The most straightforward way to detect a black hole is to find it in an X-ray binary system. Writing in last week’s issue of the journal, Nature, CfA astronomer Jaesub Hong and five colleagues used the Chandra X-ray Observatory archive to identify a dozen X-ray binaries in a density cusp within three light-years of the Galactic Center. Over the past twelve years, Chandra has observed the region for over 380 hours. The scientists analyzed X-ray images of the region between 0.65 and 12.4 light-years from the supermassive black hole (the innermost region had too many complicating sources). They found 415 X-ray point sources, and after carefully considering other possible origins for each one (novae, unusual stars, extragalactic background sources, among other possibilities) they conclude that these dozen are black hole X-ray binaries. The authors then consider the sensitivity limits, the expected abundance of single black holes relative to binaries, and other uncertainties to conclude that the observed number is in good agreement with general model expectations for black holes. They add, however, that alternative models of galaxy evolution cannot be entirely excluded. Some of the observed binaries could be the result of unique events, for example if a globular cluster and its black holes fell into the region. Reference: “A Density Cusp of Quiescent X-Ray Binaries in the Central Parsec of the Galaxy” by Charles J. Hailey, Kaya Mori, Franz E. Bauer, Michael E. Berkowitz, Jaesub Hong and Benjamin J. Hord, 5 April 2018, Nature.DOI: 10.1038/nature25029
