* Astronomy

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RE: 47 Tucanae X9

Blobrana · L

X9 in 47 Tucanae: Star Discovered in Closest Known Orbit Around Likely Black Hole

Astronomers found this extraordinarily close stellar pairing in the globular cluster named 47 Tucanae, a dense collection of stars located on the outskirts of the Milky Way galaxy, about 14,800 light years from Earth.
This particular source, known as X9, has been of interest to scientists for many years. Until a couple of years ago, astronomers thought X9 contained a white dwarf pulling material from a companion star like the Sun. (Astronomers call a pair of objects orbiting one another a 'binary' system.) However, a team of scientists in 2015 used radio data to show that X9 likely consisted instead of a black hole pulling gas from a white dwarf companion. These researchers predicted that the white dwarf would take only about 25 minutes to orbit the black hole.
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Title: The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9
Author: Arash Bahramian, Craig O. Heinke, Vlad Tudor, James C.A. Miller-Jones, Slavko Bogdanov, Thomas J. Maccarone, Christian Knigge, Gregory R. Sivakoff, Laura Chomiuk, Jay Strader, Javier A. Garcia, Timothy Kallman

47 Tuc X9 is a low mass X-ray binary (LMXB) in the globular cluster 47 Tucanae, and was previously thought to be a cataclysmic variable. However, Miller-Jones et al. (2015) recently identified a radio counterpart to X9 (inferring a radio/X-ray luminosity ratio consistent with black hole LMXBs), and suggested that the donor star might be a white dwarf. We report simultaneous observations of X9 performed by Chandra, Nustar and Australia Telescope Compact Array. We find a clear 28.18±0.02 min periodic modulation in the Chandra data, which we identify as the orbital period, confirming this system as an ultracompact X-ray binary. Our X-ray spectral fitting provides evidence for photoionised gas having a high oxygen abundance in this system, which indicates a C/O white dwarf donor. We also identify reflection features in the hard X-ray spectrum, making X9 the faintest LMXB to show X-ray reflection. We detect a ~ 6.8 day modulation in the X-ray brightness by a factor of 10, in archival Chandra, Swift, and Rosat data. The simultaneous radio/X-ray flux ratio is consistent with either a black hole primary or a neutron star primary, if the neutron star is a transitional millisecond pulsar. Considering the measured orbital period (with other evidence of a white dwarf donor), and the lack of transitional millisecond pulsar features in the X-ray light curve, we suggest that this could be the first ultracompact black hole X-ray binary identified in our Galaxy.

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