Chandra Reveals X-Ray Binary Stars at the Galactic Center
Chandra
2–8-keV image of the Galactic Centre with X-ray sources with C ≥ 100
overlaid. Thermal (HR2 > 0.3) and non-thermal (HR2 < 0.3) sources
are indicated with red and cyan circles, respectively. The colour scale
indicates the number of 2–8-keV counts per pixel. The inner and outer
yellow circles delineate the 0.2 pc < r < 1 pc region
around Sgr A*. The inner region was excluded from the analysis. The
magenta ellipse (7.8 pc × 3.9 pc, full-width at half-maximum) bounds the
region of spatially unresolved hard X-ray emission that was discovered
by NuSTAR and is due to thermal emission from intermediate polars. The
non-thermal sources cluster inside the inner parsec, whereas the thermal
sources are distributed more uniformly throughout the hard X-ray
emission. The Galactic plane runs along the semi-major axis of the
ellipse. The dearth of sources to the north of the Galactic plane is due
to strong extinction from molecular clouds (such as the circumnuclear
disk), which densely populate that region. This may reduce the estimated
number of sources by a factor of a few. Some bright sources without
circles are X-ray transients, including the outbursting neutron-star
low-mass X-ray binary AX J1745−2901 in the lower right corner. For
reference, Chandra’s angular resolution is 0.5 arcsec, which corresponds
to about 0.025 pc at the Galactic Centre. Nature (2018)
doi:10.1038/nature25029
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 stellar-mass black hole, because accretion around the black hole would generate detectable X-rays.
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.
Publication: Charles J. Hailey, et al., “A Density Cusp of Quiescent X-Ray Binaries in the Central Parsec of the Galaxy,” Nature, volume 556, pages 70–73 (2018) doi:10.1038/nature25029
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