observations of the central region. Source identification and classification were based on hardness ratios, and correlations with sources in other wavelength regimes. In follow-up work, (i) Pietsch & Haberl (2005) searched for X-ray burst sources in globular cluster (GlC) sources and candidates and identified two X-ray bursters and a few more candidates, while (ii) Pietsch et al. (2005a, hereafter PFF2005) searched for correlations with optical novae. They identified 7 SSSs and 1 symbiotic star from the catalogue of PFH2005 with optical novae, and identified anadditional XMM-Newton source with an optical nova. This work was continued and extended on archival Chandra HRC-I and ACIS-I observations by Pietsch et al. (2007, hereafter PHS2007).
Stiele et al. (2008, hereafter SPH2008) presented a time variability analysis of all of the M~31 central sources. They detected 39 sources not reported at all in PFH2005. 21 sources were detected in the July 2004 monitoring observations of the low mass X-ray binary RX J0042.6+4115 (PI Barnard), which became available in the meantime. Six sources, which were classified as “hard” sources by PFH2005, show distinct time variability and hence are classified as XRB candidates in SPH2008. The SNR classifications of three other sources from PFH2005 had to be rejected due to the distinct time variability found by SPH2008. Henze et al. (2009a) reported on the first two SSSs ever discovered in the M~31 globular cluster system, and Henze et al. (2009b) discussed the very short supersoft X-ray state of the classical nova M31N 2007-11a. A comparative study of supersoft sources detected with ROSAT, Chandra and XMM-Newton, examining their long-term variability, was presented by Stiele et al. (2010).
An investigation of the log N-log S relation of sources detected in the 2.0–10.0 keV range will be presented in a forthcoming paper (Stiele et al. 2011 in prep.). In this work the contribution of background objects and the spatial dependence of the log N-log S relations for sources of M~31 is studied.
In this paper we report on the large XMM-Newton survey of M~31, which covers the entire ellipse of M~31, for the first time, down to a limiting luminosity of erg s in the 0.2–4.5 keV band. In Sect. 2 information about the observations used is provided. The analysis of the data is presented in Sect. 3. Section 4 presents the combined colour image of all observations used. The source catalogue of the deep XMM-Newton survey of M~31 is described in Sect. 5. The results of the temporal variability analysis are discussed in Sect. 6. Cross-correlations with other M~31 X-ray catalogues are discussed in Sect. 7, while Sect. 8 discusses cross-correlations with catalogues at other wavelengths. Our results related to foreground stars and background sources in the field of M~31 are presented in Sect. 9. Individual source classes belonging to M31 are discussed in Sect. 10. We draw our conclusions in Sect. 11.
|erg cm s|
|Trinchieri & Fabbiano (1991, TF91)||E||108||–||entire set of Einstein||16 sources showed variability|
|(0.2–4 keV)||imaging observations|
|Primini et al. (1993, PFJ93)||R (HRI)||86||bulge region||18 sources variable; 3 transients|
|Supper et al. (1997, 2001)||R (PSPC)||560||–||whole galaxy||two deep surveys|
|(SPH97, SHL2001)||(0.1–2.4 keV)||491 sources not detected with Einstein|
|11 sources variable, 7 transients compared to Einstein|
|34 sources variable, 8 transients between ROSAT surveys|
|Osborne et al. (2001)||X||116||centre||examined the brightest sources for variability|
|Kong et al. (2002b)||C (ACIS-I)||204||central||observations between 1999 and 2001|
|(0.3–7 keV)||50% of the sources are variable, 13 transients|
|Kaaret (2002)||C (HRC)||142||–||centre||one 47 ks observation; 4626% of the sources|
|(0.1–10 keV)||with erg s are variable|
|Di Stefano et al. (2002)||C (ACIS-I/S)||28||–||3 disc fields||bright X-ray binaries|
|Di Stefano et al. (2004)||C (ACIS-S S3)||33||3 disc fields + centre||supersoft sources and quasisoft sources|
|Williams et al. (2004)||C (HRC)||166||–||major axis + centre||25% showed significant variability|
|Trudolyubov & Priedhorsky (2004)||C, X||43||–||major axis + centre||globular cluster study|
|Pietsch et al. (2005b, PFH2005)||X||856||–||major axis + centre||source catalogue|
|Pietsch et al. (2005a, PFF2005)||C, R, X||21||–||centre||correlations with optical novae|
|Orio (2006)||C, X||42||–||major axis + centre||supersoft sources and quasisoft sources|
|Pietsch et al. (2007, PHS2007)||C, X||46||–||centre||correlations with optical novae|
|Voss & Gilfanov (2007)||C||263||–||bulge region||low mass X-ray binary study|
|Stiele et al. (2008, SPH2008)||X||39||–||centre||re-analysis of archival and new 2004 observations|
|300||–||time variability analysis; 149 sources with a significance|
|(0.2–4.5 keV)||for variability 3; 6 new X-ray binary candidates,|
|3 supernova remnant classifications were rejected|
|Shaw Greening et al. (2009, SBK2009)||X||335||–||5 fields along||background subtracted spectra and lightcurves for|
|(0.3–10 keV)||major axis||each source; 18 HMXB candidates, selected from their|
|power law photon index|
|Stiele et al. (2010)||X||40||whole galaxy||supersoft sources; comparing ROSAT, Chandra and|
: X-ray satellite(s) on which the study is based: E for Einstein, R for ROSAT, C for Chandra, and X for XMM-Newton (EPIC)
: Number of sources
: observed luminosity range in the indicated energy band, assuming a distance of 780 kpc to M~31
Figure 1 shows the layout of the individual XMM-Newton observations over the field of M~31. The observations of the “Deep XMM-Newton Survey of M~31” (PI Pietsch) mainly point at the outer parts of M~31, while the area along the major axis is covered by archival XMM-Newton observations (PIs Watson, Mason, Di Stefano). To treat all data in the same way, we re-analysed all archival XMM-Newton observations of M~31, which were used in Pietsch et al. (2005b). In addition we included an XMM-Newton target of opportunity (ToO) observation of source CXOM31 J004059.2+411551 and the four observations of source RX J0042.6+4115 (PI Barnard).
All observations of the “Deep XMM-Newton Survey of M~31” and the ToO observation were taken between June 2006 and February 2008. All other observations were available via the XMM-Newton Data Archive111http://xmm.esac.esa.int/xsa/ and were taken between June 2000 and July 2004.
The journal of observations is given in Table 2. It includes the M~31 field name (Column 1), the identification number (2) and date (3) of the observation and the pointing direction (4, 5), while col. 6 contains the systematic offset (see Sect. 3.4). For each EPIC camera the filter used and the exposure time after screening for high background is given (see Sect. 3.1).
|M 31 field||Obs. id.||Obs. dates||Pointing direction||Offset||EPIC PN||EPIC MOS1||EPIC MOS2|
|Centre 2||(c2)||0112570601||2000-12-28||0:42:49.8||41:14:37||medium||5.82( 5.82)||medium||6.42( 6.42)||medium||6.42( 6.42)|
: Systematic offset in RA and Dec in arcsec determined from correlations with 2MASS, USNO-B1, LGGS and Chandra catalogues
: All observations in full frame imaging mode
: Exposure time in units of ks after screening for high background used for detection, for colour image in brackets
: Combination of the three observations is called b (see text), RX denotes RX J0042.6+4115
: CXOM31 denotes CXOM31 J004059.2+411
3 Data analysis
In this section, the basic concepts of the X-ray data reduction and source detection processes are described.
3.1 Screening for high background
The first step was to exclude times of increased background, due to soft proton flares. Most of these times are located at the start or end of an orbit window. We selected good time intervals (GTIs) – intervals where the intensity was lower than a certain threshold – using 7–15 keV light curves constructed from source-free regions of each observation. The GTIs with PN and MOS data were determined from the higher statistic PN light curves. Outside the PN time coverage, GTIs were determined from the combined MOS light curves. For each observation, the limiting thresholds for the count rate were adjusted individually; this way we avoided cutting out short periods (up to a few hundred seconds) of marginally increased background. Short periods of low background, which were embedded within longer periods of high background, were omitted. For most observations, the PN count rate thresholds were 2–8 cts ks arcmin.
As many of the observations were affected by strong background flares, the net exposure which can be used for our analysis was strongly reduced. The GTIs of the various observations ranged over 6–56 ks, apart from observation b2 (ObsID 0202230301) which had to be rejected, because it showed high background throughout the observation. The exposures for all three EPIC instruments are given in Cols. 8, 10 and 12 of Table 2. The observations obtained during the summer visibility window of M~31 were affected more strongly by background radiation than those taken during the winter window. The most affected observations of the deep survey were reobserved.
After screening for times of enhanced particle background, the second step was to examine the influence of solar wind charge exchange. This was done by producing soft energy ( keV) background light curves. These lightcurves varied only for 10 observations, for which additional screening was necessary. The screening of enhanced background due to solar wind charge exchange was applied to the observations only for the creation of colour images, in order to avoid that these observations will appear in the mosaic image with a tinge of red. The screening was not used for source detection.
The third and last step includes the study of the background due to detector noise. The processing chains take into account all known bad or hot pixels and columns and flag the affected pixels in the event lists. We selected data with (FLAG & 0xfa0000)=0, excluded rows and columns near edges, and searched by eye for additional warm or hot pixels and columns in each observation. To avoid background variability over the PN images, we omitted the energy range from 7.2–9.2 keV where strong fluorescence lines cause higher background in the outer detector area (Freyberg et al. 2004).
An additional background component can occur during the EPIC PN offset map calculation. If this period is affected by high particle background, the offset calculation will lead to a slight underestimate of the offset in some pixels which can then result in blocks of pixels () with enhanced low energy signal.222See also http://xmm2.esac.esa.int/docs/documents/CAL-TN-0050-1-0.ps.gz These blocks will be found by the SAS detection tools and appear as sources with extremely soft spectrum (so called supersoft sources). To reduce the number of false detections in this source class, we decided to include the task epreject in epchain, which locates the pixels with a slight underestimate of the offset and corrects this underestimate. To ensure that epreject produces reliable results, difference images of the event lists obtained with and without epreject, were created. Only events with energies above 200 eV were used. We checked whether epreject removed all pixels with an enhanced low energy signal. Only in observation ns1 the difference image still shows a block of pixels with enhanced signal. As this block is also visible at higher energies (PHA) it cannot be corrected with epreject. Additionally, we ascertained that almost all pixels not affected during the offset map calculation have a value consistent with zero in the difference images, with two exceptions discussed in Sect. 5.
For each observation, the data were split into five energy bands: (0.2–0.5) keV, (0.5–1.0) keV, (1.0–2.0) keV, (2.0–4.5) keV, and (4.5–12) keV. For the PN data, we used only single-pixel events (PATTERN 0) in the first energy band, while for the other bands, single-pixel and double-pixel events were selected (PATTERN 4). In the MOS cameras, single-pixel to quadruple-pixel events (PATTERN 12) were used. We created images, background images and exposure maps (with and without vignetting correction) for PN, MOS 1 and MOS 2 in each of the five energy bands and masked them for the acceptable detector area. The image bin size is 2. The same procedure was applied in our previous M~31 and M 33 studies (PFH2005 and Pietsch et al. 2004).
To create background images, the SAS task eboxdetect was run in local mode, in which it determines the background from the surrounding pixels of a sliding box, with box sizes of , and pixels (1010, 2020and 4040). The detection threshold is set to likemin = 15, which is a good compromise between cutting out most of the sources and leaving sufficient area to derive the appropriate background. For the background calculation, a two dimensional spline is fitted to a rebinned and exposure corrected image (task esplinemap). The number of bins used for rebinning is controlled by the parameter nsplinenodes, which is set to 16 for all but the observations of the central region, where it was set to 20 (maximum value). For PN, the background maps contain the contribution from the “out of time (OoT)” events.
3.3 Source detection
For each observation, source detection was performed simultaneously on 5 energy bands for each EPIC camera, using the XMM-SAS detection tasks eboxdetect and emldetect, as such fitting provides the most statistically robust measurements of the source positions by including all of the data. This method was also used to generate the 2XMM catalog (cf Watson et al. 2009). In the following we describe the detection procedure used.
The source detection procedure consists of two consecutive detection steps. An initial source list is created with the task eboxdetect (cf. Sect. 3.2). To select source candidates down to a low statistical significance level, a low likelihood threshold of four was used at this stage. The background was estimated from the previously created background images (see Sect. 3.2).
This list is the starting point for the XMM-SAS task emldetect (v. 4.60.1). The emldetect task performs a Maximum Likelihood fit of the distribution of source counts (based on Cash C-statistics approach; Cash 1979), using a point spread function model obtained from ray tracing calculations. If is the probability that a Poissonian fluctuation in the background is detected as a spurious source, the likelihood of the detection is then defined as .333This is a simplified description as emldetect transforms the derived likelihoods to equivalent likelihoods, corresponding to the case of two free parameters. This allows comparison between detection runs with different numbers of free parameters. The fit is performed simultaneously in all energy bands for all three cameras by summing the likelihood contribution of each band and each camera. Sources exceeding the detection likelihood threshold in the full band (combination of the 15 bands) are regarded as detections; the catalogue is thus full band selected.
The detection threshold used is 7, as in PFH2005. Some other parameters differ from the values used in PFH2005, as in this work a parameter setting optimised for the detection of extended sources was used (G. Lamer; private communication). The parameters in question are the event cut-out (ecut = 30.0) and the source selection radius (scut = 0.9) for multi-source fitting, the maximum number of sources into which one input source can be split (nmulsou = 2), and the maximum number of sources that can be fitted simultaneously (nmaxfit = 2). Multi-PSF fitting was performed in a two stage process for objects with a detection likelihood larger than ten. All of the sources were also fitted with a convolution of a -model cluster brightness profile (Cavaliere & Fusco-Femiano 1976) with the XMM-Newton point spread function, in order to detect any possible extension in the detected signal. Sources which have a core radius significantly larger than the PSF are flagged as extended. The free parameters of the fit were the source location, the source extent and the source counts in each energy band of each telescope.
To derive the X-ray flux of a source from its measured count rate, one uses the so-called energy conversion factors (ECF):
These factors were calculated using the detector response, and depended on the used filter, the energy band in question, and the spectrum of the source. As we wanted to apply the conversion factors to all sources found in the survey, we assumed a power law model with photon index and the Galactic foreground absorption of cm (Stark et al. 1992, see also PFH2005) to be the universal source spectrum for the ECF calculation.
The ECFs (see Table 3) were derived with XSPEC444http://heasarc.gsfc.gov/docs/xanadu/xspec(v 11.3.2) using response matrices (V.7.1) available from the XMM-Newton calibration homepage555http://xmm2.esac.esa.int/external/xmm_sw_cal/calib/epic_files.shtml. As all necessary corrections of the source parameters (e. g. vignetting corrections) were included in the image creation and source detection procedure666especially in the emldetect task, the on axis ECF values were derived (cf. Watson et al. 2009). The fluxes determined with the ECFs given in Table 3 are absorbed (i. e. observed) fluxes and hence correspond to the observed count rates, which are derived in the emldetect task.
During the mission lifetime, the MOS energy distribution behaviour has changed. Near the nominal boresight positions, where most of the detected photons hit the detectors, there has been a decrease in the low energy response of the MOS cameras (Read et al. 2006). To take this effect into account, different response matrices for observations obtained before and after the year 2005 were used (see Table 3).
|EPIC MOS 1||thin|
|EPIC MOS 2||thin|
|EPIC MOS 1||thin|
|EPIC MOS 2||thin|
For most sources, band 5 just adds noise to the total count rate. If converted to flux, this noise often dominates the total flux due to the small ECF. To avoid this problem we calculated count rates and fluxes for detected sources in the “XID” (0.2–4.5) keV band (bands 1 to 4 combined). While for most sources this is a good solution, for extremely hard or soft sources there may still be bands just adding noise. This, then, may lead to rate and flux errors that seem to falsely indicate a lower source significance. A similar effect occurs in the combined rates and fluxes, if a source is detected primarily by one instrument (e. g. soft sources in PN).
Sources are entered in the XMM LP-total catalogue from the observation in which the highest source detection likelihood is obtained (either combined or single observations). For variable sources this means that the source properties given in the XMM LP-total catalogue (see Sect. 5 and Table 5) are those observed during their brightest state.
We rejected spurious detections in the vicinity of bright sources. In regions with a highly structured background, the SAS detection task emldetect registered some extended sources. We also rejected these “sources” as spurious detections. In an additional step we checked whether an object had visible contours in at least one image out of the five energy bands. The point-like or extended nature, which was determined with emldetect, was taken into account. In this way, “sources” that are fluctuations in the background, but which were not fully modelled in the background images, were detected. In addition, objects located on hot pixels, or bright pixels at the rim or in the corners of the individual CCD chips (which were missed during the background screening) were recognised and excluded from the source catalogue, especially if they were detected with a likelihood larger than six in one detector only.
To allow for a statistical analysis, the source catalogue only contains sources detected by the SAS tasks eboxdetect and emldetect as described above, i. e. the few sources that were not detected by the analysis program, despite being visible on the X-ray images, have not been added by hand as it was done in previous studies (SPH2008; PFH2005).
To classify the source spectra, we computed four hardness ratios. The hardness ratios and errors are defined as:
for i = 1 to 4, where and denote count rates and corresponding errors in energy band i.
3.4 Astrometrical corrections
To obtain astrometrically-corrected positions for the sources of the five central fields we used the SAS-task eposcorr with Chandra source lists (Kong et al. 2002b; Kaaret 2002; Williams et al. 2004). For the other fields we selected sources from the USNO-B1 (Monet et al. 2003), 2MASS (Skrutskie et al. 2006) and Local Group Galaxy Survey (LGGS; Massey et al. 2006) catalogues777For the remainder of the subsection we will call all three catalogues “optical catalogues” for easier readability, although the 2MASS catalogue is an infrared catalogue..
3.4.1 Astrometry of optical/infrared catalogues
In a first step, we examined the agreement between the positions given by the various optical catalogues.888From the LGGS catalogue only sources brighter than 21 mag were used in order to be comparable to the brightness limit of the USNO-B1 catalogue. A close examination of the shifts obtained, showed significant differences between the positions given in the individual catalogues. In summary, between the USNO-B1 and LGGS catalogues we found an offset of: 0197 in R.A. and 0067 in Dec999the offset in declination is negligible; and between the USNO-B1 and 2MASS catalogues we found an offset of: 0108 in R.A. and 0204 in Dec. We chose the USNO-B1 catalogue as a reference, since it covers the entire field observed in the Deep XMM-Newton survey, and in addition it provides values for the proper motion of the optical sources.
Since the optical catalogues, as well as the Deep XMM-Newton catalogue, are composed of individual observations of sub-fields of M~31, we searched for systematic drifts in the positional zero points from region to region. However no systematic offsets were found.
Finally, we applied the corrections found to the sources in the LGGS and 2MASS catalogues, to bring all catalogues to the USNO-B1 reference frame.
The offsets found between the USNO-B1 and 2MASS catalogues can be explained by the independent determination of the astrometric solutions for these catalogues. Given that the positions provided in the LGGS catalogue are corrected with respect to the USNO-B1 catalogue (see Massey et al. 2006), the offset found in right ascension was totally unexpected and cannot be explained.
3.4.2 Corrections of the X-ray observations
From the positionally corrected catalogues, we selected sources which either correlate with globular clusters from the Revised Bologna Catalogue (V.3.4, January 2008; Galleti et al. 2004, 2005, 2006, 2007) or with foreground stars, characterised by their optical to X-ray flux ratio (Maccacaro et al. 1988) and their hardness ratio (see source selection criteria given in Table 6 and Stiele et al. 2008). For sources selected from the USNO-B1 catalogue, we used the proper motion corrected positions. We then used the SAS-task eposcorr to derive the offset of the X-ray aspect solution. Four observations did not have enough optical counterparts to apply this method. The lack of counterparts is due to the very short exposure times resulting after the screening for high background (obs. s3, ss12, ss13) and the location of the observation (obs. sn11). In these cases, we used bright persistent X-ray sources, which we correlated with another observation of the same field. We checked for any residual systematic uncertainty in the source positions and found it to be well characterised by a conservative value of 0 5. This uncertainty is due to positional errors of the optical sources as well as inaccuracy in the process of the determination of the offset between optical and X-ray sources, and is called systematic positional error. The appropriate offset, given in Col. 6 of Table 2, was applied to the event file of each pointing, and images and exposure maps were then reproduced with the corrected astrometry.
Fields that were observed at least twice are treated in a special way, which is described in the following section.
3.5 Multiple observations of selected fields
The fields that were observed more than once were the central field, the fields pointing on RX J0042.6+4115101010The combination of observations b1, b3 and b4 is called b., two fields located on the major axis of M~31 (S2, N2) and all fields of the “Large Survey” located in the southern part of the galaxy (SS1, SS2, SS3, S3, SN3, SN2, SN1). To reach higher detection sensitivity we merged the images, background images and exposure maps of observations which have the same pointing direction and were obtained with the same filter setting. Subsequently, source detection, as described in Sect. 3.3, was repeated on the merged data. For the S2 field, there are two observations with different filter settings. In this case, source detection was performed simultaneously on all 15 bands of both observations, i. e. on 30 bands simultaneously. The N2 field was treated in the same way. For the central field images, background images and exposure maps of observations c1, c2 and c3 were merged. These merged data were used together with the data of observation c4 to search for sources simultaneously; in this way it was possible to take into account the different ECFs for the different filters. One field was observed twice with slightly different pointing direction in observations sn1 and sn11; simultaneous source detection was used for these observations also.
3.6 Variability calculation
To examine the time variability of each source listed in the total source catalogue, we determined the XID flux at the source position in each observation or at least an upper limit for the XID flux. We used the task emldetect with fixed source positions when calculating the total flux. To get fluxes and upper limits for all sources in the input list we set the detection likelihood threshold to 0.
A starting list was created from the full source catalogue, which only contains the identification number and position of each source located in the field examined. To give correct results, the task emldetect has to process the sources from the brightest one to the faintest one. We, therefore, had to first order the sources in each observation by the detection likelihood. For sources not visible in the observation in question we set the detection likelihood to 0. This list was used as input for a first emldetect run. In this way we achieved an output list in which a detection likelihood was allocated to every source. For a final examination of the sources in order of detection likelihood, a second emldetect run was necessary.
We only accepted XID fluxes for detections 3 ; otherwise we used a 3 upper limit. To compare the XID fluxes between the different observations, we calculated the significance of the difference
and the ratio of the XID fluxes , where and are the maximum and minimum (or upper limit) source XID flux, and and are the errors of the maximum and minimum flux, respectively. This calculation was not performed whenever was an upper limit. Finally, the largest XID flux of each source was derived, excluding upper limits.
3.7 Spectral analysis
To extract the X-ray spectrum of individual sources, we selected an extraction region and a corresponding background region which was at least as large as the source region, was located on the same CCD at a similar off axis angle as the source, and did not contain any point sources or extended emission. For EPIC PN, we only accepted single-pixel events for the spectra of supersoft sources, while for all other spectra single and double-pixel events were used. For the EPIC-MOS detectors, single-pixel through to quadruple-pixel events were always used. Additionally, we only kept events with FLAG 0 for all three detectors. For each extraction region, we produced the corresponding response matrix files and ancillary response files.
For each source, the spectral fit was obtained by fitting all three EPIC spectra simultaneously, using the tool XSPEC. For the absorption, we used the TBabs model, with abundances from Wilms et al. (2000) and photoelectric absorption cross-sections from Bałucińska-Church & McCammon (1992) with a new He cross-section based on Yan et al. (1998).
3.8 Cross correlations
Sources were regarded as correlating if their positions overlapped within their 3 (99.73%) positional errors, defined as (Watson et al. 2009):
where is the statistical and the systematic error of the X-ray sources detected in the present study. The statistical error was derived by emldetect. The determination of the systematic error is described in Sect. 3.4. We use a value of 0 5, for all sources. The positional error of the sources in the catalogue used for cross-correlation is given by . The values of (68% error) used for the different X-ray catalogues can be found in Table 4. Exceptions to Eq. 4 are sources that are listed in more than one catalogue or that are resolved into multiple sources with Chandra. The first case is restricted to catalogues with comparable spatial resolution and hence positional uncertainty.
To identify the X-ray sources in the field of M~31 we searched for correlations with catalogues in other wavelength regimes. The XMM-Newton source catalogue was correlated with the following catalogues and public data bases:
- Globular Clusters:
Nova list of the M~31 Nova Monitoring Project111111http://www.mpe.mpg.de/~m31novae/opt/m31/M31_table.html ( is given for each individual source), PHS2007, Pietsch (2010)
- Supernova Remnants:
- Radio Catalogues:
Gelfand et al. (2005, is given for each individual source), Gelfand et al. (2004, is given for each individual source), Kimball & Ivezić (2008, ), Braun (1990, is given for each individual source), NVSS (NRAO/VLA Sky Survey121212http://www.cv.nrao.edu/nvss/NVSSlist.shtml; Condon et al. 1998, is given for each individual source)
- H II Regions, H Catalogue:
- Optical Catalogues:
- Infrared catalogues:
- Data bases:
|X-ray catalogue||X-ray catalogue|
|Ka2002||0 3||D2002||0 5|
: indicates that the catalogue provides values for each source individually
: value taken from indicated paper
: TF91: Trinchieri & Fabbiano (1991), PFJ93: Primini et al. (1993), SHP97: Supper et al. (1997), SHL2001: Supper et al. (2001), OBT2001: Osborne et al. (2001), D2002: Di Stefano et al. (2002), KGP2002: Kong et al. (2002b), Ka2002: Kaaret (2002), WGK2004: Williams et al. (2004), DKG2004: Di Stefano et al. (2004), TP2004: Trudolyubov & Priedhorsky (2004), PFH2005: Pietsch et al. (2005b), O2006: Orio (2006), WNG2006: Williams et al. (2006b), VG2007: Voss & Gilfanov (2007), SPH2008: Stiele et al. (2008), SBK2009: Shaw Greening et al. (2009), ONB2010: Orio et al. (2010)
4 Colour image
Figure 2 shows the combined, exposure corrected EPIC PN, MOS 1 and MOS 2 RGB (red-green-blue) mosaic image of the Deep Survey and archival data. The colours represent the X-ray energies as follows: red: 0.2–1.0 keV, green: 1.0–2.0 keV and blue: 2.0–12 keV. The optical extent of M~31 is indicated by the ellipse and the boundary of the observed field is given by the green contour. The image is smoothed with a 2D-Gaussian of 20 FWHM. In some observations, individual noisy MOS 1 and MOS 2 CCDs are omitted.
The images have not been corrected for the background of the detector or for vignetting.
The colour of the sources reflects their class. Supersoft sources appear in red. Thermal SNRs and foreground stars are orange to yellow. “Hard” sources (background objects, mainly AGN, and X-ray binaries or Crab-like SNRs) are blue to white.
Logarithmically scaled XMM-Newton EPIC low background images made up of the combined images from the PN, MOS 1 and MOS 2 cameras in the (0.2–4.5) keV XID band for each M~31 observation can be found in the Appendix. The images also show X-ray contours, and the sources from the XMM LP-total catalogue are marked with boxes.
5 Source catalogue (XMM LP-total)
The source catalogue of the Deep XMM-Newton survey of M~31 (hereafter XMM LP-total catalogue) contains 1 897 X-ray sources. Of these sources 914 are detected for the first time in X-rays.
The source parameters are summarised in Table 5, which gives the source number (Col. 1), detection field from which the source was entered into the catalogue (2), source position (3 to 9) with (99.73%) uncertainty radius (10), likelihood of existence (11), integrated PN, MOS 1 and MOS 2 count rate and error (12,13) and flux and error (14,15) in the (0.2–4.5) keV XID band, and hardness ratios and errors (16–23). Hardness ratios are calculated only for sources for which at least one of the two band count rates has a significance greater than . Errors are the properly combined statistical errors in each band and can extend beyond the range of allowed values of hardness ratios as defined previously (–1.0 to 1.0; Eq. 2). The “Val” parameter (Col 24) indicates whether the source is within the field of view (true or false, “T” or “F”) in the PN, MOS 1 and MOS 2 detectors respectively.
Table 5 also gives the exposure time (25), source existence likelihood (26), the count rate and error (27, 28) and the flux and error (29, 30) in the (0.2–4.5) keV XID band, and hardness ratios and errors (31–38) for the EPIC PN. Columns 39 to 52 and 53 to 66 give the same information corresponding to Cols. 25 to 38, but for the EPIC MOS 1 and MOS 2 instruments. Hardness ratios for the individual instruments were again screened as described above. From the comparison between the hardness ratios derived from the integrated PN, MOS 1 and MOS 2 count rates (Cols. 16–23) and the hardness ratios from the individual instruments (Cols. 31–38, 45–52 and 59–66), it is clear that the combined count rates from all instruments yielded a significantly larger fraction of hardness ratios above the chosen significance threshold.
In the remaining columns of Table 5, we give information extracted from the USNO-B1, 2MASS and LGGS catalogues (cf. Sect. 3.8). The information from the USNO-B1 catalogue (name, number of objects within search area, distance, B2, R2 and I magnitude of the brightest151515in B2 magnitude object) is given in Cols. 68 to 73. The 2MASS source name, number of objects within search area, and the distance can be found in Cols. 74 to 76. Similar information from the LGGS catalogue is given in Cols. 77 to 82 (name, number of objects within search area, distance, V magnitude, V-R and B-V colours of the brightest161616in B magnitude object). To improve the reliability of source classifications we used the USNO-B1 B2 and R2 magnitudes to calculate
and the LGGS V magnitude to calculate
following Maccacaro et al. (1988, see Cols. 83–86).
The X-ray sources in the XMM LP-total catalogue are identified or classified based on properties in X-rays (HRs, variability, extent) and of the correlated objects in other wavelength regimes (Cols. 87 and 88 in Table 5). For classified sources the class name is given in angled brackets. Identification and classification criteria are summarised in Table 6, which provides, for each source class (Col. 1), the classification criteria (2), and the numbers of identified (3) and classified (4) sources. The hardness ratio criteria are based on model spectra. Details on the definition of these criteria can be found in Sect. 6 of PFH2005. As we have no clear hardness ratio criteria to discriminate between XRBs, Crab-like supernova remnants (SNRs) or AGN we introduced a hard class for those sources. If such a source shows strong variability (i. e. V10) on the examined time scales it is likely to be an XRB. Compared with SPH2008 the HR2 selection criterion for SNRs was tightened (from HR2 to HR2EHR2) to exclude questionable SNR candidates from the class of SNRs. If we applied the former criterion to the survey data, 35 sources would be classified as SNRs in addition to those listed in Table 6. Most of the 35 sources are located outside the D ellipse, and none of them correlates with an optically identified SNR, a radio source, or an HII region. In addition, the errors in HR2 are of the same order as the HR2 values. It is therefore very likely that these sources do belong to other classes, since the strip between HR20 is populated by foreground stars, XRBs, background objects, and candidates for these three classes. Outcomes of the identification and classification processes are discussed in detail in Sects. 9 and 10.
The last column (89) of Table 5 contains the XMM-Newton source name as registered to the IAU Registry. Source names consist of the acronym XMMM31 and the source position as follows: XMMM31 Jhhmmss.s+ddmmss, where the right ascension is given in hours (hh), minutes (mm) and seconds (ss.s) truncated to decimal seconds and the declination is given in degrees (dd), arc minutes (mm) and arc seconds (ss) truncated to arc seconds, for equinox 2000. In the following, we refer to individual sources by their source number (Col. 1 of Table 5), which is marked with a “N” at the front of the number.
Of the 1 897 sources, 1 247 can only be classified as hard sources, while 123 sources remain without classification. Two of them (N 482, N 768) are highly affected by optical loading; both “X-ray sources” coincide spatially with very bright optical foreground stars (USNO-B1 R2 magnitudes of 6.76 and 6.74 respectively). The spectrum of source N 482 is dominated by optical loading. This becomes evident from the hardness ratios which indicate an SSS. For N 768 the hardness ratios would allow a foreground star classification. The obtained count rates and fluxes of both sources are affected by the usage of epreject, which neutralises the corrections applied for optical loading. Therefore residuals are visible in the difference images created from event lists obtained with and without epreject. As we cannot exclude the possibility that some of the detected photons are true X-rays – especially for source N 768 –, we decided to include them in the XMM LP-total catalogue, but without a classification.
|Source class||Selection criteria||identified||classified|
|fg Star||and HR2EHR and HR3EHR or not defined||40||223|
|AGN||Radio source and not classification as SNR from HR2 or optical/radio||11||49|
|Gal||optical id with galaxy||4||19|
|GCl||X-ray extent and/or spectrum||1||5|
|SSS||HR, HR2EHR or HR2 not defined, HR3, HR4 not defined||30|
|SNR||HR and HR2EHR and not a fg Star, or id with optical/radio SNR||25||31|
|XRB||optical id or X-ray variability||10||26|
|hard||HR2EHR or only HR3 and/or HR4 defined, and no other classification||1 247|
5.1 Flux distribution
The faintest source (N 526) has an XID band flux of 5.8 erg cm s. The source with the highest XID Flux (N 966, XID band flux of 3.75 erg cm s) is located in the centre of M~31 and identified as a Z-source LMXB (Barnard et al. 2003). This source has a mean absorbed XID luminosity of 2.74 erg s.
Figure 3 shows the distribution of the XID (0.2–4.5 keV) source fluxes. Plotted are the number of sources in a certain flux bin. We see from the inlay that the number of sources starts to decrease in the bin from 2.4 to 2.6 erg cm s. This XID flux roughly determines the completeness limit of the survey and corresponds to an absorbed 0.2–4.5 keV limiting luminosity of erg s.
Previous X-ray studies (Williams et al. 2004, and references therein) noted a lack of bright sources ( erg s; 0.1–10 keV) in the northern half of the disc compared to the southern half. This finding is not supported in the present study. Excluding the pointings to the centre of M~31, we found in the remaining observations 13 sources in each hemisphere that were brighter than erg s.171717The luminosity is based on XID Fluxes. Using the total 0.2–12 keV band the result does not change (23 in the northern half and 24 in the southern half). The reason our survey does not support the old results is that we found several bright sources in the outer regions of the northern half of the disk, which have not been covered in Williams et al. (2004, and references therein). In the central field of M~31, a total of 41 sources brighter than erg s (0.2–4.5 keV) were found.
Figure 4 shows the spatial distribution of the bright sources. Striking features are the two patches located north and south of the centre. The southern one seems to point roughly in the direction of M 32 (N 995), while the northern one ends in the globular cluster B 116 (N 947). However there is no association to any known spatial structure of M~31, like e. g. the spiral arms.
5.2 Exposure map
Figure 5 shows the exposure map used to create the colour image of all XMM-Newton Large Survey and archival observations (Fig. 2). The combined MOS exposure was weighted by a factor of 0.4, before being added to the PN exposure. However, this map does not quite represent the exposures used in source detection; overlapping regions were not combined during source detection.
From Fig. 5 we see that the exposure for most of the surveyed area is rather homogeneous. Exceptions are the central area, overlapping regions and observation h4.
5.3 Hardness ratio diagrams
We plot X-ray colour/colour diagrams based on the HRs (see Fig. 6). Sources are plotted as dots if the error in both contributing HRs is below 0.2. Classified and identified sources are plotted as symbols in all cases. Symbols including a dot therefore mark the well-defined HRs of a class.
From the HR1-HR2 diagram (upper panel in Fig. 6) we note that the class of SSSs is the only one that can be defined based on hardness ratios alone. In the part of the HR1-HR2 diagram that is populated by SNRs, most of the foreground stars and some background objects and XRBs are also found.
Foreground star candidates can be selected from the HR2-HR3 diagram (middle panel in Fig. 6), where most of them are located in the lower left corner. The HR3-HR4 diagram (lower panel in Fig. 6) does not help to disentangle the different source classes. Thus, we need additional information from correlations with sources in other wavelengths or on the source variability or extent to be able to classify the sources.
5.4 Extended sources
The XMM LP-total catalogue contains 12 sources which are fitted as extended sources with a likelihood of extension larger than 15. This value was chosen so as to minimise the number of spurious detections of extended sources (H. Brunner; private communication), as well as keeping all sources that can clearly be seen as extended sources in the X-ray images. A convolution of a -model cluster brightness profile (Cavaliere & Fusco-Femiano 1976) with the XMM-Newton point spread function was used to determine the extent of the sources (cf. Sect. 3.3). This model describes the brightness profile of galaxy clusters, as
where denotes the core radius; this is also the extent parameter given by emldetect.
Table 7 gives the source number (Col. 1), likelihood of detection (2), the extent found (3) and its associated error (4) in arcsec, the likelihood of extension (5), and the classification of the source (6, see Sect. 9.2) for each of the 12 extended sources. Additional comments taken from Table 5 are provided in the last column.
|141||65.08||11.22||1.29||23.68||1.45||0.20||GCl||GLG127(Gal), 37W 025A (IR, RadioS; NED)|
|252||222.05||14.64||1.12||81.60||4.40||0.49||GCl||5 optical objects in error box|
|304||299.75||15.10||0.92||133.62||2.20||0.18||GCl||B242 [CHM09]; RBC3.5: GlC|
|718||77.75||7.18||1.23||21.47||0.58||0.07||Gal||B052 [CHM09], RBC3.5|
|1 543||70.49||11.87||1.37||28.63||1.51||0.19||GCl||[MLA93] 1076 PN (SIM,NED)|
|1 795||11 416.36||18.79||0.29||4 169.74||98.87||1.43||GCl||GLG253 (Gal), [B90] 473, z=0.3 [KTV2006]|
|1 912||332.06||23.03||1.23||213.90||5.43||0.37||GCl||cluster of galaxies candidate|
: Extent and error of extent in units of 1; 1 corresponds to 3.8 pc at the assumed distance of M~31
: XID Flux and flux error in units of 1 erg cm s
: Taken from Table 5
The extent parameter found for the sources ranges from 62 to 2303 (see Fig. 7). The brightest source (N 1 795), which has the highest likelihood of extension and the second largest extent, was identified from its X-ray properties as a galaxy cluster located behind M~31 (Kotov et al. 2006). The iron emission lines in the X-ray spectrum yield a cluster redshift of . For further discussion see Sect. 9.2.
6 Variability between XMM-Newton observations
To examine the long-term time variability of each source, we determined the XID flux at the source position in each observation or at least an upper limit for the XID flux. The XID fluxes were used to derive the variability factor and the significance of variability (cf. Sect. 3.6).
The sources are taken from the XMM LP-total catalogue (Table 5). Table 8 contains all information necessary to examine time variability. Sources are only included in the table if they are observed at least twice. Column 1 gives the source number. Columns 2 and 3 contain the flux and the corresponding error in the (0.2–4.5) keV XID band. The hardness ratios and errors are given in columns 4 to 11. Column 12 gives the type of the source. All this information was taken from Table 5.
The subsequent 140 columns provide information related to individual observations in which the position of the source was observed. Column 13 gives the name of one of these observations, which we will call observation 1. The EPIC instruments contributing to the source detection in observation 1, are indicated by three characters in the “obs1_val” parameter (Col. 14, first character for PN, second MOS 1, third MOS 2), each one being either a “T” if the source is inside the FoV, or “F” if it lies outside the FoV. Then the count rate and error (15,16) and flux and error (17,18) in the (0.2–4.5) keV XID band, and hardness ratios and error (19–26) of observation 1 are given. Corresponding information is given for the remaining observations which cover the position of the source: obs. 2 (cols. 27–40), obs. 3 (41–54), obs. 4 (55–68), obs. 5 (69–82), obs. 6 (83–96), obs. 7 (97–110), obs. 8 (111–124), obs. 9 (125–138), obs. 10 (139–152). Whether the columns corresponding to obs. 3 – obs. 10 are filled in or not, depends on the number of observations in which the source has been covered in the combined EPIC FoV. This number is indicated in column 153. The maximum significance of variation and the maximum flux ratio (fvar_max) are given in columns 154 and 155. As described in Sect. 3.6, only detections with a significance greater than 3 were used, otherwise the 3 upper limit was adopted. Column 156 indicates the number of observations that provide only an upper limit. The maximum flux (fmax) and its error are given in columns 157 and 158.
In a few cases a maximum flux value could not be derived, because each observation only yielded an upper limit. There can be two reasons for this: The first reason is that faint sources detected in merged observations may not be detected in the individual observations at the 3 limit. The second reason is that in cases where the significance of detection was not much above the 3 limit, it can become smaller than the 3 limit when the source position is fixed to the adopted final mean value from all observations.
|966||1.63||49.01||46.73||0.59||XRB||1(sv,z), 2, 10(v), 12(v), 13, 14, 20, 22(v), 25(LMXB), 27, 28(1.56)|
|877||3.13||49.13||16.06||0.20||hard||1(sv), 2, 10(v), 12(v), 13, 14, 20(v), 22(v), 27, 28(3.05)|
|1 157||1.32||11.10||9.87||0.25||GlC||1(sv), 2, 5, 10, 12, 13, 14, 20, 21, 22(v), 27, 28(1.37)|
|1 060||2.13||30.00||9.04||0.14||XRB||1(sv), 2, 10, 12, 13, 14, 20(v, NS-LMXB), 22(v), 27|
|1 171||4.14||18.86||9.02||0.41||GlC||1(d,sv), 2(t, 53.4), 5, 10, 12, 13, 14, 16, 20, 22, 27, 28(2.47)|
|1 116||3.76||51.98||8.16||0.10||GlC||1(sv), 2(t, 58.6), 3(t, 33), 5, 10, 12, 13, 14, 16, 20, 21, 22(v,t), 27|
Figure 8 shows the variability factor plotted versus maximum detected XID flux. Apart from XRBs, or XRBs in GlCs, or candidates of these source classes, which were selected based on their variability, there are a few SSS candidates showing pronounced temporal variability. The sources classified or identified as AGN, background galaxies or galaxy clusters all show . Most of the foreground stars show .
Out of the 1 407 examined sources, we found 317 sources with a variability significance , i. e. 182 more than reported in SPH2008. For bright sources it is much easier to detect variability than for faint sources, because the difference between the maximum observed flux and the detection limit is larger. Therefore the significance of the variability declines with decreasing flux. This is illustrated by the distribution of the sources marked in green in Fig. 8.
Table 24 lists all sources with a variability factor larger than five. There are 69 such sources (34 in addition to SPH2008). The sources are sorted in descending order with respect to their variability factors. Table 24 gives the source number (Col. 1), maxima of flux variability (2) and maxima of the significance parameter (3). The next columns (4, 5) indicate the maximum observed flux and its error. Column 6 contains the class of the source. Sources with that were not already classified as SSS or foreground stars, were classified as XRB.
Time variability can also be helpful to verify a SNR candidate classification. If there is significant variability, the SNR classification must be rejected, and if an optical counterpart is detected, the source has to be re-classified as foreground star candidate. Column 7 contains references to the individual sources in the literature. In some cases the reference provides information on the temporal behaviour and a more precise classification (see brackets). The numbers given in connection with Voss & Gilfanov (2007) and Williams et al. (2006b) are the variability factors obtained in these papers from Chandra data. From the 69 sources of Table 24, ten show a flux variability larger than 100. With a flux variability factor source N 523 is the most variable source in our sample. Source N 57 has the largest significance of variability, with a value of . The variability significance is below 10 for just 33 sources, 15 of which show significance values below 5. Thirty-five of the variable sources are classified as XRBs or XRB candidates, and eight of them are located in globular clusters. Nine of the variable sources are SSS candidates, while six variable sources are classified as foreground stars and foreground star candidates.
Table 10 lists all “bright” sources with a maximum flux larger than 8 erg cm s and a flux variability smaller than five (the description of the columns is the same as in Table 24). All seven sources listed in Table 10 (three in addition to SPH2008) have a significance of variability . Apart from two sources, they are XRBs (three in globular clusters) or XRB candidates. The most luminous source in our sample is source N 966 with an absorbed 0.2–4.5 keV luminosity of erg s at maximum.
Figure 9 shows the relationship between the variability factor and the hardness ratios HR1 and HR2, respectively. The hardness ratios are taken from Table 5. The HR1 plot shows that the sample of highly variable sources includes SSS and XRB candidates, which occupy two distinct regions in this plot (see also Haberl & Pietsch 1999, for the LMC). The SSSs marked by triangles, appear on the left hand side, while the XRBs or XRB candidates have much harder spectra, and appear on the right. It seems that foreground stars, SSSs and XRBs can be separated, on the HR2 diagram, although there is some overlap between foreground stars and XRBs.
7 Cross-correlations with other M~31 X-ray catalogues
Cross-correlations were determined by applying Eq. 4 to the sources of the XMM LP-total catalogue and to sources reported in earlier X-ray catalogues. The list of X-ray catalogues used is given in Table 4.
7.1 Previous XMM-Newton catalogues
Previous source lists based on archival XMM-Newton observations were presented in Osborne et al. (2001), PFH2005, Orio (2006), SPH2008, and SBK2009. Of these four studies, PFH2005 covers the largest area of M~31. Table 11 lists all sources from previous XMM-Newton studies that are not detected in the present investigation.
|PFH2005 856 sources|
|103 not detected|
|6 not detected, LH100:||327 (SNR,LH2140.0), 384 (XRB,667.0), 332 (SNR,654.0),|
|316 (SNR,259.0), 312 (SNR,241.0), 281(hard,160.0)|
|10 not detected, 20LH50:||75 (SSS), 423 (fg Star), 120 (hard), 505 (hard),|
|220 (SNR), 304 (fg Star), 819 (hard), 799 (SSS), 413 (SNR), 830 (hard)|
|14 not detected, 15LH20:||427(hard), 734 (hard), 424 (hard), 518 (SSS),|
|232 (hard), 339 (hard), 446 (SSS), 219 (fg Star), 567 (hard), 256 (fg Star),|
|356 (hard), 248 (hard), 160 (hard), 399 ()|
|21 not detected, 10LH15:||375 (hard), 17 (hard), 195 (hard), 417 (SNR),|
|783 (hard), 803 (hard), 829 (hard), 135 (hard), 151 (hard), 131 (hard),|
|426 (hard), 593 (fg Star), 526 (hard), 250 (hard), 62 (hard), 67 (hard),|
|188 (hard), 186 (AGN), 510 (hard), 529 (hard), 754 (hard)|
|52 not detected, LH10:||599 (hard), 439 (hard), 809 (hard), 14 (SNR), 743 (hard),|
|433 (hard), 5 (), 210 (hard), 97 (hard), 708 (hard), 476 (), 534 (hard), 501 (),|
|170 (hard), 146 (SNR), 769 (), 838 (hard), 571 (hard), 816 (hard, 554 (), 627 (hard),|
|464 (fg Star), 811 (hard), 655 (hard), 184 (hard), 447 (hard), 380 (hard),|
|566 (hard), 137 (fg Star), 63 (), 48 (), 152 (fg Star), 291 (hard), 559 (hard),|
|102 (hard), 740 (hard), 540 (fg Star), 240 (hard), 485 (), 668 (hard), 44 (),|
|560 (hard), 836 (hard), 436 (hard), 484 (fg Star), 216 (hard), 362 (hard), 527 (), 179 (hard),|
|834 (hard), 86 (hard), 455 ()|
|SPH2008 39 sources|
|15 not detected|
|3 not detected, 50LH100:||874 (SNR,LH85.5), 895 (hard,75.9), 882 ( ,56.4)|
|6 not detected, 10LH50:||869 (), 885 (SNR), 863 (hard), 875 (SSS), 893 (hard), 866 (hard)|
|6 not detected, LH10:||870 (SNR), 891 (hard), 889 (hard), 872 (SNR), 867 (hard), 862 (SNR)|
|SBK2009 335 sources|
|31 not detected|
|4 (hard), 18 (hard), 29 (hard), 32 (hard), 34 (hard), 45 (SSS), 67 (hard),|
|102 (hard), 106 (hard), 117 (hard), 149 (hard), 152 (hard), 179 (hard),|
|183 (hard), 184 (hard), 188 (hard), 191 (hard), 192 (AGN), 202 (hard),|
|204 (fg star), 217 (hard), 249 (hard), 250 (hard), 260 (hard), 274 (hard),|
|279 (hard), 285 (hard), 295 (hard), 306 (hard), 325 (hard), 333 (hard)|
In the ten observations covering the major axis, and a field in the halo of M~31, PFH2005 detected 856 X-ray sources with a detection likelihood threshold of 7 (cf. Sect. 3.3). Of these 856 sources, only 753 sources are also present in the XMM LP-total catalogue, i. e. 103 sources of PFH2005 were not detected. This can be due to: the search strategy; the parameter settings used in the emldetect run; the determination of the extent of a source for the XMM LP-total catalogue; the more severe screening for GTIs for the XMM LP-total catalogue, which led to shorter final exposure times; the use of the epreject task and last but not least due to the SAS versions and calibration files applied. The search strategy of PFH2005 was optimised to detect sources located close to each other in crowded fields. This point especially explains the non-detection of the bright PFH2005 sources [PFH2005] 281, 312, 316, 327, 332, 384 () in the present study, as four of them ([PFH2005] 312, 316, 327, 332) are located in the innermost central region of M~31 where source detection is complicated by the bright diffuse X-ray emission, while [PFH2005] 281 and 384 lie in the immediate vicinity of two bright sources ([PFH2005] 280 and 381 at distances of 7.7and 5.5, respectively). The changes in the SAS versions and the GTIs, in particular, affect sources with small detection likelihoods ().
The improvements in the SAS detection tools and calibration files should reduce the number of spurious detections, which increase with decreasing detection likelihood. However, this does not necessarily imply that all undetected sources with of PFH2005 are spurious detections. The changes in the SAS versions, calibration files and GTIs do not only affect the source detection tasks, but also can cause changes in the background images. These changes may increase the assumed background value at the position of a source, which would result in a lower detection likelihood. Going from mlmin = 7 to mlmin = 6, but leaving everything else unchanged, we detected an additional nine sources of PFH2005. One of the previously undetected sources ([PFH2005] 75) was classified as SSS, but correlates with blocks of pixels with enhanced low energy signal in the PN offset map and was corrected by epreject. Another source classified as SSS ([PFH2005] 799) is only detected in the MOS 1 camera, but not in MOS 2. From an examination by eye, it seems that source [PFH2005] 799 is the detection of some noisy pixels at the rim of the MOS 1 CCD 6 and not a real X-ray source.
SPH2008 extended the source catalogue of PFH2005 by re-analysing the data of the central region of M~31 and also including data of monitoring observations of LMXB RX J0042.6+4115. Of the 39 new sources presented in SPH2008, 24 are also listed in the XMM LP-total catalogue, i. e. 15 sources of SPH2008 were not detected. Differences between the two studies include the detection likelihood thresholds used for eboxdetect (SPH2008: likemin=5) and emldetect (SPH2008: mlmin=6), the lower limit for the likelihood of extention (SPH2008: dmlextmin = 4; XMM LP-total: 15), the screening for GTIs, the use of the epreject task and the SAS versions and calibration files used. Concerning the GTIs, images, background images and exposure maps SPH2008 followed the same procedures as in PFH2005. The arguments given above are therefore also valid here. From the 14 undetected sources, three sources were detected in SPH2008 with mlmin 7. One source ([SPH2008] 882) was added by hand to the final source list, as SPH2008 could not find any reason why emldetect did not automatically find it. The two extended sources ([SPH2008] 863, 869) detected with extent likelihoods between 4.7 and 5.1 in SPH2008, are neither detected as extended nor as pointlike sources in the present study, where the extent likelihood has to be larger than 15.
SBK2009 re-analysed the XMM-Newton observations located along the major axis of M~31, ignoring all observations pointing to the centre of the galaxy. They used a detection likelihood threshold of ten. Of the 335 sources detected by SBK2009, 304 sources are also contained in the XMM LP-total catalogue, i. e. 31 sources are not detected. Of the 304 re-detected sources, two ([SBK2009] 298, 233) are found with a detection likelihood below ten. Of the 31 undetected sources, 27 were also not detected in PFH2005. The remaining four sources correlate with PFH2005 sources, which were not detected in the present study. SBK2009 state that they find 34 sources not present in the source catalogue of PFH2005. A possible reason for this may be that SBK2009 used different energy bands for source detection. They also had five bands, but they combined bands 2 and 3 from PFH2005 into one band in the range 0.5–2 keV, and on the other hand they split band 5 of PFH2005 into two bands from 4.5–7 keV and from 7–12 keV, respectively. This might also explain why most of the additional found sources were classified as hard.
Orio (2006) addressed the population of SSSs and QSSs based on the same archival observations as PFH2005. Orio (2006) detected 15 SSSs, 18 QSSs and 10 SNRs of which one ([O2006] Table 4, Src. 3) is also listed as an SSS ([O2006] Table 2, Src. 13). Of these sources two SSSs, four QSSs and two SNRs (among them is the source [O2006] Table 4, Src. 3) are not contained in the XMM LP-total catalogue. These seven sources are also not present in the PFH2005 catalogue.
The nine bright variable sources from Osborne et al. (2001) were all detected.
7.2 Chandra catalogues
Details of the comparison between the XMM LP-total catalogue and the different Chandra catalogues can be found in Table 12. Here, we only give a few general remarks. A non-negligible number of Chandra sources not reported in the XMM LP-total catalogue have already been classified as transient or variable sources. Thus, it is not surprising that these sources were not detected in the XMM-Newton observations (parts of: Voss & Gilfanov 2007; Williams et al. 2006b, DKG2004). One Chandra source (n1-66) lies outside the field of M~31 covered by the XMM-Newton observations. For the innermost central region of M~31, the point spread function of XMM-Newton causes source confusion and therefore only Chandra observations are able to resolve the individual sources, especially if they are faint compared to the diffuse emission or nearby bright sources (Kong et al. 2002b; Kaaret 2002; Williams et al. 2004; Di Stefano et al. 2004; Williams et al. 2006b; Voss & Gilfanov 2007). This explains why a certain number of these sources are not detected in XMM-Newton observations.
|Kong et al. (2002b) 204 sources|
|58 not detected|
|Kaaret (2002) 142 sources|
|26 not detected|
|Williams et al. (2004) 166 sources|
|28 not detected|
|Voss & Gilfanov (2007) 261 sources|
|104 not detected|
|Williams et al. (2006b) 45 sources|
|25 not detected|
|DKG2004 43 sources|
|15 not detected|
|1 not in FoV:||n1-66|
|Di Stefano et al. (2002) 28 sources|
|2 not detcted|
|2 unclassified:||17 ( r2-15), 28 ( r3-71)|
Variability information (transient, variable) is taken from the papers. “Unclassified” denotes sources which are not indicated as transient or variable sources in the papers.
Of the 28 bright X-ray sources located in globular clusters (Di Stefano et al. 2002), two were not found in the XMM-Newton data (see Table 12). They are also not included in the source catalogue of PFH2005 and SPH2008. Hence, both objects are good candidates for being transient or at least highly variable sources (cf. Sect. 10.4.2). Another study of the globular cluster population of M~31 is presented by Trudolyubov & Priedhorsky (2004). Their work is based on XMM-Newton and Chandra data and contains 43 X-ray sources. Of these sources three were not found in the present study. One of them ([TP2004] 1) is located well outside the field of M~31 covered by the Deep XMM-Newton Survey181818The source was observed with XMM-Newton on 11 January 2001. Obs. id.: 0065770101. The second source ([TP2004] 21) correlates with r3-71, which is discussed above (see Di Stefano et al. (2002) in Table 12). The transient nature of the third source ([TP2004] 35), and the fact that it was not observed in any XMM-Newton observation taken before 2004 was already reported by Trudolyubov & Priedhorsky (2004). The source was first detected with XMM-Newton in the observation from 31 December 2006.
7.3 Rosat catalogues
Of the 86 sources detected with ROSAT HRI in the central 34 of M~31 (PFJ93), all but eight sources ([PFJ93] 1,2,31,33,40,48,63,85) are detected in the XMM-Newton observations. Six of these eight sources ([PFJ93] 1,2,31,33,63,85) have already been discussed in PFH2005 and classified as transients. Sources [PFJ93] 40 and 48 correlate with [PFH2005] 312 and 332, respectively, which are discussed in Sect. 7.1. In addition to these eight sources, PFH2005 did not detect source [PFJ93] 51. This source was detected in the XMM-Newton observations centred on RX J0042.6+4115 and was thus classified as a recurrent transient (see SPH2008).
In each of the two ROSAT PSPC surveys of M~31, 396 individual X-ray sources were detected (SHP97 and SHL2001). From the SHP97 catalogue 130 sources were not detected. Of these sources 48 are located outside the FoV of our XMM-Newton M~31 survey. From the SHL2001 catalogue, 93 sources are not detected, 60 of which lie outside the XMM-Newton FoV. For information on individual sources see Table 13.
|SHP97 396 sources|
|130 not detected|
|48 outside FoV:||1,2,3,4,5,7,8,14,31,41,72,91,98,104,120,125,159,202,209,271,276,285,286,290,300,312,314,|
|21 not detected, LH12:||19,24,27,33,46,52,59,63,68,71,133,149,161,264,273,275,307,329,330,358,377|
|16 not detected, 12LH15:||12,15,49,82,93,113,114,128,196,230,262,283,334,364,372,376|
|44 not detected, LH15:||16(LH26.6),32(30.2),43(18.2),45(51.2),60(20.1),66(36.2),67(4536.2),78(20.5),80(16.3),81(26.6),|
|SHL2001 396 sources|
|93 not detected|
|60 outside FoV:||1,2,3,4,5,6,7,8,9,10,11,12,14,15,16,21,22,32,39,58,67,69,75,77,81,83,85,90,93,125,141,146,|
|4 not detected, LH12:||62,96,238,269|
|2 not detected, 12LH15:||231,361|
|27 not detected, LH15:||51(LH28.4),104(901.2),121(94.1),126(46.2),143(34.7),168(131.9),171(43.0),173(317.8),190(215.8),|
Forty-four (out of 302) sources from SHP97 and 27 (out of 293) sources from SHL2001, have ROSAT detection likelihoods larger than 15, but are not listed in the XMM LP-total catalogue. These sources have to be regarded as transient or at least highly variable.
7.4 Einstein catalogue
The list of Einstein X-ray sources in the field of M~31 reported by TF91 contains 108 sources, with 81 sources taken from the Einstein HRI data with an assumed positional error of 3 (reported by Crampton et al. 1984), and 27 sources based on Einstein IPC data with a 45 positional error. Applying the above mentioned correlation procedure to the Einstein HRI sources, 64 of these sources are also detected in this work and listed in the XMM LP-total catalogue, i. e. 17 sources are not detected ([TF91] 29, 31, 35, 39, 40, 43, 46, 50, 53, 54, 65, 66, 72, 75, 78, 93, 96). For the Einstein IPC sources only the 1 positional error was used to search for counterparts among the XMM-Newton sources. Of the 27 Einstein IPC sources six remain without a counterpart in our catalogue ([TF91] 15, 99, 100, 106, 107, 108), of which [TF91] 15 and 108 are located outside the field of M~31 covered by the XMM LP-total catalogue. Sources [TF91] 50 and 54 correlate with [PFH2005] 312 and 316, respectively. Both sources were already discussed in Sect. 7.1. Apart from [TF91] 106, which is suggested as a possible faint transient by SHL2001, the remaining 18 sources are also not detected by PFH2005. They classified those sources as transient.
8 Cross-correlations with catalogues at other wavelengths
The XMM LP-total catalogue was correlated with the catalogues and public data bases given in Sect. 3.8. Two sources (from the XMM LP-total and from the reference catalogues) were be considered as correlating, if their positions matched within the uncertainty (see Eq. 4).
However, the correlation of an X-ray source with a source from the reference catalogue does not necessarily imply that the two sources are counterparts. To confirm this, additional information is needed, like corresponding temporal variability of both sources or corresponding spectral properties. We should also take into account the possibility that the counterpart of the examined X-ray source is not even listed in the reference catalogue used (due to faintness for example).
The whole correlation process will get even more challenging if an X-ray source correlates with more than one source from the reference catalogue. In this case we need a method to decide which of the correlating sources is the most likely to correspond to the X-ray source in question. Therefore, the method used should indicate how likely the correlation is with each one of the sources from the reference catalogue. Based on these likelihoods one can define criteria to accept a source from the reference catalogue as being the most likely source to correspond to the X-ray source.
The simplest method uses the spatial distance between the X-ray source and the reference sources to derive the likelihoods. In other words, the source from the reference catalogue that is located closest to the X-ray source is regarded as the most likely source corresponding to the X-ray source.
An improved method is a “likelihood ratio” technique, were an additional source property (e. g. an optical magnitude in deep field studies) is used to strengthen the correlation selection process. This technique was applied successfully to deep fields to find optical counterparts of X-ray sources (e. g. Brusa et al. 2007). A drawback of this method is that one a priori has to know the expected probability distribution of the optical magnitudes of the sources belonging to the studied object. In our case, this means that we have to know the distribution function for all optical sources of M~31 that can have X-ray counterparts, without including foreground and background sources. Apart from the fact that such distribution functions are unknown, an additional challenge would be the time dependence of the magnitude of the optical sources (e. g. of novae) and of the connection between optical and X-ray sources (e. g. optical novae and SSSs). Therefore it is not possible to apply this “likelihood ratio” technique to the sources in the XMM LP-total survey. The whole correlation selection process becomes even more challenging if more than one reference catalogue is used.
To be able to take all available information into account, we decided not to automate the selection process, but to select the class and most likely correlations for each source by hand (as it was done e. g. in PFH2005). Therefore the source classification, and thus the correlation selection process, is based on the cross correlations between the different reference catalogues, on the X-ray properties (hardness ratios, extent and time variability), and on the criteria given in Table 6. For reasons of completeness we give for each X-ray source the number of correlations found in the USNO-B1, 2MASS and LGGS catalogues in Table 5. The caveat of this method is that it cannot quantify the probability of the individual correlations.
9 Foreground stars and background objects
9.1 Foreground stars
X-ray emission has been detected from many late-type – spectral types F, G, K, and M – stars, as well as from hot OB stars (see review by Schmitt 2000). Hence, X-ray observations of nearby galaxies also reveal a significant fraction of Galactic stars. With typical absorption-corrected luminosities of erg s, single stars in other galaxies are too faint to be detected with present instruments. However, concentrations of stars can be detected, but not resolved.
Foreground stars (fg Stars) are a class of X-ray sources which are homogeneously distributed over the field of M~31 (Fig. 10). The good positional accuracy of XMM-Newton and the available catalogues USNO-B1, 2MASS and LGGS allow us to efficiently select this type of source. The selection criteria are given in Table 6. The optical follow-up observations of Hatzidimitriou et al. (2006) and Bonfini et al. (2009) have confirmed the foreground star nature of bright foreground star candidates selected in PFH2005, based on the same selection criteria as used in this paper. Somewhat different criteria were applied for very red foreground stars, with an LGGS colour or USNO-B1 colour . These are classified as foreground star candidates, if and . A misclassification of symbiotic systems in M~31 as foreground objects by this criterion can be excluded, as symbiotic systems typically have X-ray luminosities below erg s, which is more than a factor 100 below the detection limit of our survey.
If the foreground star candidate lies within the field covered by the LGGS we checked its presence in the LGGS images (as the LGGS catalogue itself does not list bright stars, because of saturation problems). Otherwise DSS2 images were used. Correlations with bright optical sources from the USNO-B1 catalogue, with an in the range expected for foreground stars, that were not visible in the optical images were rejected as spurious. We found 223 foreground star candidates. Fourty sources were identified as foreground stars, either because they are listed in the globular cluster catalogues as spectroscopically confirmed foreground stars or because they have a spectral type assigned to them in the literature (Bonfini et al. 2009; Hatzidimitriou et al. 2006, SIMBAD).
Two of the foreground star candidates close to the centre of M~31 (N 826, N 1 110) have no entry in the USNO-B1 and LGGS catalogues, and one has no entry in the USNO-B1 R2 and B2 columns (N 976). However, they are clearly visible on LGGS images, they are 2MASS sources and they fulfil the X-ray hardness ratio selection criteria. Therefore, we also classify them as foreground stars.
The following 19 sources were selected as very red foreground star candidates: N 54, N 118, N 384, N 391, N 393, N 585, N 646, N 651, N 711, N 1 038, N 1 119, N 1 330, N 1 396, N 1 429, N 1 506, N 1 605, N 1 695, N 1 713 and N 1 747. A further 10 sources (N 210, N 269, N 278, N 310, N 484, N 714, N 978, N 1 591, N 1 908 and N 1 930) fulfil the hardness ratio criteria, but violate the criteria and are therefore marked as “foreground star candidates” in the comment column of Table 5.
Six sources (N 473, N 780, N 1 551, N 1 585, N 1 676, N 1 742), classified as foreground star candidates, have X-ray light curves that in a binning of 1 000 s showed flares (see Fig. 11). These observations strengthen the foreground star classification. A seventh source (N 714) is classified as a foreground star candidate, since its hardness ratios and its ratio in the quiescent state fulfil the selection criteria of foreground star candidates. In addition, the source shows a flare throughout observation ss3. Hence, the ratio for this observation, in which the source is brightest, is too high to be consistent with the range of values expected for foreground stars.
|N||J mag||H mag||K mag||SpT||err|
: spectral type
: error (in subtypes)
Table 14 gives the J, H and K magnitudes taken from the 2MASS catalogue for each of the six flaring foreground stars. Using the standard calibration of spectral types for dwarf stars based on their near infrared colours (from the fourth edition of Allen’s astrophysical quantities, edt. A.Cox, p.151) we derived the spectral classification for the objects, using both H-K and J-K. The spectral types (and ”error”) we give in Table 14 are derived from averaging the two classes (derived from the two colours). The spectral types are entirely consistent with those expected for flare stars (usually K and M types).
Figure 12 shows the XID flux distribution for foreground stars and foreground star candidates, which ranges from 6.9 erg cm s to 2.0 erg cm s. Most of the foreground stars and candidates (257 sources) have fluxes below 5 erg cm s.
9.1.1 Comparing XMM-Newton, Chandra and Rosat catalogues
In the combined ROSAT PSPC survey (SHP97, SHL2001) 55 sources were classified as foreground stars. Of these, 14 sources remain without counterparts in the present XMM-Newton survey. Five of these 14 sources are located outside the field observed with XMM-Newton. Forty-one ROSAT foreground star candidates have counterparts in the XMM LP-total catalogue. Of these counterparts, 16 were classified as foreground star candidates and four were identified as foreground stars (spectral type from Bonfini et al. 2009; Hatzidimitriou et al. 2006, or SIMBAD). In addition 12 sources were listed as hard, two as AGN candidates and one as a globular cluster candidate in the XMM LP-total catalogue. The counterparts of three ROSAT sources remain without classification in the XMM LP-total catalogue.
Another three ROSAT sources have more than one counterpart in the XMM-Newton data. Source [SHP97] 109 correlates with sources N 597, N 604, N 606, and N 645. The former three are classified as hard, while source N 645 is classified as a foreground star candidate. However source N 645 has the largest distance from the position of [SHP97] 109 compared to the other three XMM-Newton counterparts. Furthermore, this source had a flux below the ROSAT detection threshold (about a factor 2.6) in the XMM-Newton observations and is about a factor 3–34 fainter than the three other possible XMM-Newton counterparts. Thus it is very unlikely that [SHP97] 109 represents the X-ray emission of a foreground star.
Source [SHL2001] 156 has two XMM-Newton counterparts and is discussed in Sect. 10.1.3. The third source ([SHL2001] 374) correlates with sources N 1 922 and N 1 924. The two XMM-Newton sources are classified as hard and as a foreground star candidate, respectively. In the source catalogue of SHL2001 source [SHP97] 369 is listed as the counterpart of [SHL2001] 374. The source in the first ROSAT survey has a smaller positional error and only correlates with source N 1 924. Although this seems to indicate that source N 1 924 is the counterpart of [SHL2001] 374, we cannot exclude the possibility that [SHL2001] 374 is a blend of both XMM-Newton sources, as these two sources have similar luminosities in the XMM-Newton observations.
Kong et al. (2002b) classified four sources as foreground stars. For two sources (N 960r2-42 and N 976r3-33) the classification is confirmed by our study. The third source (N 1000r2-19) remained without classification in the XMM LP-total catalogue, as it is too soft to be classified as hard and the optical counterpart found in the LGGS catalogue does not fulfil the criteria. The fourth source (r2-46) was not detected in the XMM-Newton observations.
The foreground star classification of three sources (s1-74, s1-45, n1-82) in Williams et al. (2004) is confirmed by the XMM LP-total study (N 289, N 603, N 1 449). For source N 289 the spectral type F0 was determined (Hatzidimitriou et al. 2006).
The source list of DKG2004 contains six sources (s2-46, s2-29, s2-37, s1-45, s1-20, r3-122) that are classified as foreground stars. All six sources are confirmed as foreground star candidates by our XMM-Newton study (cf. Table 5). For source N 696 (s1-20) Hatzidimitriou et al. (2006) obtained the spectral type G0.
Of the four sources listed as foreground stars in Voss & Gilfanov (2007) only one source (N 936 [VG2007] 168) was confirmed as a foreground star, based on the entry in the RBC V3.5 and Caldwell et al. (2009). The second source (N 1 118[VG2007] 180) is listed in the RBC V3.5 and Caldwell et al. (2009) as a globular cluster. The third source (N 829[VG2007] 181) does not have a counterpart in the USNO-B1, 2MASS or LGGS catalogues, nor does it fulfil the hardness ratio criteria for foreground stars. Hence, the source is classified as hard. The fourth source ([VG2007] 81) is not spatially resolved from its neighbouring source [VG2007] 79 in our XMM-Newton observations (source N 1 078). Hence source N 1 078 is classified as hard.
9.2 Galaxies, galaxy clusters and AGN
The majority of background sources belong to the class of active galactic nuclei (AGN). This was shown by the recent deepest available surveys of the X-ray background (Mushotzky et al. 2000; Hasinger et al. 2001; Brandt & Hasinger 2005). The class of AGN is divided into many sub-sets. The common factor in all the sub-sets is that their emission emanates from a small, spatially unresolved galactic core. The small size of the emitting region is implied by the X-ray flux variability observed in many AGN, which is on time scales as short as several minutes (to years). The observed X-ray luminosities range from to erg s, sometimes even exceeding erg s. Although AGN show many different properties, like the amount of radio emission or the emission line strengths and widths, they are believed to be only different facets of one underlying basic phenomenon (cf. Urry & Padovani 1995): the accretion of galactic matter onto a supermassive black hole ( M) in the centre of the galaxy.
It is difficult and, to some extent, arbitrary to distinguish between active and normal galaxies, since most galaxies are believed to host a black hole at the position of their kinetic centre (Bender et al. 2005). In normal galaxies the accretion rate to the central supermassive BH is so low, that only weak activity can be detected – if at all. The overall thermal emission of the nuclear region is due to bremsstrahlung from hot gas. The total X-ray luminosity of a normal galaxy can reach some erg s, at most. It consists of diffuse emission and emission of unresolved individual sources.
Galaxy clusters (GCls) are by far the largest and most massive virialised objects in the Universe. Their masses lie in the range of – M and they have sizes of a few megaparsecs (Mpc). A mass-to-light ratio of M/L indicates that galaxy clusters