- ID:
- ivo://nasa.heasarc/m71cxo
- Title:
- M 71 Chandra X-Ray Point Source and Optical/Infrared Counterparts Catalog
- Short Name:
- M71CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The authors observed the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources were found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources were found within the half-mass radius r<sub>h</sub>, reaching to a limiting luminosity of 1.5 x 10<sup>30</sup> ergs/s (0.3-8 keV). From a radial distribution analysis, the authors find that 18 +/- 6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11 +/- 6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range L<sub>X</sub> = 10<sup>30</sup> - 10<sup>31</sup> ergs/s than expected by extrapolating from other studied clusters using either mass or collision frequency. In their paper, the authors explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches. The authors obtained a 52.4 ks Chandra observation (ObsID 5434) of M71 (nominal center of cluster at J2000.0 RA and Dec of 19 53 46.1 +18 46 42) on 2004 December 20-21 using the Advanced CCD Imaging Spectrometer (ACIS) in very faint (VF), timed-exposure mode with a 3.141s frame time. They searched for X-ray sources in the observed field by employing techniques described in Tennant (2006, AJ, 132, 1372) which use a circular Gaussian approximation to the point-spread function (PSF). Within twice the M71 half-mass radius (r<sub>h</sub> = 1.65 arcminutes), they set the signal-to-noise threshold (S/N) for detection to 2.0, but also required the number of source counts to be at least 5 times the statistical uncertainty in the local background estimate. The empirical relation derived by Tennant, C<sub>min</sub> = (S/N)<sup>2</sup>/0.81, then implies a point-source sensitivity limit of about 4.9 counts for r<sub>M71</sub> <= 2r<sub>h</sub> and in the energy band 0.3-8.0 keV. Because of the increase in PSF size with off-axis distance and the associated increase in background within a detection cell, for R<sub>M71</sub> > 2r<sub>h</sub> they set the S/N threshold for detection to 2.4 and again required the number of source counts to be at least 5 times the statistical uncertainty in the local background estimate. The point-source sensitivity limit thus rises to about seven counts. This table contains 63 X-ray sources and their optical/infrared counterpart information, if any, for those sources with r<sub>M71</sub> <= 2r_h which are listed in Table 1 of the reference paper: these sources have name prefixes of s01 to s63. It also contains 73 X-ray sources and their optical/infrared counterpart information, if any, for those sources with r<sub>M71</sub> > 2r_h which are listed in Table 2 of the reference paper: these sources have name prefixes of ss01 to ss59 and is01 to is14. Each entry in this table corresponds to an X-ray source if there is no counterpart information or only a single identified counterpart or to a particular X-ray source and counterpart match if there are multiple counterpart identifications. There are thus 165 entries in this HEASARC table corresponding to 136 X-ray sources. This table was created by the HEASARC in March 2011 based on the electronic version of Tables 1 and 2 of the reference paper which were obtained from the CDS (their catalog J/ApJ/687/1019 files table1.dat and table2.dat). Some of the values for the alt_name parameter in the HEASARC's implementation of this table were corrected in April 2018. This is a service provided by NASA HEASARC .
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- ID:
- ivo://nasa.heasarc/m81cxo2
- Title:
- M 81 Chandra X-Ray Point Source Catalog
- Short Name:
- M81CXO2
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains the source table from an analysis of 15 Chandra ACIS observations of the nearby spiral galaxy M81 taken over the course of six weeks in 2005 May-July. Each observation reaches a sensitivity of ~10<sup>37</sup> erg s<sup>-1</sup>. With these observations and one previous deeper Chandra observation (the properties of which are described in Table 1 and Section 2 of the reference paper), the authors have compiled a master source list of 265 point sources, extracted and fitted their spectra, and differentiated basic populations of sources through their colors. They also carried out variability analyses of individual point sources and of X-ray luminosity functions (XLFs) in multiple regions of M 81 on timescales of days, months, and years. They find that, despite measuring significant variability in a considerable fraction of sources, snapshot observations provide a consistent determination of the XLF of M81. They also fit the XLFs for multiple regions of M81 and, using common parametrization, compare these luminosity functions to those of two other spiral galaxies, M31 and the Milky Way. This table contains the 265 point sources at or above the 99.9% probability level of being real according to AE's PROB_NO_SOURCE statistic (the "master" source list), and 11 additional "borderline" sources which have 99.0-99.9% probability of being real according to AE's PROB_NO_SOURCE statistic, for a total of 276 sources whose properties were described in Tables 3 and 4 of the reference paper. The 265 "master" sources have source numbers from 1 to 265 while the 11 "borderline" sources have source numbers beginning with 'B', e.g., they have source numbers 'B1' to 'B11'. Note that only coordinates are listed for 3 sources in the master source list (source numbers 234, 241 and 262) and 2 sources in the borderline source list (B8 and B9) because they were only in the field of view (on chip) of one observation (ObsID 735). Six additional sources near the center of M81 which were found using maximum likelihood image reconstruction are not included in either the master or borderline source lists contained herein but their positions are listed in table 2 of the reference paper. This table was created by the HEASARC in August 2011 based on electronic versions of Tables 3 and 4 from the reference paper which were obtained from the ApJ web site. Some of the values for the name parameter in the HEASARC's implementation of this table were corrected in April 2018. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m87cxo
- Title:
- M 87 Chandra X-Ray Point Source Catalog
- Short Name:
- M87CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The ACIS instrument on board the Chandra X-Ray Observatory has been used to carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M 87 (NGC 4486), the giant elliptical galaxy near the dynamical center of the Virgo Cluster. These images - with a total exposure time of 154 ks - are the deepest X-ray observations obtained as of 2004 of M 87. The authors identified 174 X-ray point sources, (contained in this Browse table) of which ~ 150 are likely LMXBs. This LMXB catalog was combined with deep F475W and F850LP images taken with ACS on the Hubble Space Telescope (HST) (as part of the ACS Virgo Cluster Survey) to examine the connection between LMXBs and globular clusters in M87. Of the 1688 globular clusters in the authors' catalog, a fraction f<sub>X</sub> = 3.6% +/- 0.5% contain an LMXB. M 87 (NGC 4486) was observed with the Chandra Advanced CCD Imaging Spectrometer (ACIS) for 121 ks on 2002 July 5-6. In this table, only the S3 chip data are used. The data were processed following the CIAO data reduction threads, including a correction for charge transfer inefficiency (CTI). In addition, the authors used 38 ks of archival ACIS observations of M 87 taken on 2000 July 29. These data were processed in a fashion similar to the 2002 July data, except that no CTI correction was possible because the data were telemetered in graded mode. All reductions were carried out with CIAO, version 2.3, coupled with CALDB, version 2.21. In order to combine the event files into a single image for point-source detection, the authors obtained relative offsets by matching the celestial coordinates of two X-ray point sources. The relative offset was ~ 0.5". The total exposure time of the co-added image, excluding four background flares totaling ~ 2.5 ks, was 154 ks. This table was created by the HEASARC in March 2007 based on the CDS table J/ApJ/613/279, file table1.dat. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m37cxo
- Title:
- M 37 Chandra X-Ray Point Source Catalog
- Short Name:
- M37CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. The authors used the Chandra X-ray Observatory to study M 37 (NGC 2099), a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (P<sub>rot</sub>). M 37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities (L<sub>X</sub>), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M 37 members: 426 with P<sub>rot</sub>, 278 with X-ray detections, and 76 with both. The authors calculate Rossby numbers, R<sub>o</sub>= P<sub>rot</sub>/tau , where tau is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, L<sub>X</sub>/L<sub>bol</sub>, to minimize mass dependencies in their characterization of the rotation-coronal activity relation at 500 Myr. They find that fast rotators, for which R<sub>o</sub> < 0.09 +/- 0.01, show saturated levels of activity, with log(L<sub>X</sub>/L<sub>bol</sub>) = -3.06 +/- 0.04. For R<sub>o</sub> >= 0.09 +/- 0.01, activity is unsaturated and follows a power law of the form R<sup>beta</sup><sub>o</sub> , where beta = -2.03 (-0.14, +0.17). This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4 - 1.3 solar masses, P<sub>rot</sub> in the range 0.4 - 12.8 days, and L<sub>X</sub> in the range 10<sup>28.4</sup> - 10<sup>30.5</sup> erg s<sup>-1</sup>. These results make M 37 a new benchmark open cluster for calibrating the ARAR at ages of ~ 500 Myr. The central field of M 37 was observed five separate times between 2011 November 14 20:58 and 2011 November 1915:31 UTC for a total of 440.5 ks with the Advanced CCD Imaging Spectrometer (ACIS). The four ACIS-I chips and the ACIS-S3 chip were used in Very Faint telemetry mode to improve the screening of background events and thus increase the sensitivity of ACIS to faint sources. The exposure-weighted average aimpoint of the 16.9 x 16.9 arcmin<sup>2</sup> ACIS-I field of view is RA = 05<sup>h</sup> 52<sup>m</sup> 17.86<sup>s</sup>,Dec = +32<sup>o</sup> 33' 48.23" (J2000). The pitch angle for four observations was 103 degrees; due to scheduling constraints, it was 253 degrees for the fifth. Table 1 in the reference paper provides the basic information for the 5 Chandra observations and Figure 1 in that paper shows their footprints superimposed on a 40' x 40' i' image centered on M 37 obtained by Hartman et al. (2008, ApJ, 675, 1233, hereafter HA08) with the Megacam on the MMT telescope. This HEASARC table contains all of the data from Table 3 of the reference paper, the M 37 Chandra catalog of 774 X-ray sources, and the data from Table 5, the catalog of optical objects, for those objects which have been identified as optical counterparts to the X-ray sources. It does not contain entries for those optical objects in Table 5 which lack X-ray counterparts. This online catalog was created by the HEASARC in December 2015 based on machine-readable versions of tables 3 and 5 from the paper which were obtained from the ApJ website. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m67cxo
- Title:
- M 67 Chandra X-Ray Point Source Catalog
- Short Name:
- M67CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The M 67 Chandra X-Ray Point Source Catalog contains the results of a 47 ks Chandra/ACIS observation of the old (4 Gyr) open cluster M67. The authors detected 25 proper-motion cluster members (including ten new sources) and 12 sources (all new) that they suspect to be M 67 members from their locations close to the main sequence (1 < B-V < 1.7). Of the detected members, 23 are binaries. In addition to cluster members, about 100 background sources were detected, many of which were identified with faint objects in the ESO Imaging Survey (EIS, Momany et al. 2001, A&A, 379, 436). This table summarizes the X-ray properties of the 158 sources which were detected by Chandra in this observation, and also lists the ROSAT (Belloni et al., 1998A&A...335..517B) and optical (candidate) counterparts. This table was created by the HEASARC in January 2007 based on CDS table J/A+A/418/509/table1.dat This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m33cxoxray
- Title:
- M 33 Chandra X-Ray Point Source Catalog
- Short Name:
- ChandraM33
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This database table contains a source list for three Chandra observations of the Local Group galaxy M 33. The observations were centered on the nucleus and on the star-forming region NGC 604. A total of 261 sources were detected in an area of about 0.2 square degrees down to a flux limit of 3 x 10<sup>-16</sup> erg/s/cm<sup>2</sup>, which corresponds to a luminosity of ~2 x 10<sup>34</sup> erg/s at a distance of 840 kpc. The luminosity functions of the X-ray sources observed in M 33 have been constructed and are consistent with those of other star-forming galaxies, taking into account background contamination. In addition, the combination of X-ray color analysis and the existence of "blue" optical counterparts strongly indicates that the X-ray point source population in M 33 consists of young objects. Above 3 x 10<sup>35</sup> erg/s, there are few X-ray sources in the locus of the X-ray hardness ratio diagram that is generally populated by low-mass X-ray binaries. Notice that each of the 261 X-ray sources in the source list has 3 entries in this table, one for each separate Chandra observation, making a total of 783 entries. The Chandra datasets from which this source list was compiled are available by <a href="/db-perl/W3Browse/w3query.pl?tablehead=name%3Dheasarc_chanmaster&sortvar=obsid&bparam_obsid=786%3B+1730%3B+2023">querying CHANMASTER for obsids 786, 1730, and 2023</a>. This table was created by the HEASARC in December 2005 based on the machine-readable version of Table 3 in the above-mentioned Grimm et al. (2005) reference obtained from the ApJ Electronic Edition website. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m101cxo
- Title:
- M 101 Chandra X-Ray Point Source Catalog
- Short Name:
- M101CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- A deep (98.2ks) Chandra Cycle 1 observation has revealed a wealth of discrete X-ray sources as well as diffuse emission in the nearby face-on spiral galaxy M 101. From this rich data set, the authors have created a catalog of the 110 sources from the S3 chip which were detected with a significance of greater than 3-sigma. This detection threshold corresponds to a flux of ~ 10<sup>-16</sup> ergs/cm<sup>2</sup>/s and a luminosity of ~ 10<sup>36</sup> ergs/s for a distance to M 101 of 7.2 Mpc. The sources display a distinct correlation with the spiral arms of M101 and include a variety of X-ray binaries, supersoft sources, supernova remnants, and other objects of which only ~27 are likely to be background sources. The 29 brightest sources have enough flux (greater than 100 counts) to perform at least crude spectral modeling using the HEASARC XSPEC model fitting program. Most of the sources could be satisfactorily fitted with a simple absorbed power-law spectrum model; however, eight of the softest sources were better fitted by an absorbed blackbody model. This table was created by the HEASARC in July 2007 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/561/189">CDS Catalog J/ApJ/561/189</a> files table1.dat and table4.dat. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m17cxo
- Title:
- M 17 Chandra X-Ray Point Source Catalog
- Short Name:
- M17CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- This table contains some of the results from a Chandra ACIS observation of the stellar populations in and around the M17 H II region. The field reveals 886 sources (listed in the present table) with observed X-ray luminosities (uncorrected for absorption) between ~ 29.3 erg s<sup>-1</sup> < log L<sub>X</sub> < 32.8 erg s<sup>-1</sup>, 771 of which have stellar counterparts in infrared images. Spectral analysis results for the 598 brightest X-ray sources which have photometric significance of 2.0 or greater) are also given herein. For 546 of the X-ray sources, the fits used the "wabs(apec)" thermal plasma model in XSPEC assuming scaled 0.3 times solar photospheric abundances, while for the other 52 X-ray sources for which either the thermal model poorly described the data or required nonphysical parameters and the X-ray source was not identified with a known stellar counterpart, the fits used the "wabs(powerlaw)" model in XSPEC. In addition to the comprehensive tabulation of X-ray source properties, several other results were presented in the reference paper: 1. The X-ray luminosity function is calibrated to that of the Orion Nebula Cluster population to infer a total population of roughly 8000-10,000 stars in M17, one-third lying in the central NGC 6618 cluster. 2. About 40% of the ACIS sources are heavily obscured with A<sub>V</sub> > 10 mag. Some are concentrated around well-studied star-forming regions -- IRS 5/UC1, the Kleinmann-Wright Object, and M17-North -- but most are distributed across the field. As previously shown, star formation appears to be widely distributed in the molecular clouds. X-ray emission is detected from 64 of the hundreds of Class I protostar candidates that can be identified by near- and mid-infrared colors. These constitute the most likely protostar candidates known in M17. 3. The spatial distribution of X-ray stars is complex: in addition to the central NGC 6618 cluster and well-known embedded groups, we find a new embedded cluster (designated M17-X), a 2 pc long arc of young stars along the southwest edge of the M17 H II region, and 0.1 pc substructure within various populations. These structures may indicate that the populations are dynamically young. 4. All (14/14) of the known O stars but only about half (19/34) of the known B0-B3 stars in the M17 field are detected. These stars exhibit the long-reported correlation between X-ray and bolometric luminosities of L<sub>X</sub> ~ 10<sup>-7</sup> L<sub>bol</sub>. While many O and early-B stars show the soft X-ray emission expected from microshocks in their winds or moderately hard emission that could be caused by magnetically channeled wind shocks, six of these stars exhibit very hard thermal plasma components (kT > 4 keV) that may be due to colliding wind binaries. More than 100 candidate new OB stars are found, including 28 X-ray detected intermediate- and high-mass protostar candidates with infrared excesses. 5. Only a small fraction (perhaps 10%) of X-ray selected high- and intermediate-mass stars exhibit K-band-emitting protoplanetary disks, providing further evidence that inner disks evolve very rapidly around more massive stars. This table was created by the HEASARC in December 2007 based on electronic versions of Tables 1, 2, 3, 4 and 5 of the reference paper which were obtained from the electronic ApJ web site. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m83cxo
- Title:
- M 83 Chandra X-Ray Point Source Catalog
- Short Name:
- M83CXO
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The authors of this table have obtained a series of deep X-ray images of the nearby (4.61 Mpc) galaxy M 83 using Chandra, with a total exposure of 729 ks. Combining the new data with earlier archival observations totaling 61 ks, they find 378 point sources within the D<sub>25</sub> contour of the galaxy. The authors find 80 more sources, mostly background active galactic nuclei (AGNs), outside of the D<sub>25</sub> contour. Of the X-ray sources, 47 have been detected in a new radio survey of M 83 obtained using the Australia Telescope Compact Array (ATCA). Of the X-ray sources, at least 87 seem likely to be supernova remnants (SNRs), based on a combination of their properties in X-rays and at other wavelengths. The authors attempt to classify the point source population of M 83 through a combination of spectral and temporal analysis. As part of this effort, in the reference paper they carry out an initial spectral analysis of the 29 brightest X-ray sources. The soft X-ray sources in the disk, many of which are SNRs, are associated with the spiral arms, while the harder X-ray sources, mostly X-ray binaries (XRBs), do not appear to be. After eliminating AGNs, foreground stars, and identified SNRs from the sample, the authors construct the cumulative luminosity function (CLF) of XRBs brighter than 8 x 10<sup>35</sup> erg s<sup>-1</sup>. Despite M 83's relatively high star formation rate, the CLF indicates that most of the XRBs in the disk are low mass X-ray binaries (XRBs). The X-ray observations of M 83 in this survey were all carried out with the ACIS-S in order to maximize the sensitivity to soft X-ray sources, such as SNRs, and to diffuse emission. The nucleus of M 83 was centered in the field of the back-illuminated S3 chip to provide reasonably uniform coverage of M 83. In addition to the S3 chip, data were also obtained from chips S1, S2, S4, I2, and I3. All of the observations were made in the "very faint" mode to optimize background subtraction. Observations were spaced over a period of one year from 2010 December to 2011 December, as indicated in Table 1 of the reference paper. The only difference among observations was the roll orientation of the spacecraft and the differing exposure times. All of the observations were nominal, and yielded a total of 729 ks of useful data. In order to maximize their sensitivity and more importantly to improve their ability to identify time variable sources, the authors included in their analysis earlier Chandra observations of M 83 in 2000 and 2001 totaling 61 ks which were obtained by G. Rieke (Prop ID. 1600489) and by A. Prestwich (Prop ID. 267005758). These data were obtained in a very similar manner to that of the present survey, and increased the total exposure to 790 ks. The authors used ACIS EXTRACT (AE) to derive net count rates from the sources in various energy bands: 0.35 - 8.0 keV (total or T), 0.35 - 1.1 keV (soft or S), 1.1 - 2.6 keV (medium or M), 2.6 - 8.0 keV (hard or H), 0.5 - 2.0 keV ("normal" soft band) and 2.0 - 8.0 keV ("normal" hard band). Their choice of these bands was based on a variety of overlapping goals. The broad 0.35 - 8.0 keV band samples the full energy range accessible to Chandra observations. The three bands S, M and H provide energy ranges intended to classify sources on the basis of their hardness ratios. The boundary at 1.1 keV, in particular, is just above the region containing strong features due to Ne and Fe seen in the spectra of most SNRs. The 0.5 - 2.0 keV and 2.0 - 8.0 keV bands are needed because number counts of active galactic nuclei (AGNs) and of X-ray binary populations are normally carried out in these bands and because the 0.5 - 2.0 keV band, encompassing the peak of the response curve, provides better statistics for some purposes than S+M. The AE count rates were used to establish which of the sources in the candidate list were statistically valid. The authors retained any source that had a probability-of-no-source < 5 x 10<sup>-6</sup> in any one of these bands in the total data set. For their final run of AE, their list of source candidates had 847 potential sources. Of those, they find a total of 458 valid point sources, whose properties are listed in this table. Of the 458 point sources, 378 are located within the area defined by the D<sub>25</sub> ellipse of the galaxy (which the authors take to have a major axis diameter of 12.9 arcminutes), and the remaining 80 are outside this region. There were 43 sources in the nuclear region (defined to be any source within a projected radius of 0.5 kpc from the optical nucleus). This table was created by the HEASARC in July 2014 based on electronic versions of Tables 3, 4 and 5 from the reference paper which were obtained from the ApJS website. This is a service provided by NASA HEASARC .
- ID:
- ivo://nasa.heasarc/m51cxo2
- Title:
- M 51 Deep Chandra ACIS X-Ray Point Source Catalog
- Short Name:
- M51CXO2
- Date:
- 07 Mar 2025
- Publisher:
- NASA/GSFC HEASARC
- Description:
- The authors obtained a deep X-ray image of the nearby galaxy M 51 using the Chandra X-Ray Observatory. Here the catalog of X-ray sources detected in these observations is presented, while an overview of the properties of the point-source population is provided in the reference paper. The authors find 298 sources within the D<sub>25</sub> radii (the apparent major isophotal galactic radii measured at or reduced to the surface brightness level mu<sub>B</sub> = 25.0 B-mag per square arcsecond) of NGC 5194 and NGC 5195, of which 20% are variable, a dozen are classical transients, and another half dozen are transient-like sources. The typical number of active ultraluminous X-ray sources in any given observation is ~5, and only two of those sources persist in an ultraluminous state over the 12 years of observations. Given reasonable assumptions about the supernova remnant population, the luminosity function is well described by a power law with an index between 1.55 and 1.7, only slightly shallower than that found for populations dominated by high-mass X-ray binaries (HMXBs), which suggests that the binary population in NGC 5194 is also dominated by HMXBs. The luminosity function of NGC 5195 is more consistent with a low-mass X-ray binary dominated population. This deep study of M51 is composed of 107 ks of archival Chandra observations, to which the authors added another 745 ks of observations. The Chandra ObsIDs and parameters of all of the observations used in this study (which span from June 2000 to October 2012) are given in Table 2 of the reference paper. All of the observations were made with the ACIS-S array. The authors used the ACIS Extract software package (AE) to perform the photometry. For each source, AE extracted a source region whose size and shape were based on the local PSF, and a background region whose size and shape were based on the size of the local PSF and the location of nearby sources. Source properties were then calculated in a standard manner. Of particular importance in this analysis is the prob_no_source parameter, which is the probability that one could measure the observed count rate in the absence of a source. The authors took a source to be significant only if this parameter was < 5 x 10<sup>-6</sup>. At this probability threshold, one would expect a single spurious source per field, or roughly 1.5 spurious sources within the D<sub>25</sub> regions. As they used the same value in their analysis of M83 (Long et al. 2014, ApJS, 212, 21, the source catalog from which is available in the HEASARC database as the <a href="/W3Browse/chandra/m83cxo.html">M83CXO</a> table), the two catalogs are directly comparable. This table was created by the HEASARC in January 2017 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJ/827/46">CDS Catalog J/ApJ/827/46</a> files table4.dat, table5.dat and table6.dat. This is a service provided by NASA HEASARC .
