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

TL;DR

This study uses extensive Chandra X-ray observations to identify and analyze a population of quiescent and transient X-ray binaries in the Galactic Center, revealing their properties, distribution, and implications for gravitational wave events.

Contribution

It provides the first detailed characterization of the XRB population in the Galactic Center, estimating their numbers, orbital periods, and spatial distribution, based on long-term X-ray monitoring data.

Findings

Identified ~240-630 black hole low-mass X-ray binaries in the GC.

Most sources are quiescent with long-term variability.

The population is disk-like and aligned with the nuclear star cluster.

Abstract

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 $\Gamma$~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 hour orbital periods, consistent with the stellar/binary dynamics modelling 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.