The Chandra Source Catalog (CSC) is a virtual X-ray astrophysics facility that enables both detailed individual source studies and statistical studies of large samples of X-ray sources detected in Advanced CCD Imaging Spectrometer and High Resolution Camera-I imaging observations obtained by the Chandra X-ray Observatory. The catalog provides carefully curated, high-quality, and uniformly calibrated and analyzed tabulated positional, spatial, photometric, spectral, and temporal source properties, as well as science-ready X-ray data products. The latter includes multiple types of source- and field-based FITS format products that can be used as a basis for further research, significantly simplifying follow-up analysis of scientifically meaningful source samples. We discuss in detail the algorithms used for the CSC Release 2 Series, including CSC 2.0, which includes 317,167 unique X-ray sources on the sky identified in observations released publicly through the end of 2014, and CSC 2.1, which adds Chandra data released through the end of 2021 and expands the catalog to 407,806 sources. Besides adding more recent observations, the CSC Release 2 Series includes multiple algorithmic enhancements that provide significant improvements over earlier releases. The compact source sensitivity limit for most observations is similar to 5 photons over most of the field of view, which is similar to 2x fainter than Release 1, achieved by coadding observations and using an optimized source detection approach. A Bayesian X-ray aperture photometry code produces robust fluxes even in crowded fields and for low-count sources. The current release, CSC 2.1, is tied to the Gaia-CRF3 astrometric reference frame for the best sky positions for catalog sources.

The Chandra Source Catalog has identified over 315,000 unique sources, across 10,382 observations. The aperture photometry system was created to characterize energy and photon fluxes for these observed sources by fitting a point-spread function matrix and observed counts to calculate actual intensities. To support overlapping point-spread functions of adjacent sources, multiple sources are fitted simultaneously. Sources are fitted per observation, across all observations to create master properties, and across subsets of similar observations, using a Bayesian blocking algorithm applied to the per-observation aperture photometry results. Populating the catalog, aperture photometry is run a great many times - approximately 2.9 million times on the current detection list. As such, optimization in the dimensions of both time and memory is crucial. Herein we will discuss the challenges and decisions made as we moved from requirements analysis, prototyping, and development into production.

S. M. Doe1, A. L. Siemiginowska1, B. L. Refsdal1, I. N. Evans1, C. S. Anderson1, N. R. Bonaventura1, J. C. Chen1, J. E. Davis2, J. D. Evans1, G. Fabbiano1, E. Galle1, D. G. Gibbs1, K. J. Glotfelty1, J. D. Grier1, R. Hain1, D. M. Hall1, P. N. Harbo1, X. He1, J. C. Houck2, M. Karovska1, J. Lauer1, M. L. McCullough1, J. C. McDowell1, J. B. Miller1, A. W. Mitschang1, D. L. Morgan1, J. S. Nichols1, M. A. Nowak2, D. A. Plummer1, F. A. Primini1, A. H. Rots1, B. A. Sundheim1, M. S. Tibbetts1, D. W. Van Stone1, S. L. Winkelman1, P. Zografou1

CXOM31 J004252.030+413107.87 is one of the brightest X-ray sources within the D-25 region of M31, and associated with a globular cluster known as B135; we therefore call this X-ray source XB135. XB135 is a lowmass X-ray binary (LMXB) that apparently exhibited hard state characteristics at 0.3-10 keV luminosities 4-6 x 10(38) erg s(-1), and the hard state is only observed below similar to 10% Eddington. If true, the accretor would be a high-mass black hole (BH) (greater than or similar to 50 M-circle dot); such a BH may be formed from direct collapse of a metal-poor, high-mass star, and the very low metallicity of B135 (0.015 Z(circle dot)) makes such a scenario plausible. We have obtained new XMM-Newton and Chandra HRC observations to shed light on the nature of this object. We find from the HRC observation that XB135 is a single point source located close to the center of B135. The new XMM-Newton spectrum is consistent with a rapidly spinning similar to 10-20 M-circle dot BH in the steep power law or thermal dominant state, but inconsistent with the hard state that we previously assumed. We cannot formally reject three component emission models that have been associated with high luminosity neutron star (NS) LMXBs (known as Z-sources); however, we prefer a BH accretor. We note that deeper observation of XB135 could discriminate against an NS accretor.

Over approximately the last five years, we have identified similar to 35 black hole candidates (BHCs) in M31 from their X-ray spectra. Our BHCs exhibited 0.3-10 keV spectra consistent with the X-ray binary (XB) hard state at luminosities that are above the upper limit for neutron star (NS) XBs. When our BHC spectra were modeled with a disk blackbody + blackbody model for comparison with bright NS XBs, we found that the BHCs inhabited a different parameter space than the NS XBs. However, BH XBs may also exhibit a thermally dominated (TD) state that has never been seen in NS XBs; this TD state is most often observed in X-ray transients. We examined the similar to 50 X-ray transients in our Chandra survey of M31 and found 13 with spectra suitable for analysis. We also examined two BHCs outside the field of view of our survey in the globular clusters B045 and B375. We have 42 strong BHCs and 8 plausible BHCs that may benefit from further observation. Of our 15 BHCs in globular clusters, 12 differ from NS spectra by >5 sigma. Due to improvements in our analysis, we have upgraded 10 previously identified plausible BHCs to strong BHCs. The mean maximum duty cycle of the 33 X-ray transients within 6 ' of M31* is 0.13; we estimate that >40% of the XBs in this region contain BH accretors. Remarkably, we estimate that BHCs contribute >90% of those XBs > 10(38) erg s(-1).

We present a rigorous description of the general problem of aperture photometry in high-energy astrophysics photon-count images, in which the statistical noise model is Poisson, not Gaussian. We compute the full posterior probability density function for the expected source intensity for various cases of interest, including the important cases in which both source and background apertures contain contributions from the source, and when multiple source apertures partially overlap. A Bayesian approach offers the advantages of allowing one to (1) include explicit prior information on source intensities, (2) propagate posterior distributions as priors for future observations, and (3) use Poisson likelihoods, making the treatment valid in the low-counts regime. Elements of this approach have been implemented in the Chandra Source Catalog.

We identified a new X-ray transient CXOM31 004252.457+411631.17 (T13) in M31 during a 2013 June Chandra observation. This system is particularly exciting because it is located within 100 '' of M31*; it is thought that this region of the bulge is sufficiently dense to form X-ray binaries dynamically, but only systems with black hole accretors and/or short periods are expected to survive. A follow-up XMM-Newton observation yielded a soft spectrum, well described by a 0.39 +/- 0.02 keV disk blackbody; applying this model to the Chandra observation yielded an observed 0.3-10 keV luminosity peak of 6.2 +/- 0.6x10(37) erg s(-1) (4.7x10(36) erg s(-1) in the 2.0-10 keV band). Observing with Hubble Space Telescope/Advanced Camera for Surveys did not reveal an optical counterpart, but allowed us to place an upper limit of B > 26.9, corresponding to an absolute V band magnitude >2.0. From the 2-10 keV luminosity and absolute V magnitude, we estimate an orbital period <5 h from an empirical relation. Fitting a disk blackbody + blackbody model allows us to reject a neutron star accretor at a 14 sigma level.

We have created 0.3–10 keV, 13 yr, unabsorbed luminosity lightcurves for 528 X-ray sources in the central 20′ of M31. We have 174 Chandra observations spaced at ∼1 month intervals due to our transient monitoring program, deeper observations of the M31 nucleus, and some public data from other surveys. We created 0.5–4.5 keV structure functions (SFs) for each source for comparison with the ensemble SF of active galactic nuclei (AGN). We find 220 X-ray sources with luminosities ≳1035 erg s−1 that have SFs with significantly more variability than the ensemble AGN SF, and which are likely X-ray binaries (XBs). A further 30 X-ray sources were identified as XBs using other methods. We therefore have 250 probable XBs in total, including ∼200 new identifications. This result represents great progress over the ∼50 XBs and ∼40 XB candidates previously identified out of the ∼2000 X-ray sources within the D25 region of M31; it also demonstrates the power of SF analysis for identifying XBs in external galaxies. We also identify a new transient black hole candidate, associated with the M31 globular cluster B128.