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Journal Article

The X-ray binary population in the Galactic Center revealed through multi-decade observations


Ponti,  Gabriele
High Energy Astrophysics, MPI for Extraterrestrial Physics, Max Planck Society;

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Mori, K., Hailey, C. J., Schutt, T. Y. E., Mandel, S., Heuer, K., Grindlay, J. E., et al. (2021). The X-ray binary population in the Galactic Center revealed through multi-decade observations. The Astrophysical Journal, 921(2): 148. doi:10.3847/1538-4357/ac1da5.

Cite as: https://hdl.handle.net/21.11116/0000-0009-A331-7
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.