The supermassive black hole at a galaxy’s center is expected gradually to accumulate many small, stellar-mass black holes around it. In the case of our own galaxy, as many as 20,000 black holes may have settled around the central few light-years. So far, however, no such density cusp has been reported. One of the best ways to look for such black holes is via binary stars in which one member is a stellar-mass black hole, because accretion around the black hole would generate detectable X-rays. Harvard-Smithsonian Center For Astrophysics (CfA) astronomer Jaesub Hong was a member of a six-person team that used the Chandra X-ray Observatory to search for such binaries. They examined the equivalent of several weeks’ worth of archival Chandra observations obtained over twelve years, in an area corresponding to a volume that stretches to about sixty light-years from the galactic nucleus. In this region, thousands of X-ray point sources are seen, produced by a range of processes including hot gas, stellar atmospheres, binaries with white dwarf star members, neutron stars, and black holes. The innermost region itself, out to about twelve light-years, has hundreds of sources. (For comparison, the nearest star to the Sun is four light-years away.) The X-ray energies of the sources can be used to diagnose their character, but in this dense complex source confusion was a challenge. To minimize the confusion, the team focused on relatively bright sources, about one hundred of them, and also used simulations as a reality check. They found that twelve of the sources in the central dozen light-years had relatively “soft” X-ray spectra consistent with these sources being black-hole binaries. Although some alternative explanations cannot be ruled out (for example, a class of pulsars), the observed X-ray properties of these sources are the first strong evidence for the population of black hole binaries predicted to settle near the galactic center. The results suggest there are a larger number of (still undetected) isolated BHs present, and not least emphasize the complex and fascinating nature of this unique location in our galaxy. 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
