A four-year XMM-Newton/Chandra monitoring campaign of the Galactic Centre: analysing the X-ray transients
Key Words.:X-rays: binaries - Stars: neutron - Accretion, accretion discs - Galaxy: centre - X-rays: individuals: AX J1745.6–2901, CXOGC J174535.5–290124, GRS 1741–2853, XMM J174457–2850.3, CXOGC J174538.0–290022, KS 1741–293, GRO J1744–28, SAX J1747.0–2853, AX J1742.6–2901, XMMU J174654.1–291542.
We report on the results of a four-year long X-ray monitoring campaign of the central 1.2 square degrees of our Galaxy, performed with Chandra and XMM-Newton between 2005 and 2008. Our study focuses on the properties of transient X-ray sources that reach 2–10 keV luminosities of for an assumed distance of 8 kpc. There are 17 known X-ray transients within the field of view of our campaign, eight of which were detected in outburst during our observations: the transient neutron star low-mass X-ray binaries GRS 1741–2853, AX J1745.6–2901, SAX J1747.0–2853, KS 1741–293 (all four are also known X-ray bursters), and GRO J1744–28 (a 2.1 Hz X-ray pulsar), and the unclassified X-ray transients XMM J174457–2850.3, CXOGC J174535.5–290124 and CXOGC J174541.0–290014. We present their X-ray spectra and flux evolution during our campaign, and discuss our results in light of their historic activity. Our main results include the detection of two thermonuclear X-ray bursts from SAX J1747.0–2853 that were separated by an unusually short time interval of 3.8 min. Investigation of the lightcurves of AX J1745.6–2901 revealed one thermonuclear X-ray burst and a 1600-s long X-ray eclipse. We found that both XMM J174457–2850.3 and GRO J1744–28 displayed weak X-ray activity above their quiescent levels at , which is indicative of low-level accretion. We compare this kind of activity with the behaviour of low-luminosity X-ray transients that display 2–10 keV peak luminosities of and have never been seen to become brighter. In addition to the eight known X-ray transients, we discovered a previously unknown X-ray source that we designate XMMU J174654.1–291542. This object emits most of its photons above 2 keV and appears to be persistent at a luminosity of , although it exhibits strong spectral variability on a time scale of months. Based on its X-ray properties and the possible association with an infrared source, we tentatively classify this object as a cataclysmic variable. No new transients were found during our campaign, reinforcing the conclusion of previous authors that most X-ray transients recurring on a time scale of less than a decade have now been identified near the Galactic centre.
The region around Sgr A, the dynamical centre of our Galaxy, has been observed at various spatial scales and in different energy bands by many past and present X-ray missions. At early times dedicated monitoring campaigns using Einstein (Watson et al. 1981), Granat (Churazov et al. 1994; Pavlinsky et al. 1994), ROSAT (Sidoli et al. 2001), ASCA (Sakano et al. 2002) and BeppoSAX (Sidoli et al. 1999; in ’t Zand et al. 2004) have led to the discovery of several X-ray point sources located within the central degrees of our Galaxy.
More recently, an intensive monitoring campaign carried out with Chandra between 1999 and 2006 has resolved thousands of distinct X-ray sources in a field of around the Galactic centre (GC; Wang et al. 2002; Baganoff et al. 2003; Muno et al. 2003b, 2004, 2006, 2009). Furthermore, starting in 2006 the inner around Sgr A has been monitored on an almost daily basis with Swift (Kennea & The Swift/XRT team 2006; Degenaar & Wijnands 2009, 2010), whereas a region subtending many degrees has been regularly scanned by RXTE since 1999 (Swank & Markwardt 2001) and by INTEGRAL since 2005 (Kuulkers et al. 2007b).
The plethora of X-ray sources found in the direction of the innermost parts of our Galaxy encompasses a variety of objects (e.g. Muno et al. 2004). The population of X-ray sources with luminosities of (2–10 keV) is thought to be dominated by accreting white dwarfs and active stars (e.g. Verbunt et al. 1984, 1997; Muno et al. 2006; Revnivtsev et al. 2009).
Both LMXBs and HMXBs can be transient: such systems spend the majority of their time (years/decades) in a quiescent state during which the X-ray luminosity is generally , while their intensity typically reaches up to during short (weeks/months) outburst episodes. This large X-ray variability is ascribed to changes in the mass-accretion rate onto the compact primary. In LMXBs this is thought to be caused by instabilities in the accretion disk or the Roche-lobe overflowing companion star. In HMXBs the compact primary accretes matter that is expelled by the companion star via a wind or a circumstellar disk, and transient behaviour may be the result of variations in the mass-loss rate of the companion or the binary geometry.
1.1 Low-luminosity X-ray transients
Repeated observations of the region around Sgr A with Chandra, XMM-Newton and Swift have revealed a population of transient X-ray sources that have 2–10 keV peak luminosities of (e.g. Muno et al. 2005b; Sakano et al. 2005; Porquet et al. 2005b; Degenaar & Wijnands 2009). Their outburst amplitudes and spectral properties suggest that these objects are X-ray binaries in which the compact object accretes at a very low rate from its companion star, thereby causing a relatively low X-ray luminosity (e.g. Pfahl et al. 2002; Belczynski & Taam 2004; Muno et al. 2005b; Wijnands et al. 2006a). Earlier X-ray missions already provided a glimpse of such low-luminosity X-ray transients (Sunyaev 1990; in ’t Zand et al. 1991; Maeda et al. 1996), but the current generation of instruments exploiting sensitive and high spatial resolution X-ray imaging have considerably improved our understanding of the number and behaviour of such objects.
Observations of X-ray binaries accreting at low X-ray luminosities can address several questions related to stellar and binary evolution, as well as accretion flows at low rates. For instance, constraining the number and nature of low-luminosity X-ray transients allows us to gain more insight into the statistics of different source classes, and can serve as an important calibration point for population synthesis models (e.g. Pfahl et al. 2002; Belczynski & Taam 2004). Furthermore, the mass-accretion rate averaged over thousands of years, , plays an important role in the evolution of LMXBs. First studies have shown that values of put tight constraints on the possible evolutionary paths, and might require an unusual type of binary such as neutron stars accreting from a hydrogen-depleted or planetary companion (King & Wijnands 2006). Monitoring observations are an important tool for estimating the time-averaged mass-accretion rates of low-luminosity transients (Degenaar & Wijnands 2009, 2010). Repeated non-detections allow us to place more stringent upper limits on their long-term averaged accretion rates and can therefore be as interesting as actual detections.
Furthermore, thermonuclear X-ray bursts observed from slowly accreting neutron stars have provided important new insight into the physics of nuclear burning on the surface of these compact objects (e.g. Cornelisse et al. 2002; in ’t Zand et al. 2005; Cooper & Narayan 2007; Peng et al. 2007; Degenaar et al. 2010a). Moreover, studying the low-luminosity transients gives insight into the accretion process at low mass-accretion rates. Some bright transient X-ray binaries that exhibit accretion outbursts with intensities of have also been observed to undergo episodes of low-level accretion with X-ray luminosities of (e.g. Wijnands et al. 2001, 2002; Linares et al. 2008; Sidoli et al. 2008; Degenaar & Wijnands 2009; Degenaar et al. 2011a; Fridriksson et al. 2011; Romano et al. 2011). This provides an interesting comparison with the outbursts of the low-luminosity X-ray transients.
1.2 This work
Dedicated surveys aiming to search for and to identify low-luminosity X-ray transients have the potential to unveil rare types of accreting compact objects, and can provide valuable input for theoretical models. Here, we report on the results from a joint Chandra and XMM-Newton monitoring campaign that covered a region of 1.2 square degrees around Sgr A and was carried out between 2005–2008. Wijnands et al. (2006a) reported on the initial results of our monitoring observations, discussing the first series of data obtained in 2005 June–July.
The main goal of this programme is to investigate the X-ray properties (below 10 keV) of transient objects that have low 2–10 keV peak luminosities of . Owing to the high concentration of X-ray point sources in the inner square degree around Sgr A, as well as sensitivity limitations, such systems are often inaccessible to the current monitoring instruments. Our Chandra/XMM-Newton campaign provides the means to refine our understanding of currently known low-luminosity systems (e.g. characterise their outburst behaviour, improve estimates of their duty cycles and time-averaged mass-accretion rates), to search for new X-ray transients and to capture low-level accretion activity in bright X-ray binaries.
We present our work as follows. We describe the setup of the programme in Section 2, and proceed by detailing the reduction and analysis procedures in Section 3. The results of our temporal and spectral analysis of ten different X-ray sources are presented on a case-by-case basis in Appendix A, while in Section 4 we summarise the main results and highlight a few individual sources. We end in Section 5 where we give an overview of all transient X-ray sources located in the region covered by our campaign. We discuss the implications of our findings for understanding the nature of the GC X-ray transients and low-level accretion activity.
2 Description of the monitoring programme
Our choice to monitor the central square degree of our Galaxy was motivated by the fact that this region is populated by nearly 20 known X-ray transients, several of which undergo sub-luminous accretion episodes (Muno et al. 2005a; Wijnands et al. 2006a; Kennea et al. 2006; Degenaar & Wijnands 2009; Degenaar et al. 2011b). The relatively wide field of view (FOV; ) and large collecting area (1 100 at 1 keV) of XMM-Newton make it an excellent facility for surveying sky regions down to relatively faint flux levels.
We used the data obtained with the European Photon Imaging Camera (EPIC), which consists of one PN (Strüder et al. 2001) and two MOS (Turner et al. 2001) detectors that are sensitive in the 0.1–15 keV range and have spectral imaging capabilities. The PN is an array of 12 CCDs ( pixels each), while the MOS units are composed of an array of seven CCDs, each consisting of pixels. A micrometeorite strike damaged one of the CCDs of the MOS1, which is operated with only six detectors since then (Abbey et al. 2006).
The XMM-Newton observations are complemented by Chandra pointings that provide high spatial (sub-arcsec) resolution and a very low X-ray background within an energy band of 0.1–10 keV. We chose the High Resolution Camera (HRC; Kenter et al. 2000) as our prime Chandra instrument, because it provides the largest FOV (), comparable in size to XMM-Newton. The HRC-I is a square micro-channel plate detector (made up of pixels) that has an effective area of at 1 keV and is designed for imaging observations. Because the energy resolution of the HRC is poor, we obtained a few additional pointings with the Advanced CCD Imaging Spectrometer (ACIS; Garmire et al. 2003) to follow up active transients, aiming to obtain spectral information. The ACIS-I consists of a four-chip imaging array (each having pixels), providing an effective area of at 1 keV and a FOV of .
Our Chandra/XMM-Newton campaign covers 1.2 square degrees around Sgr A, sub-divided into seven different pointing directions (named GC-1, GC-2, GC-3, GC-4, GC-5, GC-6 and GC-7; Wijnands et al. 2006a).
2.2 Sensitivity and X-ray images
The exposure time of individual observations was typically 5–10 ks. Depending on the spectral properties, a 5 ks Chandra/HRC-I pointing can detect sources (near aimpoint) down to a 2–10 keV luminosity of for photon indices of and hydrogen column densities of . The XMM-Newton observations are a factor of a few more sensitive. With our programme we reached X-ray luminosities that are a factor of deeper than the sensitivity of wide-field monitoring instruments (e.g. RXTE/ASM, RXTE/PCA, Swift/BAT, MAXI, INTEGRAL), which are typically limited to in their instrument passbands. In addition, XMM-Newton and Chandra provide (sub-) arcsecond spatial resolution, while these other instruments typically yield positional uncertainties of tens of arcseconds to arcminutes.
The entire programme spanned a period of 39 months (3.25 yr), for a cumulative exposure time of 412.7 ks (168.1 ks with Chandra, 244.6 ks with XMM-Newton). Subsequent pointings were separated by 2–10 months (see Table 1). The total exposure time reached in the different pointing directions is ks. Mosaic images of the Chandra/HRC-I and XMM-Newton/PN data are shown in Fig. 1. Apart from diffuse X-ray structures (e.g. around Sgr A), these images reveal several X-ray point sources (see also Section 4). The locations of active transients and two persistent X-ray binaries (1E 1743.1–2843 and 1A 1742–294) are indicated by circles and the cross-hair in the centre of the images shows the position of the Sgr A complex. Fig. 1 also includes a zoomed Chandra/HRC image of the inner around Sgr A, where three active X-ray transients were detected.
3 Data analysis
For the present study we were only interested in (candidate) transient X-ray binaries. We therefore focused on transient X-ray sources that have a 2–10 keV peak luminosity for an assumed distance of 8 kpc, since there will be a high fraction of cataclysmic variables among the fainter objects (Verbunt et al. 1997; Muno et al. 2003c, 2009). We searched for transient sources in our Chandra and XMM-Newton data by comparing images of different epochs with one another.
The objects detected in our observations were correlated with the SIMBAD database to identify the known X-ray sources in our sample based on positional coincidence. Furthermore, we overlaid the positions of sources found in our campaign on an optical image from the Digital Sky Survey (DSS) and an infra-red image from the Two Micron All Sky Survey (2MASS), to filter out likely foreground objects (e.g. active stars). Spectral information obtained from our XMM-Newton and Chandra/ACIS-I observations also aids to identify transients that are located near or beyond the GC (i.e., at a distance of kpc). These sources will appear relatively hard in X-rays, since the softer photons (below keV) will be strongly absorbed by the interstellar medium in the direction of the GC (hydrogen column densities of several times atoms are typical in this region). X-ray sources with detectable emission below 2 keV are likely to be foreground X-ray active stars or cataclysmic variables that are located within a few kiloparsecs from the Sun.
To characterise the X-ray spectra and to calculate source fluxes, we fitted the obtained spectral data between 0.5–10 keV using XSpec (v. 12.6; Arnaud 1996). We used a simple powerlaw model (POWERLAW), modified by interstellar absorption (PHABS). For this we used the default abundances and cross-sections available in XSpec. Using the tool grppha, we grouped the spectra of the brightest sources to contain a minimum number of 20 photons per bin, whereas fainter objects were binned into groups of at least 10 or 5 photons. For a small number of counts we also fitted the unbinned spectra without background subtraction using Cash-statistic (CSTAT in XSpec). Since this yielded spectral parameters and fluxes that were consistent with those obtained using the minimum -method, we only report on the results obtained using the latter.
Whenever a source was detected during multiple observations, we fitted the spectra simultaneously with the hydrogen column density tied between the individual observations. We converted the deduced unabsorbed 2–10 keV fluxes into luminosities by adopting a distance of 8 kpc, unless better distance estimates were available for sources, e.g. as inferred from type-I X-ray burst analysis. Finally, we created long-term lightcurves for each transient source detected during our campaign.
The detailed data reduction and analysis procedures for both satellites are discussed in the next sections. Our analysis includes the Chandra observations performed in 2005 June (HRC) and July (ACIS), which were previously reported by Wijnands et al. (2006a).
Data reduction and source detection
The Chandra data were reduced and analysed using the ciao tools (v. 4.2). The ACIS-I observations were carried out in the faint data mode with the nominal frame time of 3.2 s. As an initial step, we reprocessed the HRC and ACIS level-1 data files following the standard data preparation procedures.
We searched the data for X-ray sources by employing the WAVDETECT tool with the default ”Mexican Hat” wavelet (Freeman et al. 2002). To take into account sensitivity variations across the chips, we generated an exposure map for each observation, evaluated at an energy of 4 keV (the approximate energy at which we expect to detect the largest number of photons for X-ray binaries). For each HRC-I observation, we generated images with a binning of 4, 16 and 32 pixels and ran the detection algorithm on each of the separate images with the default input parameters. This approach allowed us to cover a range of source sizes, accommodating the variation of the point spread function (PSF) as a function of off-axis angle. We adopted a recommended significance threshold that is approximately the inverse of the total number of pixels in the image (, and for HRC images binned by a factor of 4, 16 and 32 respectively), which should correspond to about one expected spurious source per image (Freeman et al. 2002). We compiled a master source list for each observation by combining the objects detected at each image resolution.
The ACIS-I images were binned by a factor of 2 and we employed WAVDETECT using a detection significance threshold of . We ran the detection routine separately for the 0.5–2 and 2–10 keV bands, to be able to distinguish between soft and hard X-ray sources.
Count rates, lightcurves and spectra
We extracted net count rates and lightcurves for each source employing the tool DMEXTRACT (0.5–10 keV). We employed extraction regions centred on the positions found by the WAVDETECT routine and containing 95% of the source counts. Depending on the brightness of the sources and their offset from the aimpoint of the observations, this corresponded to extraction radii of . Background events were collected from a source-free region that had a radius of three times that of the source region. We visually inspected lightcurves with bin times of 5, 10 and 100 s to search for features such as thermonuclear X-ray bursts.
For the ACIS-I data, we extracted source and background spectra using PSEXTRACT. Redistribution matrices (rmf) and ancillary response files (arf) were subsequently generated using the tasks MKACISRMF and MKARF, respectively. Since the HRC provides poor energy resolution, we converted the HRC-I count rates to 2–10 keV unabsorbed fluxes employing pimms (v. 4.1) and using either the spectral information deduced from our Chandra/ACIS and XMM-Newton observations, or values reported in literature (see Table 3). If a transient source was not detected, we obtained a Bayesian statistical upper limit on the source count rate using the ciao tool ASPRATES.
Both AX J1745.6–2901 and GRS 1741–2853 caused pile-up in the ACIS data. When left uncorrected, pile-up typically causes the broad-band count rate to be underestimated and the spectrum to become harder. In an attempt to circumvent these effects we used an iterative approach in which we extracted source photons from annular extraction regions with increasingly large parts of the core PSF excluded. Once the spectral parameters remained unchanged after increasing the annular radius, we assumed that the piled-up inner regions (which distort the spectral shape) were sufficiently excluded. The necessary aperture correction to the arf file was administered using ARFCORR.
Data reduction and source detection
In all XMM-Newton observations the EPIC cameras were operated in full window mode. Data reduction and analysis was carried using the Science Analysis Software (sas; v. 10.0.0) and following standard analysis threads.
Source detection was carried out with the task EDETECTCHAIN, adopting the default detection likelihood of 10. We searched in two different energy bands of 0.5–2 and 2–10 keV for the PN and the MOS2. We did not include the MOS1 for source detection, since one of the CCD units was damaged by a micrometeoroid strike (Abbey et al. 2006).
Count rates, lightcurves and spectra
We extracted count rates for all objects in our source list using the task EREGIONANALYSE (0.5–10 keV). We also employed this tool to determine the optimum extraction region (achieving the highest signal to noise ratio) for source lightcurves and spectra. This yielded source regions with radii of and a typical enclosed energy fraction of percent. For the extraction of background events we used regions with a radius three times larger than that of the source, positioned on a source-free portion of the CCD. For the observations in which our transient sources were not detected, we obtained a upper limit on the count rate using EREGIONANALYSE.
We created background-corrected lightcurves at a resolution of 5, 10 and 100 s for the PN and both MOS cameras using the tools EVSELECT and EPICLCCORR. Source and background spectra, as well as the associated rmf and arf files, were generated using the meta task ESPECGET. The spectral data were fitted within XSpec with all model parameters tied between the three EPIC detectors.
During our observations, both AX J1745.6–2901 and SAX J1747.0–2853 became bright enough to cause pile-up in the EPIC instruments (see Section 4). We used the sas task EPATPLOT to evaluate the level of pile-up in the MOS and PN data, using annular regions of increasing size. Once the observed pattern distribution matched the expected one, we chose that annular size to extract source photons.
|bursting neutron star LMXB||2005-06-05||HRC-I||GC-2||2.0 fix|
|2||AX J1745.6–2901||1.25 (4185)|
|bursting neutron star LMXB||2006-02-27||PN||GC-2|
|bursting neutron star LMXB||2005-10-20||HRC-I||GC-3||2.6 fix|
|bursting neutron star LMXB||2007-09-06||PN||GC-7|
|pulsating neutron star LMXB||2005-10-20||HRC-I||GC-4||2.8 fix|
|6||XMM J174457–2850.3||7.5 fix||(14)|
|8||CXOGC J174541.0–290014||2005-10-20||HRC-I||GC-2||18.8 fix||2.0 fix|
|9||XMMU J174654.1–291542||1.07 (159)|
|likely weak persistent||2006-02-27||PN||GC-6|
|10||AX J1742.6–2901||2008-07-16||HRC-I||GC-5||1.6 fix||2.9 fix|
|likely weak persistent|
Note. – Quoted errors represent 90% confidence levels. and denote the absorbed and unabsorbed 2–10 keV model fluxes. The hydrogen column density of XMM J174457–2850.3 (#6) was fixed to the value reported by Degenaar & Wijnands (2009). For CXOGC J174541.0–290014 (#8) we adopted the spectral parameters given by Muno et al. (2004) and for AX J1742.6–2901 (#10) those reported by Degenaar et al. (2012), since these objects were only detected in our Chandra/HRC observations.
4 Results summary and highlights
Each of our Chandra and XMM-Newton observations reveals tens of distinct X-ray sources. Amongst the detected objects are the Arches cluster and Sgr A, which are both complexes of X-ray point sources combined with diffuse emission structures. Several of the X-ray point sources found in our observations could be identified with known stars or had clear DSS/2MASS counterparts, which renders them likely foreground sources.
We detected two persistent X-ray binaries during our campaign: 1E 1743.1–2843 and 1A 1742–294. The former is an LMXB black hole candidate (e.g. Porquet et al. 2003), whereas 1A 1742–294 is a neutron star LMXB that displays type-I X-ray bursts (e.g. Pavlinsky et al. 1994). Our monitoring observations caught a total of six type-I X-ray bursts from this source.
During our programme we detected activity from eight previously known X-ray transients, several of which exhibited multiple outbursts during our campaign. The results of our spectral and temporal analysis are presented in detail for each of the individual sources in Appendix A, where we also touch upon their long-term X-ray behaviour. In addition to the eight previously known transients, we found two X-ray sources that were detected only in a subset of our observations. Although this possibly indicates a transient nature, we argue that both are likely weak persistent X-ray sources with intensities close to the detection limit of our observations (see Section 4.4).
The results of our spectral analysis of these ten X-ray sources are summarised in Table 3. This table gives an overview of all observations in which a particular source was detected. For clarity we have assigned source numbers that correspond to the individual subsections of Appendix A. All fluxes listed in Table 3 and reported elsewhere in this work are given for the 2–10 keV energy band. Unless stated otherwise, luminosities were calculated from the unabsorbed flux by assuming a source distance of 8 kpc. All quoted errors refer to 90% confidence levels. Some of the detected transients were within the FOV of two different pointing directions. In these cases we report only the information extracted from the observations in which the source is nearest to the aim point. In the following sections we highlight some of the results of our campaign.
4.1 Lightcurve features of AX J1745.6–2901
The transient neutron star LMXB AX J1745.6–2901 was active during several of our observations and exhibited two distinct accretion outbursts during our campaign (2006 and 2007–2008; see Appendix A.2). Investigation of the X-ray lightcurves revealed two prominent features that we discuss below.
During the Chandra/ACIS-I observation performed on 2007 April 6, we detected a strong reduction in the X-ray flux of AX J1745.6–2901 (left panel of Fig. 2). This event has every characteristic of an X-ray eclipse, which is caused by temporal obscuration of the central X-ray emitting region by the companion star. X-ray eclipses allow for a direct determination of the orbital period and can be used to study any possible evolution of the binary orbit (e.g. Wolff et al. 2009). Only a handful of neutron star LMXBs are known to display X-ray eclipses.
We barycentred the lightcurve using the tool BARYCEN to determine the time at which the eclipse occurred. The eclipse started on 2007 April 6 around 16:03 UTC and had a total duration of 1600 s. The ingress of the eclipse lasted for 400 s, while the egress time was 200 s. The eclipse was not total, with a residual count rate of 20% of the out-of-eclipse emission (0.5–10 keV). This profile matches the description of the eclipses that were seen by ASCA and the mid-eclipse time is consistent within the uncertainties of the ephemeris derived by Maeda et al. (1996). This leaves no doubt that this X-ray transient seen active during our campaign is AX J1745.6–2901. Other authors also reported on X-ray eclipses with similar characteristics seen during XMM-Newton (2007 March–April) and Chandra (2008 May) observations (Porquet et al. 2007; Heinke et al. 2008). A detailed study of the ASCA-detected eclipses suggested that the binary is seen at an inclination angle of and harbours a G-dwarf companion star in an 8.4-h orbit (Maeda et al. 1996).
We extracted lightcurves in two different energy bands to investigate whether the eclipse properties are energy-dependent. We compare the 0.5–5.0 and 5.0–10 keV energy bands, because this yields similar intensities for the non-eclipsed emission. Figure 2 (left) compares the eclipse profiles. The duration, ingress and egress time is similar for both energy ranges, but the eclipse is deeper in the harder band. At the base of the eclipse the intensity in the 0.5–5.0 keV band is 30% of the out-of-eclipse emission, whereas this is only 10% for the 5.0–10 keV band. This may indicate that the softer X-ray photons come from a region that is more extended than the emission site of the harder photons. Similar eclipse behaviour has been observed for other LMXBs and explained in terms of a dust-scattering halo (e.g. Homan et al. 2003; Ferrigno et al. 2011).
The Chandra/HRC data obtained on 2008 May 10 revealed an X-ray flare from AX J1745.6–2901 that started around 2008 May 11 at 00:04 UT and had a duration of 50 s (right plot in Fig. 2). The fast rise and exponential decay shape, combined with the fact that this source is a known X-ray burster, strongly suggest that this event was a type-I X-ray burst. These are bright flashes of thermal X-ray emission that are caused by unstable thermonuclear burning of the accreted matter on the surface of the neutron star. Unfortunately, the HRC data does not allow for a spectral confirmation.
Using (see Table 3), and assuming a blackbody temperature typically seen for type-I X-ray bursts ( keV), the observed HRC peak count rate translates into a 0.01–100 keV luminosity of for an assumed distance of kpc. The duration and peak intensity of the flare match other thermonuclear bursts detected from AX J1745.6–2901 (Maeda et al. 1996; Degenaar & Wijnands 2009). The duration suggests that helium is ignited in a hydrogen-rich environment (cf. Galloway et al. 2008). The persistent accretion luminosity at the time of the X-ray burst was (see Table 3), corresponding to of the Eddington rate of a neutron star.
4.2 Transient nature and X-ray bursts of SAX J1747.0–2853
The neutron star LMXB SAX J1747.0–2853 has been detected on numerous occasions and by different satellites ever since its discovery in 1998 (Appendix A.3). Wijnands et al. (2002) observed the source with Chandra in between two bright outburst and detected it at a 0.5–10 keV luminosity of . Since this is 2–3 orders of magnitude higher than the typical quiescent X-ray luminosity of neutron star LMXBs, the transient nature of SAX J1747.0–2853 was cast in doubt (Wijnands et al. 2002). However, an apparent quenching of type-I X-ray bursts suggested that the accretion was suppressed at least during some intervals (Werner et al. 2004).
SAX J1747.0–2853 was active during several of our monitoring observations, but there were also epochs during which the source went undetected (Appendix A.3). We infer upper limits on the 2–10 keV luminosity of for individual observations. This favours a classification as transient X-ray source. We further improve these constraints by using archival Chandra/ACIS-I observations performed on 2006 October 22 (obs ID 7157, ks exposure) and 2007 February 14 (obs ID 7048, ks). The combined ACIS-I image shows an excess of photons at the location of SAX J1747.0–2853 with a significance of . For an absorbed powerlaw model with and (see Table 3), the net source count rate of translates into a 2–10 keV luminosity of . This clearly establishes the transient nature of SAX J1747.0–2853. During the Chandra observations presented by Wijnands et al. (2002) the source was thus likely detected in a low-level accretion state.
Short recurrence time X-ray bursts
SAX J1747.0–2853 is a known X-ray burster and we detected two such events during our campaign. The lightcurve extracted from the 2006 February XMM-Newton observation revealed a pair of type-I X-ray bursts occurring around 07:57 and 08:01 UT (Fig. 3). The time elapsed between the end of the first and the start of the second burst is s (3.8 min). Both show the typical fast rise, exponential decay shape and are of similar duration (40 s) and peak intensity (Fig. 3). We analysed the spectra of the X-ray bursts using only the MOS cameras, because the PN switched off during the bursts (possibly due to the high count rate). To avoid pile-up we extracted events from an annulus with inner radius of and an outer radius of .
Limited by statistics, we cannot perform time-resolved spectroscopy; therefore we extracted the full-burst spectra (spanning 40 s of data). A spectrum obtained from an interval of 100 s preceding the first burst was used as a background to account for the underlying persistent emission. We fitted both burst spectra to an absorbed blackbody model with the hydrogen column density fixed to the value obtained from fitting the persistent emission (; Table 3). For the first X-ray burst this yields a blackbody temperature of keV and an emitting radius of km. For the second burst we obtain comparable values of keV and km.
Extrapolation of the blackbody fits to the energy range of 0.01–100 keV yields an estimate of the bolometric luminosity of and for the first and second burst, respectively. Using the count rate to flux conversion factor inferred from fitting the average burst spectra, we estimate that the bolometric peak luminosities of both bursts reached . This is close to the Eddington limit for neutron stars. Fitting the spectral data of the persistent emission shows that the source was accreting at 1% of the Eddington rate when the burst doublet occurred. The duration of the bursts (40 s) is typical of helium ignited in a hydrogen-rich environment (cf. Galloway et al. 2008).
The short time-interval of 3.8 min between the two consecutive type-I X-ray bursts is amongst the shortest measured for burst doublets.
Current theoretical models predict that over 90% of the accreted hydrogen/helium is burned during a type-I X-ray burst, which implies that it would take at least a few hours to accumulate enough matter to power a new burst (Woosley et al. 2004). This is at odds with the detection of X-ray bursts that have short recurrence times and thus suggests that some of the initial fuel is preserved after ignition of the first burst (Galloway et al. 2008; Keek et al. 2010). One possible explanation for the occurrence of burst doublets could be that the matter is confined to certain parts of the neutron star surface, e.g. the magnetic poles, and that the bursts of a pair ignite at different locations. Alternatively, the bursts might ignite in separate layers that lie on top of each other. In this scenario the first burst causes unburned fuel to be mixed down to larger depth, which can cause the ignition of a new burst (e.g. Keek et al. 2009).
A systematic study of short recurrence time bursts shows that, on average, the second burst of a pair is shorter, less bright, cooler, and less energetic than the first (Boirin et al. 2007; Keek et al. 2010). This is suggestive of a reduced hydrogen content after the first burst has ignited and would favour the explanation that the bursts are resulting from different envelope layers rather than different areas of the neutron star (Boirin et al. 2007; Keek et al. 2010). In contrast, the two type-I X-ray bursts observed from SAX J1747.0–2853 were of similar duration ( s) and similar intensity, which points towards a similar fuel content.
4.3 Low-level accretion activity
Two of the X-ray transients covered by our monitoring observations were detected at low X-ray luminosities of . This is a factor of above their quiescent levels, yet well below their maximum X-ray intensities of . Below we briefly discuss our findings, which indicate that these sources likely exhibit low-level accretion activity outside their regular (i.e., bright) accretion outbursts. We discuss this in more detail in Section 5.3.
XMM J174457–2850.3 is a faint, unclassified transient source that displays a quiescent luminosity of and exhibits X-ray outbursts with a peak luminosity of (Appendix A.6). We detected one such outburst during our campaign (in 2005). Several of our observations, however, detect the source at an intensity of (see Table 3 and Fig. A.6). This is a factor 10–100 higher than its quiescent emission level, yet considerably weaker than the full outbursts it displays.
As detailed in Appendix A.6, our Chandra/XMM-Newton monitoring observations support previous findings that the bright episodes of this source last only shortly and that the source spends most of its time at luminosities that are a factor 10–100 above its quiescent level. This peculiar behaviour has led to the speculation that this unclassified X-ray source is possibly a wind-accreting X-ray binary (Degenaar & Wijnands 2010), although some neutron star LMXBs also appear to display low-level accretion activity (Section 5.3). One such example is GRO J1744–28 (see below).
The neutron star LMXB GRO J1744–28, also known as “the bursting pulsar” (Appendix A.5), undergoes outbursts that reach up to (Woods et al. 1999) and has a quiescent luminosity of (Wijnands & Wang 2002; Daigne et al. 2002). GRO J1744–28 is detected during all five XMM-Newton observations of GC-4. During the first four data sets, the source intensity was found to be . Within the errors this is consistent with the source being in quiescence. During the final observation (2008 September), however, the source intensity was enhanced to (Appendix A.5). Indications of enhanced activity above the quiescent level have been reported from this source on other occasions as well (Muno et al. 2007).
4.4 Two weak persistent objects
XMMU 174654.1–291542: a new X-ray source
In all XMM-Newton observations covering the field GC-6, we detect an X-ray point source at =, = (J2000) with an uncertainty of . This object is only detected above 2 keV and has no counterpart in the SIMBAD astronomical database or in DSS/2MASS images.
Fitting all XMM-Newton spectra simultaneously to an absorbed powerlaw yields and photon indices varying between (see Table 3). This suggests that there is considerable spectral variation between the different data sets. This is illustrated by Fig. 4 (left), which compares the PN spectra obtained in 2006 September and 2008 May. If the different spectra are fitted individually, we obtain a considerable spread in hydrogen column densities of and powerlaw indices of , although the errors on both parameters are large.
We also fitted the spectra simultaneously with the powerlaw index tied between the different observations, whereas the hydrogen column density was left to vary. This resulted in and , with typical errors of (for for 159 d.o.f.). Regardless of the chosen approach, we obtain 2–10 keV luminosities lying in a range of (assuming kpc).
Additional evidence in support of spectral variations is provided by Fig. 4 (right), where we plot the ratio of PN source counts in the 4–10 keV and 0.5–4 keV energy bands versus the count rate over the full 0.5–10 keV range, using all five XMM-Newton observations. The biggest outlier in this graph concerns the 2006 September data set, during which the count rate in the full energy band was a factor 2 higher than for the other observations. The count rates in the soft 0.5–4 keV band are similar for all five observations, so this difference can be completely attributed to the intensity in the harder 4–10 keV band.
XMMU J174654.1–291542 is not detected in the individual HRC-I observations, which constrains the source luminosity during those epochs to . In the merged HRC image, the source is weakly detected at a count rate of . For and , this implies a luminosity of . Given the small difference between the XMM-Newton detections and the Chandra upper limits, we cannot asses whether this object is truly transient or rather a persistent source that displays intensity variations by a factor of a few (see left panel of Fig. 5).
We searched through archival data to shed more light on the long-term variability of this newly identified X-ray source. We found two Chandra/ACIS pointings that cover the source region, carried out on 2006 November 2 and 2008 May 10 (obs IDs 7163 and 9559 with exposure times of 14.3 and 14.8 ks, respectively). During both observations XMMU J174654.1–291542 is one of the brightest objects in the field, yielding 350–400 net photons per observation. We extracted source and background spectra for these two data sets.
There are no prominent spectral differences between the two archival Chandra observations. We obtain similar photon indices of and hydrogen column densities of and for the 2006 and 2008 data, respectively. The inferred luminosity is for both observations. The archival Chandra observations thus detect XMMU J174654.1–291542 at similar intensity levels as seen during our monitoring programme. This likely points towards a persistent nature. However, all detections occurred within a time frame of 2.5 yr and we cannot exclude that the source is a weak transient that underwent a long outburst.
The strong spectral variation that we observe for XMMU J174654.1–291542 is reminiscent of the behaviour seen for systems that harbour a compact primary accreting from the wind of a companion star. Wind accretion occurs e.g. in neutron star or black hole HMXBs, symbiotic X-ray binaries (a subclass of LMXBs that contain a neutron star and an M-type giant) or symbiotics in which the accreting object is a white dwarf (e.g. Luna & Sokoloski 2007; Masetti et al. 2007; Heinke et al. 2009b; Romano et al. 2011). In such systems the changing spectral properties are attributed to intrinsic variations in the absorption column density due to the wind of the companion star.
Our spectral analysis of XMMU J174654.1–291542 indicates that there might be a large spread in hydrogen column densities between the different observations. The obtained spectral index of is also quite typical of wind-accreting binaries, whereas Roche-lobe overflowing LMXBs usually have softer X-ray spectra. Its X-ray spectral properties thus suggest that XMMU J174654.1–291542 may harbour a compact object accreting from the wind of its companion star.
We found a possible infrared counterpart in the Spitzer/IRAC catalogue that is consistent with our position of XMMU J174654.1–291542 (Ramírez et al. 2008). This object is relatively bright with an apparent magnitude of 10.7 mag at 3.6 micron. A quick comparison with WISE data (Wright 2008) of the recurrent nova T Coronae Borealis (T CrB), which has an absolute 3.6-micron magnitude of approximately mag, suggests that the Spitzer source might be a white dwarf binary located at kpc. If this object is associated with XMMU J174654.1–291542, our X-ray source might thus indeed be a symbiotic. At a distance of 6 kpc, the X-ray fluxes inferred from our campaign translate into luminosities of .
The unclassified X-ray source AX J1742.6–2901
During the final series of Chandra/HRC observations, performed in 2008 July, we detected an X-ray point source located at , (J2000). The source is weak (a total number of 35 net counts) and detected at a large offset angle () from the aimpoint of the observation, which strongly decreases the accuracy of the source localisation. Using equation 5 of Hong et al. (2005), we tentatively estimate a positional uncertainty of .
Within our estimated error, the Chandra coordinates are consistent with the Swift localisation of AX J1742.6–2901. This unclassified X-ray source (not to be confused with the neutron star LMXB AX J1745.6–2901 discussed in Section 4.1) was discovered during ASCA observations of the GC in 1998 September (Sakano et al. 2002). The ASCA source was tentatively associated with the ROSAT object 2RXP J174241.8–290215, which was detected at a count rate of during a 2-ks PSPC observation in 1992 March.
AX J1742.6–2901 was detected only once during our campaign, with a HRC-I count rate of . The source is located just outside the FOV of our XMM-Newton pointings (see Fig. 1) and was not covered by the Chandra survey of Muno et al. (2009). However, the source region was observed with Swift/XRT as part of a Swift follow-up programme of unclassified ASCA sources (Degenaar et al. 2012). During the XRT observations, performed on 2008 March 7 (i.e., four months prior to our Chandra/HRC detection), a single X-ray source was detected within the FOV. The coordinates inferred from the Swift data coincide with our Chandra/HRC position. Analysis of the XRT spectral data yielded and , resulting in a luminosity of (Degenaar et al. 2012).
We used the reported Swift/XRT spectral parameters to convert our observed Chandra/HRC count rate into 2–10 keV fluxes (see Table 3). Assuming a distance of 8 kpc, the unabsorbed flux translates into a luminosity of . For these spectral parameters the ROSAT/PSPC and ASCA/GIS count rates correspond to and , respectively. Given that AX J1742.6–2901 has been detected at comparable intensity levels with ROSAT (1992), ASCA (1998), Swift and Chandra (both 2008), and that the source is close to the detection limit of our HRC observations (see Fig. 5), we consider it likely that this is not a transient object but rather a weak persistent X-ray source that displays a factor of a few variability and peaks near . We cannot classify the source further.
4.5 Possible transient reported by Wijnands et al. (2006a)
Wijnands et al. (2006a) reported on the possible discovery of a new sub-luminous X-ray transient, which was detected during our HRC-I observation of 2005 June 5 at a count rate of . This object is also detected in the XMM-Newton observations carried out in 2008 September, displaying a count rate of , whereas the other XMM-Newton observations yield intensity upper limits of . This suggests a possible transient nature. However, the XMM-Newton observations reveal that the source is detected only below 2 keV. It is therefore not a candidate transient X-ray binary, but more likely a flaring star.
We have presented the results of a four-year monitoring campaign of the GC carried out with the Chandra and XMM-Newton observatories. We have covered a field of 1.2 square degrees around Sgr A, which was targeted on ten different epochs between 2005 June and 2008 September. Our study focused on the behaviour of transient X-ray sources that reach 2–10 keV peak luminosities when in outburst. We detected activity of eight previously known X-ray transients during our campaign. On average, six of these were seen active each year (see Table 3). We studied their X-ray spectra and long-term lightcurves. All are highly absorbed (), indicating source distances near or beyond the GC.
We detected type-I X-ray bursts from the neutron star LMXBs AX J1745.6–2901 and SAX J1747.0–2853 (see Sections 4.1 and 4.2). For the former we also observed a 1600-s long eclipse in one of the X-ray lightcurves, which had similar properties as the eclipses that had previously been seen by ASCA (Maeda et al. 1996). For SAX J1747.0–2853 we found a pair of a type-I X-ray bursts that have a waiting (recurrence) time of only 3.8 min. Their similar peak intensity and duration suggests that the fuel that powered both bursts was likely of similar composition. This is an important pointer to understand the mechanism that is responsible for the unusually short recurrence time. We determined the quiescent luminosity of SAX J1747.0–2853 of , which firmly establishes its classification as X-ray transient (cf. Werner et al. 2004).
We uncovered episodes of low-level accretion in a luminosity range of from two X-ray transients that are known to reach considerably higher intensities of in full outburst (GRO J1744–28 and XMM J174457–2850.3; see Section 5.3). Two other transient sources that were detected during our campaign (CXOGC J174535.5–290124 and CXOGC J174538.0–290022) exhibit outbursts with similar intensities of , but have never been observed in a brighter state (see Appendices A.7 and A.8).
In addition to the eight known transient systems, we detected two weak unclassified X-ray sources that, despite being undetected at some epochs during our campaign, are likely persistent and may be variable by a factor of a few (see Section 4.4). The first is a previously unknown X-ray source that we designate XMMU J174654.1–291542. Inspection of archival data suggests that the source may be persistent at a luminosity of (assuming kpc). Despite its relatively steady X-ray intensity, we observed significant changes in the source spectrum on a time scale of months. Based on its X-ray spectral properties and the possible association with a Spitzer/IRAC infrared object, we tentatively classify this new X-ray source as a cataclysmic variable. The unclassified X-ray source AX J1742.6–2901 was detected with ROSAT, ASCA, Swift, and Chandra at similar intensity levels of between 1992 and 2008, which suggests that the source is likely persistent.
5.1 The population of Galactic centre X-ray transients
In Table 4 we list all known X-ray transients that are located in the region covered by our campaign. This table is an update from Wijnands et al. (2006a), in which we have included three new transients that were discovered by Swift in 2006 (Swift J174553.7–290347 and Swift J174622.1–290634; Degenaar & Wijnands 2009) and in 2011 (Swift J174535.5–285921; Degenaar et al. 2011b). For each source we list the angular distance from Sgr A, the minimum and maximum X-ray luminositiy, and any other relevant information that characterises the source. We indicate which sources were active during our campaign and whether the transients displayed multiple outbursts in the past decade.
There are five confirmed and two candidate LMXBs amongst the 17 transients covered by our campaign (Table 4). The confirmed LMXBs contain a neutron star primary, as demonstrated by the detection of X-ray pulsations (GRO J1744–28) or type-I X-ray bursts (GRS 1741–2853, AX J1745.6–2901, KS 1741–293 and SAX J1747.0–2853). The candidate LMXB CXOGC J174540.0–290031 displays eclipses in its X-ray lightcurve that indicate a high inclination and an orbital period of 7.9 h. This source has been suggested to harbour a black hole based on the detection of strong radio emission (Muno et al. 2005a; Porquet et al. 2005a). Similarly, the transient X-ray source 1A 1742–289 that erupted in 1975 has been tentatively classified as a black hole LMXB based on its X-ray properties and the apparent association with a strong radio source (Davies et al. 1976; Branduardi et al. 1976).
The remaining ten X-ray transients are unclassified, although their large outburst amplitudes (a factor ) and spectral properties render it likely that these are accreting neutron stars or black holes. A lack of infrared counterparts with mag, in turn suggests that these transients are either LMXBs or HMXBs with companions fainter than a B2 V star (Muno et al. 2005b; Mauerhan et al. 2009). Tentative X-ray pulsations were reported for two of the unclassified transients: XMM J174457–2850.3 (5 s; Sakano et al. 2005) and XMMU J174554.4–285456 (172 s; Porquet et al. 2005b). These spin periods fall within the range of HMXBs. However, both results require confirmation (Sakano et al. 2005; Porquet et al. 2005b).
It is remarkable that the ten unclassified transients all have maximum 2–10 keV luminosities of and are thus fainter than the seven confirmed LMXBs (disregarding the LMXB CXOGC J174540.0–290031, which has been argued to be intrinsically brighter and possibly strongly obscured; Muno et al. 2005a). Apart from the low outburst intensity, the X-ray spectra, outburst profiles and recurrence times are not notably different from the brighter transients (see Table 4, see also Muno et al. 2004; Degenaar & Wijnands 2009). If they are X-ray binaries, their sub-luminous character requires an explanation.
Based on statistical arguments, Wijnands et al. (2006a) argued that it is unlikely that all these objects are viewed at high inclination (thereby reducing their observed X-ray emission) and that most of them must be intrinsically faint. It has been proposed that these transients might consist of compact objects accreting from the wind of a main sequence companion (Pfahl et al. 2002). This would be a relatively inefficient process that may account for the observed low X-ray luminosities. An alternative explanation is that these are LMXBs with tight orbits and unusual donor stars (e.g. a white dwarf, brown dwarf or planet) that can only accommodate a small accretion disk (King & Wijnands 2006; in’t Zand et al. 2009). Comparing the low-luminosity transients with weak outbursts observed from the brighter transients suggests that there might be examples of both these types of objects amongst the Galactic centre transients (see Section 5.3).
Out of the 17 transients listed in Table 4, we have detected eight in an active state during our campaign. Moreover, extensive monitoring of the GC region during the past decade has shown that 11 of these transients (i.e., 65%) recurred between 1999 and 2012 (see the column labelled ”Rec?”), six of which even experienced three or more distinct outbursts during this epoch (the neutron star LMXBs GRS 1741–2853, AX J1745.6–2901, KS 1741–293 and SAX J1747.0–2853, and the unclassified transients CXOGC J174535.5–290124 and XMM J174457–2850.3).
No new X-ray transients with a 2–10 keV peak luminosity of were found during our campaign. This confirms suggestions from previous authors that the majority of sources that recur on time scales less than a decade, and undergo outbursts of at least a few days, have now been identified in this region in ’t Zand et al. (2004); Muno et al. (2009); Degenaar & Wijnands (2010). Although a new transient was discovered recently in July 2011 (Degenaar et al. 2011b, c; Chakrabarty et al. 2011; Servillat et al. 2011), previous discoveries date back to 2006 in spite of extensive monitoring with various X-ray satellites (Degenaar & Wijnands 2009).
5.2 Quiescent luminosities
In addition to the maximum 2-10 keV intensity reached during outburst, Table 4 also includes information on the quiescent luminosity of the 17 transients covered by our campaign. The listed values correspond to the minimum X-ray luminosity observed for each source and were taken from the literature. Only for SAX J1747.0–2853, KS 1741–293 and Swift J174622.1–290634 we did not find reported quiescent properties. We therefore searched the Chandra and XMM-Newton data archives for observations that covered the quiescent states of these three transients. We presented the results for SAX J1747.0–2853 in Section 4.2, whereas the other two are discussed below.
We note that the new transient that was discovered in 2011 (Swift J174535.5–285921) is likely associated with CXOGC J174535.6–285928 (Chakrabarty et al. 2011). This faint X-ray source was discovered in the Chandra survey of the GC, during which it was detected at a luminosity of and did not display any variability on long or short time scales (Muno et al. 2009). CXOGC J174535.6–285928 likely represents the quiescent counterpart of the new 2011 transient and therefore we quote its luminosity as the quiescent level of Swift J174535.5–285921.
KS 1741–293: To determine the quiescent luminosity of KS 1741–293, we used archival Chandra observations performed in 2001, 2006 and 2007 (obs IDs 2267, 2272, 7038, and 8459) that amount to a total exposure time of 56 ks. The composite spectrum was obtained using the tool COMBINESPECTRA and yields a 2–10 keV quiescent luminosity of for an assumed distance of 8 kpc. The quiescent spectral properties of KS 1741–293 will be discussed in more detail in a separate work.
Swift J174622.1–290634: We found a total of seven archival Chandra/ACIS-I observations that covered the position of Swift J174622.1–290634. This source exhibited an outburst in 2006 that was first detected with Swift in mid-May. In late June, the source went undetected by Swift/XRT, indicating an upper limit on the 2–10 keV quiescent luminosity of a few times (Degenaar & Wijnands 2009). We found that the source is weakly detected in Chandra observations carried out on 2006 July 4 (obs ID 6642, 5 ks) and 2006 August 24 (obs ID 7037, 40 ks).
By fitting the spectra of these two Chandra observations simultaneously with the Swift outburst data, we found a joint value of and photon indices of (Swift), and (Chandra; 2006 July and August, respectively). It appears that the spectrum was softening, although the source continued to be detected above 2 keV (at lower energies the source photons are strongly absorbed). The corresponding 2–10 keV luminosities are , and , respectively.
We can obtain additional constraints on the quiescent luminosity of Swift J174622.1–290634 by using archival Chandra data obtained in 2001 and 2004 (obs IDs 2282, 2291, 4683, and 4684; total exposure time of 120 ks). This allows us to estimate a pre-outburst upper limit on the 2–10 keV luminosity of (for an absorbed powerlaw model with and , assuming kpc).
There are caveats to consider when interpreting the quiescent luminosities listed in Table 4. Although it is generally assumed that accretion is strongly suppressed during quiescence, it may not have fully come to a halt. Indeed, several transient neutron star and black hole X-ray binaries have been found to display considerable variation in their quiescent X-ray luminosity, which is attributed to residual accretion (for recent examples, see Kong et al. 2002; Hynes et al. 2004; Cackett et al. 2010, 2011; Fridriksson et al. 2011; Degenaar & Wijnands 2012). This implies that some of the minimum X-ray luminosities listed in Table 4 might correspond to low-level accretion. Literature values often only reflect a snapshot of the quiescent state and might therefore not necessarily represent the absolute minimum X-ray luminosity of these transient sources.
It is worth noting that several transients have quiescent luminosities in the range of (see Table 4). Examples include the neutron star LMXBs GRS 1741–2853, AX J1745.6–2901, KS 1741–293 and GRO J1744–28. Transient neutron star LMXBs often have quiescent X-ray spectra that are composed of a soft, thermal component that dominates the spectrum below 2 keV, accompanied by a hard emission tail that can be described by a simple powerlaw. Since any soft thermal emission is strongly absorbed by the high hydrogen column densities inferred for our sources, this implies that they must have considerable hard powerlaw emission in quiescence (cf. Jonker et al. 2003). We note that detections and upper limits of about (such as obtained for SAX J1747.0–2853) do not provide strong constraints on the temperature of the neutron star, since the soft thermal emission is so heavily absorbed.
5.3 Low-level accretion activity
Although several transients covered by our campaign are known to exhibit outbursts with 2–10 keV peak luminosities of (see Table 4), these sources also display activity well below that level. We already discussed in Section 4.3 that the unclassified transient XMM J174457–2850.3 and the pulsating neutron star LMXB GRO J1744–28 were both detected during our campaign at a luminosity of , which is intermediate between their quiescent state and their full X-ray outbursts. For the former, Swift monitoring observations of the GC have shown that this low-luminosity state can persist for several weeks (Degenaar & Wijnands 2010). Indeed, this X-ray transient is more often detected at a low X-ray luminosity than in full outburst.
The two neutron star LMXBs GRS 1741–2853 and KS 1741–293 have both shown short, weak outbursts ( week, ) that preceded longer and brighter accretion episodes with a duration of several weeks/months and 2–10 keV luminosities peaking at (see Appendices A.1 and A.4). Albeit on a different scale, one can argue that the neutron star LMXB AX J1745.6–2901 displayed something similar; a major outburst with a 2–10 keV peak luminosity of was detected from this system in 2007–2008, whereas a few months earlier (in 2006) it underwent an accretion episode that had a duration and intensity that were both a factor 5 lower (see also Degenaar & Wijnands 2010). Finally, the neutron star LMXB SAX J1747.0–2853 was found to display activity at on several occasions, which is also well below its maximum outburst intensity (Wijnands et al. 2002; Campana et al. 2009). Another firm indication of low-level accretion activity is the detection of type-I X-ray bursts from SAX J1747.0–2853 and KS 1741–293 at accretion luminosities of (Chenevez et al. 2009; Linares et al. 2011; see also Kuulkers et al. 2009 for similar examples).
Taken together, it appears that all five neutron star LMXBs covered by our campaign exhibited multiple luminous outbursts with in the past decade, but also frequently display low-level accretion activity at . For GRS 1741–2853, KS 1741–293 and AX J1745.6–2901 it was found that the weaker outbursts are shorter. The disk instability model, which is thought to provide the framework to explain the transient behaviour of LMXBs, provides a possible explanation (e.g. King & Ritter 1998; Lasota 2001). Sub-luminous outbursts may occur when only a small fraction of the accretion disk becomes ionized and accreted onto the compact object, whereas a larger part of the disk is consumed during the more luminous outbursts (see also Degenaar & Wijnands 2010).
Our campaign covered two unclassified X-ray transients that appear active at levels of without becoming brighter. Despite being frequently active, both CXOGC J174535.5–290124 (Appendix A.7) and CXOGC J174541.0–290014 (Appendix A.8) have never been observed at a luminosity (Muno et al. 2005b). Strikingly, as many as ten out of 17 transients covered by our campaign have 2–10 keV peak luminosities that are well below and have never been observed in a brighter state (see Table 4).
These very-faint X-ray transients are possibly connected to the ”burst-only” sources: a group of neutron star LMXBs that became detectable only when exhibiting a type-I X-ray burst, while no persistent emission could be detected down to a limit of (Cornelisse et al. 2002; Wijnands et al. 2006a; Campana 2009). In analogy with the weak outbursts seen for the brighter neutron star LMXBs, we may interpret the low X-ray luminosities in terms of a small amount of matter accreted from the disk. The fact that these sources have never been observed in a brighter state could in turn imply that the entire disk (and hence the binary orbit) is relatively small. It is of note that it takes a long time to accumulate enough fuel to power an X-ray burst when the mass-accretion rate is very low. These are therefore expected to occur much less frequently than for higher accretion rates (e.g., in’t Zand et al. 2009; Degenaar et al. 2010a).
Long-lasting activity at , such as seen for the unclassified transient XMM J174457–2850.3, is difficult to accomodate within the disk instability model. This behaviour might find a more natural explanation in terms of wind-accretion (see the discussion in Degenaar & Wijnands 2010). Similarly, some of the other unclassified transients (e.g. CXOGC J174535.5–290124 and CXOGC J174538.0–290022) appear to spend long times at relatively low intensity levels (Muno et al. 2005b; Degenaar & Wijnands 2010). This might be an indication that in these systems the companion star does not fill its Roche lobe, but supplies matter to the compact object via a wind. On the other hand, there are a few examples of LMXBs in which the donor star does overflow its Roche lobe and that appear to persist at low X-ray luminosities of (e.g. Del Santo et al. 2007; in’t Zand et al. 2009; Campana 2009; Degenaar et al. 2010a). At least one of these harbours a hydrogen-rich companion and must thus have a relatively large orbit (Degenaar et al. 2010a). The sub-luminous character therefore remains a puzzle.
5.4 Ultra-faint X-ray transients
While our present work focused on transient X-ray sources with luminosities , we detected a number of objects that appear variable by a factor of but remain below (assuming kpc). We found several such sources in our XMM-Newton observations. They are hard (most photons emitted above 2 keV) and have no DSS/2MASS counterparts, which effectively rules out that these are foreground stars.
Two such examples are CXOGC J174451.7–285308 and CXOGC J174423.4–291741 from the Chandra catalogue of Muno et al. (2009). Both objects were detected once during our campaign at 2–10 keV luminosities of . CXOGC J174451.7–285308 is indicated by Muno et al. (2009) as exhibiting long-term variability by a factor of 70. CXOGC J174423.4–291741, on the other hand, is listed in this catalogue as a weak persistent source displaying a luminosity of (using the conversion factor from photon to energy flux quoted by these authors).
As discussed in Section 5.3, several confirmed and candidate X-ray binaries display activity at similar intensity levels. Furthermore, Heinke et al. (2009a) found a transient object with a peak luminosity of in the globular cluster M15, which is likely a neutron star LMXB. Although a significant fraction of these “ultra-faint X-ray transients” found in our XMM-Newton data might be accreting white dwarfs (Verbunt et al. 1997), this suggests that there could also be X-ray binaries amongst them.
|Source name||Offset from||Comments||Rec?||Ref.|
|Sgr A ()||()||()|
|CXOGC J174540.0–290031||0.05||LMXB black hole candidate, radio source||No||1,2,3|
|h, high inclination|
|1A 1742–289||0.92||LMXB black hole candidate, radio source||No||6,7,19|
|Swift J174535.5–285921||1.36||unclassified (CXOGC J174535.6–285928)||No||19,20,21,22|
|AX J1745.6–2901*||1.37||neutron star LMXB (burster), h||Yes||4,5,8|
|Swift J174553.7–290347||4.50||unclassified (CXOGC J174553.8–290346)||No||4|
|XMMU J174554.4–285456||6.38||unclassified, possible 172-s X-ray pulsar||No||2,23|
|GRS 1741–2853*||10.00||neutron star LMXB (burster), kpc||Yes||4,5,9,10|
|Swift J174622.1–290634||11.04||unclassified (CXOGC J174622.2–290634)||No||4|
|XMM J174457–2850.3*||13.78||unclassified, possible 5-s X-ray pulsar||Yes||4,5,11|
|SAX J1747.0–2853*||19.55||neutron star LMXB (burster), kpc||Yes||12,13|
|GRO J1744–28*||21.71||neutron star LMXB (0.5-s X-ray pulsar)||Yes||14,15,16|
|KS 1741–293*||22.09||neutron star LMXB (burster)||Yes||17,18|
Note.– Table updated from Wijnands et al. (2006a). Sources marked by an asterisk were detected in an active state during our campaign. and represent the peak and quiescent luminosities, respectively. These values were taken from the literature and converted into the 2–10 keV energy band, or determined in this work. A distance of 8 kpc was assumed, except for GRS 1741–2853 and SAX J1747.0–2853, which have estimated distances of 7.2 and 6.7 kpc, respectively. The column labelled ”Rec?” indicates whether a source recurred (i.e., displayed more than one distinct outburst) between 1999 and 2012. References: 1=Muno et al. (2005a), 2=Muno et al. (2005b), 3=Porquet et al. (2005a), 4=Degenaar & Wijnands (2009), 5=Degenaar & Wijnands (2010), 6=Davies et al. (1976), 7=Branduardi et al. (1976), 8=Maeda et al. (1996), 9=Muno et al. (2003a), 10=Trap et al. (2009), 11=Sakano et al. (2005), 12=Werner et al. (2004), 13=Wijnands et al. (2002), 14=Giles et al. (1996), 15=Wijnands & Wang (2002), 16=Daigne et al. (2002), 17=in ’t Zand et al. (1991), 18=de Cesare et al. (2007), 19=Muno et al. (2009), 20=Degenaar et al. (2011b), 21=Degenaar et al. (2011c), 22=Chakrabarty et al. (2011), 23=Porquet et al. (2005b).
Acknowledgements.This work was supported by the Netherlands organisation for scientific research (NWO) and the Netherlands Research School for Astronomy (NOVA). ND is supported by NASA through Hubble Postdoctoral Fellowship grant number HST-HF-51287.01-A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. RW acknowledges support from a European Research Council (ERC) starting grant. This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-ray Center (CXC) in the application package CIAO. This work was in part based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. We also made use of the Swift public data archive.
Appendix A Results on individual sources
In this appendix we describe the properties of the eight X-ray transients that were detected during our campaign in more detail. For each source we describe the X-ray spectra and lightcurves of individual observations, as well as the flux evolution during our campaign. In particular, we attempt to constrain the luminosity, duration and recurrence time of the observed outbursts, sometimes using literature reports and archival X-ray observations. We also briefly review the historic outburst behaviour of each source, as well as any activity reported elsewhere during and after our campaign.
As mentioned in Section 1, the inner around Sgr A (corresponding to field GC-2 in our campaign) has been covered with Swift’s XRT starting in 2006 (Kennea & The Swift/XRT team 2006; Degenaar & Wijnands 2009, 2010). Those quasi-daily observations provide a partial overlap with our campaign. For sources located in this region we will therefore compare reports on the Swift data with results from our Chandra/XMM-Newton observations. Sources located at larger angular distances () are covered by the INTEGRAL Galactic bulge monitoring programme, which provides observations every few days during two 4-month windows per year (Kuulkers et al. 2007b).
We summarised the results of our spectral analysis in Table 3. Below, the individual sources are discussed in the same order as they appear in Table 3, so that the numbering of the following sections corresponds to the numbered labels in that table. To directly compare the different sources all spectra are plotted in the same energy range (2–10 keV) and all long-term lightcurves on the same intensity scale (). In all plots, triangles represent XMM-Newton observations, while bullets and squares are used for Chandra/ACIS and HRC data, respectively. The last two objects listed in Table 3 are likely weak persistent X-ray sources and were already discussed in detail in Section 4.4.
a.1 Grs 1741–2853
Brief historic overview: This transient neutron star LMXB was discovered by the Granat observatory in 1990 (Sunyaev 1990) and has been detected in an active state many times since then (for a detailed overview, see Trap et al. 2009). The source is known to display type-I X-ray bursts (e.g. Cocchi et al. 1999), from which a distance of 7.2 kpc can be inferred (Trap et al. 2009). It is frequently active and typically exhibits outbursts that reach a 2–10 keV peak luminosity of , and last for a few weeks (e.g. Degenaar & Wijnands 2010). In quiescence, the source is detected at (Muno et al. 2003a).
Activity during our campaign: We detected activity from GRS 1741–2853 in two different epochs (in 2005 and 2007). The source was first detected in outburst during the Chandra/HRC observations performed on 2005 June 5 and was also seen in the follow-up ACIS pointing of 2005 July 1 (Wijnands et al. 2006a). Over the one month time span separating these two observations, the source intensity decreased by a factor 4 (see Table 3) from to . This may indicate that the activity was ceasing. The rise of this outburst was caught by INTEGRAL in 2005 in early April (Kuulkers et al. 2007b). If the decrease in flux signalled by the Chandra data was due to a transition towards quiescence, the duration of the 2005 outburst was therefore 13 weeks.
We found the source again in outburst during our Chandra observations of 2007 March 12, when it was bright enough to cause pile-up of the ACIS instrument. We therefore extracted the source spectrum using a annulus, avoiding the inner piled-up part of the PSF (see Section 3.2). The inferred luminosity was . During the subsequent observation performed on 2007 April 6, GRS 1741–2853 had nearly faded by one order of magnitude (see Table 3), whereas the source was not detected on April 18 with an upper limit of . This indicates that the source had returned to the quiescent state at that time (see Fig. A.1). The 2007 outburst of GRS 1741–2853 was covered by different satellites (Kuulkers et al. 2007a; Wijnands et al. 2007; Muno et al. 2007; Porquet et al. 2007). Swift/XRT observations detected the source with a peak luminosity of and constrain the outburst duration to be weeks (Degenaar & Wijnands 2010).
X-ray spectra: A joint fit to the ACIS spectral data obtained during the two outbursts of 2005 and 2007 (Fig. A.1) yields and (see Table 3). These values are comparable to those found for other outbursts of GRS 1741–2853 (Muno et al. 2003a; Trap et al. 2009; Degenaar & Wijnands 2009, 2010). The count rate detected during the 2005 June Chandra/HRC pointing and upper limits inferred from observations in which the source was not detected, were converted to 2–10 keV unabsorbed fluxes using and .
Constraints on the weak 2006 outburst: Apart from outbursts reaching , GRS 1741–2853 also undergoes low-level accretion activity. Swift/XRT monitoring observations of the GC exposed a weak, short outburst from GRS 1741–2853 between 2006 September 14–20, during which the source did not become brighter than (2–10 keV; Degenaar & Wijnands 2009). This is about three orders of magnitude lower than the maximum outburst luminosity exhibited by this source, yet still a factor above its quiescent level.
The source region is covered by one of our XMM-Newton observations on 2006 September 8, which is just one week before the sub-luminous outburst detected by Swift/XRT. During these observations, GRS 1741–2853 was not detected and we can infer a upper limit on the PN count rate of . Using pimms with and , we can estimate that the 2–10 keV luminosity of GRS 1741–2853 was at that time. Therefore, one week prior to the peculiar short 2006 outburst there were no indications of enhanced activity above the quiescent level.
Activity after our campaign: Swift monitoring observations detected a new outburst from GRS 1741–2853 in 2009 September–November, which had a duration of 4–5 weeks and reached up to (Degenaar & Wijnands 2009). The source was again active for 12 weeks in 2010 July–October at an average luminosity of (Degenaar et al. 2010b). This further underlines the frequent activity of this neutron star LMXB, as is illustrated by the results from our Chandra/XMM-Newton monitoring campaign. It was noted by Degenaar & Wijnands (2010) that despite differences in duration and maximum intensity, the fluency of the bright outbursts of GRS 1741–2853 are very similar (with the exception of the unusual faint, short 2006 outburst).
a.2 Ax j1745.6–2901
Brief historic overview: Another transient that was frequently detected during our monitoring campaign is located at an angular distance of from Sgr A, at a position consistent with the Chandra localisation of AX J1745.6–2901 (Heinke et al. 2008). This neutron star LMXB was discovered in 1993 by the ASCA observatory, exhibits type-I X-ray bursts and its X-ray lightcurve displays eclipses that recur every 8.4 h, corresponding to the orbital period of the binary (Maeda et al. 1996; Kennea & Skinner 1996). The source is detected in quiescence at a luminosity of (Degenaar & Wijnands 2009). Following detections by ASCA in 1993 and 1994, AX J1745.6–2901 was never reported in outburst again until 2006, despite extensive monitoring of the source region.
Activity during our campaign and outburst constraints: We detected two distinct outbursts from AX J1745.6–2901 (in 2006 and 2007–2008). The source was first active during our XMM-Newton observations of 2006 February 27, when it displayed a luminosity of (assuming a distance of kpc). In the subsequent observation performed on 2006 September 8, the source was not detected with an upper limit of (assuming and ). This is consistent with results obtained with Swift/XRT, which indicated that the source was active for three months between 2006 February and June, but resided in quiescence thereafter (Degenaar & Wijnands 2009).
It is unclear when the 2006 outburst of AX J1745.6–2901 started, since the position of the Sun with respect to the GC rendered this region unobservable between 2005 November and 2006 February. Since the source was not detected during our 2005 monitoring observations, the outburst must have started after 2005 October 20 (see Table 1). The time span between the Chandra observations and the first detection of AX J1745.6–2901 on 2006 February 24 (with Swift; Kennea et al. 2006) is 4 months. This constrains the outburst duration to 3–7 months.
Renewed activity of the source was reported in 2007 February, as seen by various instruments (Kuulkers et al. 2007a; Wijnands et al. 2007; Porquet et al. 2007; Degenaar & Wijnands 2009). AX J1745.6–2901 is detected at similar luminosities of in all our observations carried out between 2007 February and 2008 May (see Table 3). We picked up a likely type-I X-ray burst and an X-ray eclipse during these observations (see Section 4.1). The source intensity had decreased by nearly a factor 10 in 2008 July, and it went undetected in 2008 September. This indicates that the source had returned to the quiescent state (see Fig. A.2).
The Swift/XRT observations of the GC also suggest that AX J1745.6–2901 was continuously active since 2007 February, until it returned to quiescence in 2008 in early September (Degenaar & Wijnands 2010). The Swift observations suggest that this long outburst must have commenced between 2006 November 3 and 2007 March 6 (when the GC was unobservable due to Sun-angle constraints). This constrains the total duration of the outburst to months ( yr). The 2007–2008 outburst was a factor of 5 longer and a factor of 5 more luminous than the 2006 outburst (Table 3 and Fig. A.2, see also Degenaar & Wijnands 2010).
X-ray spectra: Spectral analysis of the Chandra/ACIS and