Spatial distribution of black hole binaries within ~ 1 pc of the Galactic Center revealed by Chandra and NuSTAR X-ray observations
Abstract
The recent discovery of a dozen quiescent X-ray binaries in the vicinity of Sgr A* suggests a density cusp of hundreds of black hole (BH) LMXBs (Hailey et al. 2018), indicating over 10,000 isolated stellar-mass BHs, as predicted by the theoretical models of compact objects interacting dynamically with the supermassive BH. We report three on-going investigations identifying BH binaries and determining their spatial distribution in the central pc region. (1) We are analyzing additional Chandra ACIS-S data of the GC point sources and identify them by both spectral and timing properties. For example, further Chandra ACIS analysis detected another quiescent BH-LMXB candidate, which is highly variable with non-thermal X-ray spectra, at r~1 pc. (2) We present NuSTAR observations of two new X-ray transients detected by the Neil Gehrels Swift Observatory in the central parsec region in 2016. Their broadband (3-79 keV) spectral and timing characteristics are consistent with those of BH binaries, confirming an overabundance of X-ray transients including another BH transient detected with radio jet (Muno et al. 2005). (3) We also present our systematic study of all the previously detected X-ray transients within ~50 pc in the GC, particularly, on their outburst history. Frequent monitoring of the GC by Swift and other X-ray telescopes over the past two decades surveyed the outburst history of all GC transients, including very faint X-ray transients with peak luminosity below 1036 erg/s, more thoroughly than elsewhere in our Galaxy (e.g., globular clusters). For example, all six NS-LMXBs identified within the central ~ 50 pc region have had recurrent X-ray outbursts with a frequency of ~5 years or less, whereas all BH-LMXBs outbursted only once or never in the past two decades. A combination of the 13 quiescent BH-LMXBs discovered by Chandra and the three BH transients, allows us to investigate their spatial distribution in the central 1 pc region. Our results have strong implications for binary formations in galactic nuclei and test the predictions from recent theoretical models.
- Publication:
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AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019
- Bibcode:
- 2019HEAD...1711273H