The X-Ray Binary Population in the Galactic Center Revealed through Multi-decade Observations

• Published in: The Astrophysical journal 2021-11, Vol.921 (2), p.148
• Main Authors: Mori, Kaya, Hailey, Charles J., Schutt, Theo, Mandel, Shifra, Heuer, Keri, Grindlay, Jonathan E., Hong, Jaesub, Ponti, Gabriele, Tomsick, John A.
• Format: Article
• Language: English
• Subjects: Astronomical models; Astrophysics; Binary stars; Emission lines; Galactic center; Galactic nuclei; Globular clusters; Gravitational waves; Low-mass X-ray binary stars; Neutron stars; Orbits; Outbursts; Star clusters; Stellar mass black holes; Stellar models; X ray binaries; X ray sources; X ray stars; X-ray astronomy; X-ray transient sources; X-rays
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac1da5

We present an investigation of the quiescent and transient X-ray binaries (XRBs) of the Galactic Center (GC). We extended our Chandra analysis of the non-thermal X-ray sources, located in the central parsec, from Hailey et al. (2018), using an additional 4.6 Msec of ACIS-S data obtained in 2012–2018. The individual Chandra spectra of the 12 sources fit to an absorbed power-law model with a mean photon index Γ ≈ 2 and show no Fe emission lines. Long-term variability was detected from nine of them, confirming that a majority are quiescent XRBs. Frequent X-ray monitoring of the GC revealed 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 six known neutron star LMXBs in the GC, which have all undergone multiple outbursts with ≲ 5 year recurrence time on average. Based on the outburst history data of the broader population of X-ray transients, we conclude that the 16 sources represent a population of ∼240–630 tightly bound BH-LMXBs with ∼4−12 hr orbital periods, consistent with the stellar/binary dynamics modeling in the vicinity of Sgr A*. The distribution of the 16 BH-LMXB candidates is disk-like (at 87% CL) and aligned with the nuclear star cluster. Our results have implications for XRB formation and the rate of gravitational wave events in other galactic nuclei.