Observations of the X-ray Source Population in the Galactic Center

Intensive observations of the Galactic Center (GC) have been conducted over the last decade by Chandra, XMM-Newton, Swift and NuSTAR. Analysis of the multi-million second Chandra ACIS data has led to the identification of more than one hundred X-ray sources in the central < 5 pc region and revealed a concentration of 12 non-thermal X-ray sources within a parsec of Sgr A*. Their spectral and timing properties point to a population of quiescent X-ray binaries (XRB) with either a neutron star (NS) or black hole (BH). Frequent X-ray observations of the central r < 30 pc region, such as daily Swift-XRT monitoring since 2006, has detected 20 X-ray transients, including very faint X-ray transients (VFXTs), and XRBs with NS or BH. The outburst characteristics, spatial and spectral properties of those objects within the central few parsecs are compared with those farther out. The comparison suggests that the 12 quiescent XRBs and four other centrally-located, one-time bursting XRBs are most likely BH-low mass X-ray binaries (BH-LMXBs), or a peculiar group of NS-LMXBs with long burst recurrence time. In contrast to the GC population, the Galactic Ridge, which has been extensively studied by INTEGRAL/IBIS, Suzaku and NuSTAR, appears dominated by some combination of dwarf novae, polars and possibly an unusual type of mCV. Deeper observations of the GC with Chandra and NuSTAR and further monitoring with Swift, along with potential future observations with Lynx, will further elucidate these issues. The recently launched Spectrum-RG observatory will provide additional source statistics on X-ray populations in both the GC and Galactic Ridge.