The X-ray Binary Population of the Galactic Center as Revealed Through Decades of Observations

We present a comprehensive investigation of the X-ray binaries (XRBs) of the Galactic Center (GC). Our analysis of the 12 non-thermal X-ray sources in the central parsec utilized a combined total of 6 Msec of Chandra ACIS-S and ACIS-I data, obtained over the course of 15 years. The individual Chandra spectra of these 12 sources fit to an absorbed power-law model with mean photon index Γ ~ 2, and don’t show the characteristic Fe emission lines typical of CVs. Significant long-term variability was detected from nine of the sources, confirming that most if not all are quiescent XRBs. Frequent X-ray monitoring of the GC reveals that the 12 non-thermal X-ray sources, as well as four X-ray transients, have shown at most a single outburst over the last two decades. They are distinct from the known neutron star LMXBs in the GC, which have all undergone multiple outbursts, with average recurrence time ≲ 5 years. A review of the properties of the broader Galactic X-ray transient populations indicates that these 16 sources represent ~240-630 tightly bound BH-LMXBs with ~4-12 hr orbital periods, consistent with recent stellar/binary dynamics modeling in the vicinity of Sgr A*. Our results have significant implications for XRB formation and the rate of gravitational wave events in other galactic nuclei.