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Register a new userA Chandra Catalog of X-ray Sources in the Central 150 pc of the Galaxy
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M. P. Muno
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We present the catalog of X-ray sources detected in a shallow Chandra survey of the inner 2 by 0.8 degrees of the Galaxy, and in two deeper observations of the Radio Arches and Sgr B2. The catalog contains 1352 objects that are highly-absorbed (N_H > 4e22 cm^-2 and are therefore likely to lie near the Galactic center (D~8 kpc), and 549 less-absorbed sources that lie within <6 kc of Earth. Based on the inferred luminosities of the X-ray sources and the expected numbers of various classes of objects, we suggest that the sources with L_X < 1e33 erg/s that comprise ~90% of the catalog are cataclysmic variables, and that the ~100 brighter objects are accreting neutron stars and black holes, young isolated pulsars, and Wolf-Rayet and O stars in colliding-wind binaries. We find that the spatial distribution of X-ray sources matches that of the old stellar population observed in the infrared, which supports our suggestion that most of the X-ray sources are old cataclysmic variables. However, we find that there is an apparent excess of ~10 bright sources in the Radio Arches region. That region is already known to be the site of recent star formation, so we suggest that the bright sources in this region are young high-mass X-ray binaries, pulsars, or WR/O star binaries. We briefly discuss some astrophysical questions that this catalog can be used to address.
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(abridged) We present a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17-by-17 arcminute field around Sgr A*. This field encompasses a physical area of 40 by 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic center is 10^{31} erg s^{-1} (2.0--8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGN. The surface density of absorbed sources (not detected below 1.5 keV) falls off as 1/theta away from Sgr A*, in agreement with the distribution of stars in infrared surveys. Point sources brighter than our completeness limit produce 10% of the flux previously attributed to diffuse emission. The log(N)-log(S) distribution of the Galactic center sources is extremely steep (power-law slope alpha = 1.7). If this distribution extends down to a flux of 10^{-17} erg cm^{-1} s^{-1} (10^{29} erg s^{-1} at 8 kpc, 2.0--8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. Therefore, the 2.0--8.0 keV luminosity distribution must flatten between 10^{29} - 10^{31} erg s^{-1}. Finally, the spectra of more than half of the Galactic center sources are very hard, and can be described by a power law ($E^{-Gamma}) with photon index Gamma < 1. Such hard spectra have been seen previously only from magnetically accreting white dwarfs and wind-accreting neutron stars, suggesting that there are large numbers of these systems in our field.
We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 by 0.8 degree field around the Galactic center. We increase the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Msec of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities >4e32 erg/s (0.5-8.0 keV; 90% confidence) over an area of one square degree, and up to an order of magnitude more sensitive in the deepest exposure (1.0 Msec) around Sgr A*. The positions of 60% of our sources are accurate to <1 (95% confidence), and 20% have positions accurate to <0.5. We search for variable sources, and find that 3% exhibit flux variations within an observation, 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7-285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically-accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8 sigma evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l~0.1 degree.
Globular clusters host a variety of lower-luminosity ($L_X<10^{35}$ erg s$^{-1}$) X-ray sources, including accreting neutron stars and black holes, millisecond pulsars, cataclysmic variables, and chromospherically active binaries. In this paper, we provide a comprehensive catalog of more than 1100 X-ray sources in 38 Galactic globular clusters observed by the Chandra X-ray Observatorys ACIS detector. The targets are selected to complement the MAVERIC surveys deep radio continuum maps of Galactic globular clusters. We perform photometry and spectral analysis for each source, determine a best-fit model, and assess the possibility of it being a foreground/background source based on its spectral properties and location in the cluster. We also provide basic assessments of variability. We discuss the distribution of X-ray binaries in globular clusters, their X-ray luminosity function, and carefully analyze systems with $L_X > 10^{33}$ erg s$^{-1}$. Among these moderately bright systems, we discover a new source in NGC 6539 that may be a candidate accreting stellar-mass black hole or a transitional millisecond pulsar. We show that quiescent neutron star LMXBs in globular clusters may spend ~2% of their lifetimes as transitional millisecond pulsars in their active ($L_X>10^{33}$ erg s$^{-1}$) state. Finally, we identify a substantial under-abundance of bright ($L_X>10^{33}$ erg s$^{-1}$) intermediate polars in globular clusters compared to the Galactic field, in contrast with the literature of the past two decades.
We present the X-ray point-source catalog produced from the Chandra Advanced CCD Imaging Spectrometer (ACIS-I) observations of the combined sim3.2 deg2 DEEP2 (XDEEP2) survey fields, which consist of four ~0.7-1.1 deg2 fields. The combined total exposures across all four XDEEP2 fields range from ~10ks-1.1Ms. We detect X-ray point-sources in both the individual ACIS-I observations and the overlapping regions in the merged (stacked) images. We find a total of 2976 unique X-ray sources within the survey area with an expected false-source contamination of ~30 sources (~1%). We present the combined logN-logS distribution of sources detected across the XDEEP2 survey fields and find good agreement with the Extended Chandra Deep Field and Chandra-COSMOS fields to f_{X,0.5-2keV}sim2x10^{-16} erg/cm^2/s. Given the large survey area of XDEEP2, we additionally place relatively strong constraints on the logN-logS distribution at high fluxes (f_{X,0.5-2keV}sim3x10^{-14} erg/cm^2/s), and find a small systematic offset (a factor ~1.5) towards lower source numbers in this regime, when compared to smaller area surveys. The number counts observed in XDEEP2 are in close agreement with those predicted by X-ray background synthesis models. Additionally, we present a Bayesian-style method for associating the X-ray sources with optical photometric counterparts in the DEEP2 catalog (complete to R_AB < 25.2) and find that 2126 (~71.4pm2.8%) of the 2976 X-ray sources presented here have a secure optical counterpart with a <6% contamination fraction. We provide the DEEP2 optical source properties (e.g., magnitude, redshift) as part of the X-ray-optical counterpart catalog.
We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7$-$290015 (T15) and SWIFT J174540.2$-$290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within $rsim1$ pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3-79~keV luminosities of $8times10^{36}$ and $3times10^{37}$ erg/s, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of $L_X lesssim 2 times10^{31}$ erg/s on their quiescent 2-8 keV luminosities. No pulsations, significant QPOs, or type I X-ray bursts were detected in the NuSTAR data. While T15 exhibited no significant red noise, the T37 power density spectra are well characterized by three Lorentzian components. The declining variability of T37 above $ u sim 10$ Hz is typical of black hole (BH) transients in the hard state. NuSTAR spectra of both transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe atomic features, and a continuum component well described by Comptonization models. Their X-ray reflection spectra are most consistent with high BH spin ($a_{*} gtrsim 0.9$) and large disk density ($n_esim10^{21}$ cm$^{-3}$). Based on the best-fit ionization parameters and disk densities, we found that X-ray reflection occurred near the inner disk radius, which was derived from the relativistic broadening and thermal disk component. These X-ray characteristics suggest the outbursting BH-LMXB scenario for both transients and yield the first BH spin measurements from X-ray transients in the central 100 pc region.
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