Initial results from NuSTAR observations of the Norma Arm
Results are presented for an initial survey of the Norma Arm gathered with the focusing hard X-ray telescope NuSTAR. The survey covers 0.2 deg of sky area in the 3–79 keV range with a minimum and maximum raw depth of 15 ks and 135 ks, respectively. Besides a bright black-hole X-ray binary in outburst ( (catalog 4U 1630$-$47)) and a new X-ray transient ( (catalog NuSTAR J163433$-$473841)), NuSTAR locates three sources from the Chandra survey of this region whose spectra are extended above 10 keV for the first time: (catalog CXOU J163329.5$-$473332), (catalog CXOU J163350.9$-$474638), and (catalog CXOU J163355.1$-$473804). Imaging, timing, and spectral data from a broad X-ray range (0.3–79 keV) are analyzed and interpreted with the aim of classifying these objects. (catalog CXOU J163329.5$-$473332) is either a cataclysmic variable or a faint low-mass X-ray binary. (catalog CXOU J163350.9$-$474638) varies in intensity on year-long timescales, and with no multi-wavelength counterpart, it could be a distant X-ray binary or possibly a magnetar. (catalog CXOU J163355.1$-$473804) features a helium-like iron line at 6.7 keV and is classified as a nearby cataclysmic variable. Additional surveys are planned for the Norma Arm and Galactic Center, and those NuSTAR observations will benefit from the lessons learned during this pilot study.
Subject headings:X-rays: binaries; cataclysmic variables; stars: binaries, general; stars: neutron; stars: novae
The Norma Arm is among the most active regions of massive star formation in the Milky Way (Bronfman et al., 2000). It is not surprising that this region is also densely populated with the evolutionary byproducts of massive stars, neutron stars (NSs) and black holes (BHs). Many of these compact objects belong to binary systems and accrete matter from a normal stellar companion. These systems are called X-ray binaries (XRBs) and they represent laboratories for studying the physics of matter subjected to extreme gravitational and electromagnetic potentials. Their numbers can be used to constrain rates of massive star formation (e.g., Antoniou et al., 2010), while their spatial distributions are important for studies of stellar evolution (e.g., Bodaghee et al., 2012a).
One advantage of surveying the Norma Arm is that it represents an intersection of molecular clouds, star-forming regions, and accreting compact objects, thereby providing X-ray source populations at various stages of evolution. These populations can then be compared with large populations residing in other active regions of the Galaxy such as the Galactic Center (Muno et al., 2009) and Carina Arm (Townsley et al., 2011).
Thus, the Norma Arm has been the subject of recent observing campaigns seeking to uncover its X-ray populations. In the soft X-rays (10 keV), the Chandra telescope discovered 1100 sources in a 1.3 deg section of this field. The largest source groups are cataclysmic variables (CVs), background active galactic nuclei (AGN), and stars (flaring, foreground, or massive), with other source types represented in smaller numbers (e.g., XRBs, young massive clusters, and supernova remnants: Fornasini et al., 2014, subm.). In the hard X-rays, INTEGRAL (e.g., Bird et al., 2010; Krivonos et al., 2012) discovered a few dozen sources in the Norma Arm, almost all of which are XRBs.
With the advent of the hard X-ray focusing telescope NuSTAR (Harrison et al., 2013), it is now possible to map this region with unprecedented angular ( full-width-half-maximum, 58 half-power diameter) and spectral resolution (400 eV) around 10 keV. This paper presents results from a NuSTAR survey of a small section of the Norma Arm that took place in 2013 February. Section 2 describes the analysis procedures employed on the NuSTAR data and on selected data from Chandra, as well as some of the challenges inherent in X-ray observations of this field. In Section 3, results from imaging, spectral, and timing analyses are presented for X-ray sources detected in the survey. Their implications on source classifications for these objects are discussed in Section 4.
|observation ID||shorthand||pointing R.A. (J2000)||pointing decl. (J2000)||position angle [deg]||start date [UTC]||exposure time [s]|
2. Observations & Data Analysis
2.1. NuSTAR data
The NuSTAR data consist of nine pointings whose details are summarized in Table 1. These nine pointings are comprised of two focal plane modules A and B (FPMA and FPMB) each having a field-of-view (FOV) of . To increase sensitivity, adjacent pointings were tiled with significant overlap (50%) resulting in sky region covered by the survey of around 0.2 deg (), centered at (J2000.0) R.A. and decl. . In Galactic coordinates, this is and .
Data analysis relied on HEASoft 6.14 and the NuSTAR Data Analysis Software (NuSTARDAS 1.2.0111http://heasarc.gsfc.nasa.gov/docs/nustar/analysis/nustardas_swguide_v1.5.pdf) with the latest calibration database files (CALDB: 2013 August 30). Raw event lists from FPMA and FPMB were reprocessed using nupipeline222http://heasarc.gsfc.nasa.gov/docs/nustar/analysis in five energy bands: 3–10 keV, 3–79 keV, 10–40 keV, 10–79 keV, and 40–79 keV.
2.1.1 Image cleaning
Given the density of bright sources and the high level of diffuse background, the Norma Arm presents a number of unique challenges for NuSTAR. The first challenge is from the telescope mast which allows photons to land on the detector without having passed through the focusing optics. These are known as stray-light photons (a.k.a. 0-bounce photons) which originate from bright sources situated a few degrees outside the FOV of each module. Fortunately, these pixels are easily modeled and excluded by creating polygonal region files in ds9 that correspond to the geometric patterns expected from stray light of known bright sources near the FOV. The main source of stray light for these Norma survey observations is (catalog IGR J16320$-$4751) (Tomsick et al., 2003; Rodriguez et al., 2006), a variable but persistent supergiant X-ray binary located between 0.2 and 0.5 outside the FOV.
The second, and more daunting, challenge was that (catalog 4U 1630$-$47), a black-hole X-ray binary, was undergoing an outburst which means it was especially bright during our observations of this field (0.3 Crab in 3–10 keV: Bodaghee et al., 2012b, see also King et al. (2014)). When the source is outside the FOV, photons can still arrive on the detector modules without being properly focused. Such photons are called ghost-rays (a.k.a. 1-bounce photons), and their pattern is not completely understood (Koglin et al., 2011; Harrison et al., 2013).
In order to generate a mosaic image of the entire field where such effects could be minimized, we created new event lists (and exposure maps) in which we excluded regions with pixels contaminated by either stray light or ghost rays. This was done by visually examining the event lists of each observation (showing only those pixels, binned in blocks of 4, with more than 20 counts) and creating a polygonal region file in ds9 that encompasses clusters of pixels (from both modules) on which ghost rays had fallen. By design, the regions were a few pixels wider than necessary to account for both the slightly different sky fields seen by each detector module and to account for the slight jitter due to the motion of the telescope mast. These cleaned event lists were used to generate an exposure-corrected mosaic image in the five energy bands listed above. Vignetting corrections were not applied to these mosaic images.
|name||R.A. (deg)||decl. (deg)||90% confidence radius||detection significance ()||offset|
2.1.2 Systematic offset of detected sources
We ran wavdetect on individual event lists, and on the mosaic images, in order to create lists of detected sources in each energy band. In all cases, we assumed: a point-spread function (PSF) with a constant full-width at half-maximum (FWHM) of 18 (Harrison et al., 2013); scale sizes of 1, 2, 4, and 8 pixels; and a threshold of . This threshold implies around 1 spurious source per observation. For the mosaics, we used the cleaned (non ghost-ray removed) images assuming a background map that mimics the observed low-frequency (i.e., large scale) ghost ray patterns with wavelet scales with characteristic sizes of 8–32 pixels (Slezak et al., 1994; Starck & Murtagh, 1994; Vikhlinin et al., 1997). Each pixel is 2 wide, so the wavelet scales are –, i.e., larger than the high-frequency scales expected for point sources. The lack of high-frequency scales in the background map leads to a poor modeling of the sharp edges and dark dips of the ghost ray pattern. This results in a large number of source detections that align with artifacts in the image, and we conclude that they are likely spurious.
We visually inspected the event lists and the mosaic images (in each band) searching for NuSTAR-detected sources that were coincident with Chandra sources (see § 2.2). There are 3 NuSTAR-detected sources that have probable Chandra counterparts. The NuSTAR-derived positions show a systematic offset (i.e., with a similar direction and magnitude) with respect to the Chandra positions. In physical coordinates, this offset is pixels (39) and pixels (95) in the and directions, respectively, found by averaging the offsets of the Chandra sources. This is consistent with the expected performance from NuSTAR (Harrison et al., 2013). Therefore, we registered the mosaic images to the Chandra sources by subtracting these offset values from the reference pixel. We reran wavdetect to determine a final source position, positional uncertainty (quoted at 90% confidence), and detection significance in the 3–79 keV band. These values are reported in Table 2.
2.1.3 Spectral and timing analyses
Source spectra and light curves were extracted from the cleaned event lists of each module using a 30-radius circle centered on the Chandra position while the background count rates were taken from a 90-radius circle on the same detector chip: away from the source extraction region, but with a similar background pattern. The effects of vignetting on exposure were accounted for in the response matrices and spectra. The spectra were fit in Xspec (Arnaud, 1996) assuming Wilms et al. (2000) abundances and photo-ionization cross-sections of Verner et al. (1996).
While this extraction radius covers roughly 40% of the enclosed energy, the NuSTAR PSF has a relatively narrow peak (18 FWHM) superimposed on broad wings, which means source extraction radii wider than this (at the off-axis angles considered here) have the undesired result of adding more background relative to the gain in source counts. Results of the spectral analysis showed that the sources emitted few counts above 20 keV, and so the energy band used for NuSTAR timing and spectral analyses was restricted to 3–24 keV. All NuSTAR source spectra were binned to contain at least 20 net (i.e., background-subtracted) source counts and a minimum significance of 2.
2.2. Chandra Data
In 2011, Chandra observed a section of the Norma Arm, a subset of which is the NuSTAR survey region described in this paper. Of the 1100 X-ray sources detected by Fornasini et al. (2014, subm.), we excluded all objects outside the 0.2 deg NuSTAR survey region, and then rejected those whose ratio between net source counts in the 2–10 keV and 0.5–2 keV energy bands was less than 0.8. This yields a catalog of 22 relatively hard sources that are suitable low-energy X-ray counterpart candidates to sources detected at higher energies with NuSTAR.
Observations used in this study are ObsID ADS/Sa.CXO#obs/12532 (catalog 12532) and ObsID ADS/Sa.CXO#obs/12532 (catalog 12533). Reprocessing and reduction of this data relied on CIAO v.4.5. Spectra were extracted from each event list in the 0.3–10 keV band for a source region centered on the Chandra position (a circle of radius ), and for a source-free background region (a rectangle with dimensions: ) on the same detector chip. Spectral data were grouped to contain a minimum of 20 sourcebackground counts per bin.
The flux map (counts map divided by the exposure map) of the broad-band energy range (3–79 keV) is presented in Figure 1. The surveying strategy, which tiled the pointings so that they contained significant overlap in their observed fields, as well as the redundancy of having two detector modules whose FOVs are slightly shifted, leads to a mosaic image that is practically free of gaps, despite the exclusion of a large fraction of pixels with stray light and ghost rays (10%–50% of the pixels in each module). The photon-free region (black wedge) at the upper-left or northeast of (catalog 4U 1630$-$47) is due to the exclusion of pixels with ghost rays with no redundant observations that can compensate for the lack of exposure. The median exposure time is 24 ks with the deepest regions having 96 ks of exposure.
Although the effects of stray light and ghost rays have been minimized, the background level remains high and inhomogeneous throughout the image. The exclusion of contaminated pixels leads to artifacts that are visible as bright arcs concentric around (catalog 4U 1630$-$47). Increasing the size of the exclusion region leads to exposure gaps in the mosaic. Bright fringes that appear along the right edge of the mosaic image are due to secondary ghost rays from (catalog 4U 1630$-$47). The contaminated pixels are situated in a “halo” whose inner radius is from (catalog 4U 1630$-$47). We did not attempt to correct for this a posteriori due to insufficient exposure redundancy in the affected regions.
The objects in the survey region that are most easily perceptible are (catalog 4U 1630$-$47) and (catalog NuSTAR J163433$-$473841). Their properties are discussed in separate papers, but highlights include: the discovery of reflection from the inner accretion disk of (catalog 4U 1630$-$47) yielding a black hole spin of 0.985(3), and an iron absorption feature at 7.03(3) keV suggesting a magnetically-driven disk wind (King et al., 2014); and the discovery of a hard X-ray source ( (catalog NuSTAR J163433$-$473841)) which underwent a 1-day long X-ray flare serendipitously during our NuSTAR survey, but was never seen in any wavelength before or since those observations. This suggests that (catalog NuSTAR J163433$-$473841) is a new fast X-ray transient that could be a magnetar or an active stellar binary (Tomsick et al., 2014).
In addition to these objects, there are 3 significantly detected hard X-ray sources whose positions are compatible with sources seen at lower energies by Chandra: CXOU J163329.5473332, CXOU J163350.9474638, and CXOU J163355.1473804 (Fig. 1). Their basic properties are listed in Table 2. Uncertainties are quoted at 90% confidence, unless noted otherwise.
3.1. Cxou j163329.5473332
NuSTAR detects a source at the 8.3- level whose position (Table 2) is 19 away from, and compatible with, that of (catalog CXOU J163329.5$-$473332). The source appears in NuSTAR ObsID 5 and ObsID 6. However, it falls in the chip gap and among the ghost rays during ObsID 5, and so only data from ObsID 6 was used for spectral and timing analyses (Fig. 2).
Chandra spectral data from ObsID ADS/Sa.CXO#obs/12533 (catalog 12533) were fit with an absorbed power law yields cm and (). There were 12512 net source counts in 0.3–10 keV, distributed as 144 cts (0.3–3 keV) and 11111 cts (3–10 keV). Using Cash (1979) statistics and Pearson (1900) test statistics on unbinned Chandra data give consistent results.
An absorbed power law was then fit to the NuSTAR data only. With fixed to the best-fit value from Chandra, we measure a photon index that is consistent with the one from Chandra. The source emitted 12022 net counts in the NuSTAR energy band (3–24 keV), and nearly all of them (10520) were recorded below 10 keV.
We fit the combined spectra from Chandra and NuSTAR using a cross-instrumental constant fixed at unity for the Chandra data, and allowed to vary for the NuSTAR data. The best-fitting model parameters for the power-law model are cm and (Fig. 3 and Table 3). A fit of equivalent quality () is obtained with an absorbed blackbody model of temperature keV, and a lower column density cm. The instrumental constant ( for the power law, for the blackbody), which is consistent with pre-flight expectations for NuSTAR (Harrison et al., 2013) and with joint Chandra-NuSTAR spectral fits of other Galactic objects (e.g., Gotthelf et al., 2014), indicates little variability in source flux between observations taken nearly 2 years apart. Fitting the joint spectral data with a bremsstrahlung model leads to an unconstrained plasma temperature ( keV).
Figure 4 presents the light curve (3–24 keV, 100-s resolution) of (catalog CXOU J163329.5$-$473332). We searched for periods in the range of 0.004 s (i.e., twice the time resolution of the light curve data used in this fine timing analysis) to 18959 s (i.e., the observation duration), and we did not detect a significant pulsation signal in the soft (3–8 keV), hard (8–24 keV), or broad energy band (3–24 keV).
The field of (catalog CXOU J163329.5$-$473332) was observed by XMM-Newton and this object was detected as part of the XMM-Newton Serendipitous Survey Catalog (Watson et al., 2009), although it appears faint and far off-axis (10 arcmin). We analyzed observation ID 0654190201 (rev. 2051) which was taken in 2011 February, with a total exposure time of 22 ks. The parameters from the XMM-Newton spectral fit of this source, i.e., the observed flux, column density, photon index, and blackbody temperature, are consistent with those derived from fits to the Chandra, NuSTAR, and combined Chandra-NuSTAR spectra.
We observed the near-infrared counterpart to (catalog CXOU J163329.5$-$473332) with the NEWFIRM telescope and its magnitudes are 15.290.07 mag, 11.920.10 mag, and 10.130.06 mag (Rahoui et al., 2014). The infrared spectrum displays strong CO lines in absorption (at 16198 and 22957 ), a number of weak emission lines, and no Br- line. This spectrum is typical of an early MIII-type star feeding a small accretion disk (Rahoui et al., 2014).
|name||model||aaInstrumental constant fixed to 1 for the Chandra data and allowed to vary for the NuSTAR data.||bbColumn density in units of cm.||or ccPhoton index of the power law (PL) model, or plasma temperature (in keV) for the blackbody (BB) and bremsstrahlung (FF) models.||norm.ddModel normalization ().||dof eeReduced over degrees of freedom (dof).||ffNet source counts from both NuSTAR modules combined in the soft () band: 3–10 keV.||ggNet source counts from both NuSTAR modules combined in the hard () band: 10–24 keV.||HR hhHardness ratio defined as .||obs. flux iiObserved flux (i.e., not corrected for absorption) in units of erg cm s in the 0.3–24 keV band.||unabs. flux jjAbsorption-corrected flux in units of erg cm s in the 0.3–24 keV band.|
|CXOU J163355.1473804 $\dagger$$\dagger$The best fitting model for (catalog CXOU J163355.1$-$473804) requires a Gaussian component at 6.7 keV.||PL||1.00.3||61||1.50.3||1.5||2.0/35||25626||5212||0.70.2||32.83.2||43.27.1|
3.2. Cxou j163350.9474638
The Chandra spectral data (ObsID ADS/Sa.CXO#obs/12532 (catalog 12532)) were fit with an absorbed power law to give cm and (). A thermal blackbody model ( keV) fit to the data yields a similar column ( cm) and fit quality (). There are 19014 net source counts in the 0.3–10 keV range, with 306 counts having energies below 3 keV, and the rest (15913) are above 3 keV.
We then fit an absorbed power law to the NuSTAR data alone while fixing to the best-fit value from Chandra. The fit quality is decent () and the photon index ( 3.30.3) is consistent with the value measured with Chandra. An absorbed blackbody provides an acceptable fit () with a temperature 1.10.1 keV, similar to that measured with Chandra. The source emitted 40030 net counts in 3–24 keV, with most of them (37528) below 10 keV.
The spectra from Chandra and NuSTAR were jointly fit with an absorbed power law yielding cm and 3.70.5 (Fig. 3 and Table 3). Although the fit quality is good (), the cross-instrumental constant is which indicates significant variability on year-long timescales.
Adding an exponential cutoff constrains the break energy ( keV). However, this component is not required by the data since it returns a similar with 2 less dof. The measured is larger in the joint fit than in the Chandra data alone, and fixing the column density to the Chandra value leads to a poorer fit ().
Thermal models also provide good fits to the data. A blackbody model () gives a lower column density than for the power law ( cm), and has a temperature of keV. A bremsstrahlung model () has an absorbing column consistent with the power law model ( cm), and a plasma temperature of keV (a value that is not constrained with the Chandra data alone).
The 3–24-keV light curve binned at 100 s is presented in Fig. 4, and it shows (catalog CXOU J163350.9$-$474638) to be a relatively soft source that displays low variability on short timescales. The background is mostly due to (catalog 4U 1630$-$47) whose ghost-ray halo covers the extraction regions used to produce the light curves. The apparent decrease in background counts is not significant. There are no periodicities detected in the range of 0.004 s to 18407 s in any energy range. An upper limit of 30% (at 90% confidence) is derived for the fractional r.m.s. expected for a periodic signal.
3.3. Cxou j163355.1473804
This source appears in two Chandra observations (ObsID ADS/Sa.CXO#obs/12532 (catalog 12532) and ObsID ADS/Sa.CXO#obs/12533 (catalog 12533)); the spectral data from these observations were summed to give 54624 net source counts (0.3–10 keV), divided into 16813 and 37720 net counts in the 0.3–3 keV and 3–10 keV bands, respectively. A power law model fit to the binned spectral data provides an adequate fit () with cm and a flat photon index: . A blackbody of temperature keV improves the fit slightly ().
The likely hard X-ray counterpart to (catalog CXOU J163355.1$-$473804) is detected at the 8.7- level (3–79 keV) in the NuSTAR mosaic image. Ghost-ray photons contaminate the region around the source in ObsID 4, and so spectral and timing analysis relied only on data from ObsID 5. The 32128 net source counts (3–24 keV) were distributed as 25626 net counts in 3–10 keV, and 5212 in 10–24 keV.
We fit the NuSTAR data with power law and blackbody models holding fixed to the best-fit value from Chandra, and derived a steeper photon index ( 1.90.3) or a plasma temperature consistent with that of Chandra ( 2.20.3 keV), with both models giving poor fits ( and , respectively).
Jointly fitting the Chandra and NuSTAR data gives a poor fit () when using only an absorbed power law: cm and 1.50.3 (Fig. 3 and Table 3). The fit quality improves () with a blackbody model having keV and a lower column density ( cm), or with a power law and exponential cutoff () where cm, , and the cutoff energy is keV. In both cases, the instrumental constant is 1.10.2 suggesting little variability over yearlong timescales.
Residuals remain around 6.7 keV where emission from the fluorescence of ionized iron is expected. Indeed, the best spectral fits are obtained when a Gaussian component () is added to either the cutoff power law or the blackbody model. In order to analyze this line, we rebinned the NuSTAR spectra to have at least 20 source background counts and a minimum significance of 2. For the power law (), cm, with and an exponential cutoff at keV. The line centroid is keV with an equivalent width of 500 eV (unconstrained).
For the blackbody model (), the line centroid is keV with an equivalent width of eV. The column density and blackbody temperature are cm and keV, respectively. The radius of the emitting region implied by the blackbody model is very small (0.03–0.16 km assuming source distances of 2–10 kpc). Either the source is very distant (20 kpc) or the blackbody is not the right model.
We replaced the blackbody continuum with a bremsstrahlung () and obtained cm, keV, a line energy of keV, and an equivalent width of eV. We also modeled the continuum with apec () and the resulting iron abundance is at least 40% greater than Solar () with a plasma temperature of keV.
The NuSTAR light curve (3–24 keV) for (catalog CXOU J163355.1$-$473804) is presented in Fig. 4. No coherent pulsations were detected for search periods ranging from 2 ms to 21 ks.
The likely infrared counterpart to (catalog CXOU J163355.1$-$473804) was observed with NEWFIRM giving magnitudes of 16.430.07 mag, 15.450.10 mag, and 14.990.09 mag (Rahoui et al., 2014). With a weak CO line at 16198 , a strong CO line at 22957 , and weak Br- emission, the infrared spectrum is typical of a late GIII-type star.
4.1. Cxou j163329.5473332
The Chandra position for (catalog CXOU J163329.5$-$473332) is encompassed by the 21 uncertainty radius of an INTEGRAL-detected source named IGR J163364733 (Krivonos et al., 2010) which was also detected in a short observation by Swift (Landi et al., 2011). The flux recorded by Swift-XRT (2–10 keV) and by NuSTAR (3–10 keV) translate to X-ray luminosities of erg s, and erg s, respectively. The available X-ray data of (catalog CXOU J163329.5$-$473332) show it to be a faint, absorbed ( cm), and relatively hard X-ray source (the bulk of its photons are emitted in 3–10 keV).
Thus, (catalog CXOU J163329.5$-$473332) could be a faint low-mass X-ray binary (LMXB: e.g., Degenaar & Wijnands, 2009) or a cataclysmic variable (Kuulkers et al., 2006) of the intermediate polar (IP: e.g., Patterson, 1994) variety due to the hard X-ray detection. The detection of (catalog CXOU J163329.5$-$473332) out to 20 keV with a moderately steep photon index () and low X-ray luminosity is consistent with both classifications. Another possibility is a binary system in which the compact object is a non-accreting magnetar (e.g., Thompson & Duncan, 1996).
4.2. Cxou j163350.9474638
These NuSTAR observations of (catalog CXOU J163350.9$-$474638) extend the source spectrum beyond 10 keV. However, the source demonstrates significant variability in intensity (by at least a factor of 4) over the two years separating the Chandra and NuSTAR observations, which makes it difficult to draw firm conclusions from joint-fitting of the broadband X-ray spectral energy distribution.
Nevertheless, it is possible to compare the spectral parameters derived from single-instrument fits. The photon index is steeper in the NuSTAR data (by 50%) compared with the value measured with Chandra. This is not uniquely due to the fact that NuSTAR covers higher X-ray energies, since 90% of the photons recorded by NuSTAR were below 10 keV, i.e., in an energy range covered by Chandra. On the other hand, thermal models also fit the data well, and the blackbody temperature ( 1.20.2 keV) and column density ( cm) are in agreement for both Chandra and NuSTAR spectra.
There are no catalogued IR/optical objects from Vizier333http://vizier.u-strasbg.fr or in the Vista Variables in the Via Lactae Survey (Minniti et al., 2010) compatible with the Chandra position. Thus, (catalog CXOU J163350.9$-$474638) lacks a stellar counterpart which would rule out a CV or XRB located nearby, while the steep power law disfavors an AGN. Given its thermal spectrum, its long-term variability, and the absence of multi-wavelength counterparts, we conclude that (catalog CXOU J163350.9$-$474638) could be a low-mass X-ray binary situated a large distance away, or perhaps an isolated, magnetized NS (i.e., a magnetar).
4.3. Cxou j163355.1473804
Prior to the NuSTAR survey, Chandra found (catalog CXOU J163355.1$-$473804) to be a relatively bright X-ray source with a hard spectral continuum. As the brightest of the three objects in this study, this permitted us to measure the source’s broadband X-ray spectrum with relatively high precision. The spectrum combining Chandra and NuSTAR data is consistent with a cutoff power law of and keV. Thermal models such as a blackbody with keV or a bremsstrahlung with keV also describe the data well, although the implied size of the emission region is not consistent with the blackbody model. The column density required by the best-fitting models ( cm) is lower than measured for the two other sources in the study, indicating that the source is either less intrinsically absorbed than the others, or more likely, that it is closer to us.
With NuSTAR, we are able to confirm the detection of an iron line that is hinted at in the Chandra data. The line energy of 6.7 keV suggests thermal emission from highly ionized, helium-like iron (Fe XXV) in the optically thin plasma around an accreting white dwarf, i.e., a cataclysmic variable (e.g., Hellier & Mukai, 2004; Pandel et al., 2005; Kuulkers et al., 2006). For example, (catalog EX Hya) and (catalog V405 Aur) are CVs that show a 6.7 keV line with equivalent widths 400–900 eV, i.e., consistent with the equivalent width measured in (catalog CXOU J163355.1$-$473804) (Hellier et al., 1998).
The identification of the infrared counterpart as a cool, GIII star supports the CV classification. Another factor favoring a CV nature for (catalog CXOU J163355.1$-$473804) is the apparent lack of change in intensity or spectrum during the two years separating the Chandra and NuSTAR surveys, with no indication from all-sky X-ray monitors that the system underwent a major outburst ( erg s) in that time (or at any time in the past few decades).
Its lower absorbing column compared with the other sources in the survey suggests that (catalog CXOU J163355.1$-$473804) is at a distance of 2 or 3 kpc at most, i.e., in the Crux Arm, or in the nearest arc of the Norma Arm. At an assumed distance of 3 kpc, the absorption-corrected flux (0.3–79 keV) of the bremsstrahlung model translates to an X-ray luminosity of erg s. This is consistent with the persistent X-ray luminosity expected from a CV (e.g., Muno et al., 2004; Kuulkers et al., 2006).
4.4. Undetected Chandra sources
Of the 22 hard Chandra sources in the survey region, 3 were detected by NuSTAR, and they ranked first, second, and fourth in order of the number of hard X-ray (3 keV) counts recorded by Chandra. The third brightest source in the hard Chandra band is (catalog CXOU J163358.9$-$474214). This source was not detected in the NuSTAR event lists and mosaic images, despite the fact that it was located in a relatively ghost-ray free and stray-light free part of the image in ObsID 1. This indicates a variable nature for this object (significant variability was also observed with Chandra), and we establish a 3- upper limit of erg cm s on the absorbed source flux in the 3–10 keV range, i.e., higher than the average flux registered by Chandra in a similar energy band (2–10 keV: Fornasini et al., 2014, subm.). The Chandra error circle for this source contains a counterpart candidate seen in the near-IR by 2MASS, and in the mid-IR by Spitzer and WISE. The X-ray variability and the possible association with an IR-emitting source suggest a low-mass X-ray binary or a cataclysmic variable.
All other Chandra sources in the NuSTAR survey region had less than 35 cts in the hard Chandra band which means they are too faint to be detected by NuSTAR given the exposure depth of this survey.
Sections of the Norma field have been observed by XMM-Newton and source candidates found therein are listed in the XMM-Newton Serendipitous Survey Catalog (Watson et al., 2009). Of the 150 sources in the field, 22 of these are both relatively bright (flux in the 0.2–12 keV band erg cm s) and hard (hardness ratio between the 2–4.5 keV and 4.5–12 keV bands 0.0). Only one of them coincides spatially with the error circle of a NuSTAR source: (catalog CXOU~J163329.5$-$473332). It is one of the hardest sources (ranked 6th hardest out of 22), but it is also among the faintest (ranked 19th in flux out of 22).
4.5. Lessons learned from this pilot study
Besides the analysis of X-ray sources, one of the primary goals of this pilot study is to optimize the strategy for future observations. Our experiences with this mini-survey showed us that some of our strategic choices were sound and some can be improved.
Based on the results of the Chandra survey, we knew that the mini-survey region contained several sources that NuSTAR could detect. As was done here, observers should select regions in such a way that they encompass the largest number of hard Chandra sources (or, when available, XMM-Newton sources) that are relatively bright, but not so bright that their ghost rays and stray light contaminate adjacent observations. With the exposures available in this survey (10–100 ks), NuSTAR was able to detect three out of four X-ray sources that had more than 100 cts in the hard Chandra band (3 keV). The non-detection of the fourth source still gives the useful result that the source is variable. While this Chandra hard-band count rate could be used as a rule-of-thumb for a source’s detectability in a typical mini-survey such as this, it is no guarantee since it does not account for X-ray sources that are variable, or that were in the soft state during the NuSTAR survey.
Another factor that led to the selection of this region was that we expected it to contain a relatively low level of stray light given the satellite’s roll angle at the time the observations were performed. Even if stray light were to affect one or both of the modules, substitute coverage is available from the overlapping module and/or adjacent observation(s).
The value of exposure redundancy, not only thanks to the two modules but also by tiling observations with significant overlap (50% shifts), can not be overstated for eliminating or reducing imaging artifacts. This is an important factor that greatly facilitated the analysis of the faint sources in this study. Further improvements in this direction can be made by dividing up the 25 ks exposures into two or three 10–15 ks exposures tiled with slightly more overlap (roughly ) between adjacent pointings. While data with more overlap will take more time to analyze (i.e., the spectra from separate observations will need to be merged to obtain meaningful statistics) the tradeoff is increased exposure redundancy in case pixels need to be discarded due to ghost rays or stray light.
Observers who wish to use NuSTAR for galactic surveys can prevent or reduce the effects of stray light and ghost rays in two ways: 1) by using opportunistic observations gathered only when known transients are off or emitting at low levels according to wide-field X-ray monitors such as MAXI, Swift-BAT, and INTEGRAL-ISGRI; and 2) by increasing the exposure redundancy. While we underestimated its effects during the planning of this survey, we now know more about the brightness and extent of the ghost-ray pattern from objects such as (catalog 4U~1630$-$47) which will help guide the selection of future surveys.
An open question is whether NuSTAR should continue to survey “regions” rather than using the observing time to place the most promising targets from these regions on axis. However, it is important to note that a targeted approach might have missed the discovery of the new X-ray transient (catalog NuSTAR J163433$-$473841).
While there are technical challenges, there are also tremendous scientific benefits from surveying the Galaxy with NuSTAR. Understanding the disk-wind connection in (catalog 4U 1630$-$47), the serendipitous discovery of (catalog NuSTAR J163433$-$473841), and insights into the faint members of the galactic X-ray population are primary among these. Surveys allow NuSTAR to offer a complete picture of the Inner Milky Way which will add to our knowledge of the content of our host galaxy and unlock new mysteries.
5. Summary & Conclusions
An initial NuSTAR survey of the Norma Arm gave insights into the hard X-ray spectral and timing behavior of five sources, three of which are described for the first time in this paper. These three sources have unclassified soft X-ray counterparts from Chandra, so the broadband 0.3–79 keV data (including IR follow-up observations) allow us to propose their likely classifications.
As a faint, hard X-ray source with a low-mass companion, (catalog CXOU J163329.5$-$473332) is shown to be either a cataclysmic variable or a faint low-mass X-ray binary. The intensity variations on year-long timescales and the lack of a clear multi-wavelength counterpart indicate that (catalog CXOU J163350.9$-$474638) could be a distant X-ray binary or possibly a magnetar. We discovered a helium-like iron line at 6.7 keV in the NuSTAR spectrum of (catalog CXOU J163355.1$-$473804), and so it is classified as a nearby cataclysmic variable given the low mass of its IR counterpart.
With NuSTAR we are granted unprecedented views into the hard X-ray populations of our Galaxy. While NuSTAR can perform surveys, its observations can be affected by ghost rays and stray light. These effects can be diminished by planning observations to avoid bright sources located just outside the field of view, and by increasing the exposure redundancy. More NuSTAR surveys are planned for the Norma Arm and other crowded fields such as the Galactic Center, and those observations will benefit from the lessons learned during this pilot study.
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