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The Ultracompact Nature of the Black Hole Candidate X-Ray Binary 47 Tuc X947 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. 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 photoionized gas having a high oxygen abundance in this system, which indicates a CO 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 an approx. 6.8-d 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.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Bahramian, Arash
(Michigan State Univ. East Lansing, MI, United States)
Heinke, Craig O.
(Alberta Univ. Edmonton, Alberta, Canada)
Tudor, Vlad
(Curtin Univ. of Technology Perth, Australia)
Miller-Jones, James C. A.
(Curtin Univ. of Technology Perth, Australia)
Bogdanov, Slavko
(Columbia Univ. New York, NY, United States)
Maccarone, Thomas J.
(Texas Tech Univ. Lubbock, TX, United States)
Knigge, Christian
(Southampton Univ. United Kingdom)
Sivakoff, Gregory R.
(Alberta Univ. Edmonton, Alberta, Canada)
Chomiuk, Laura
(Michigan State Univ. East Lansing, MI, United States)
Strader, J.
(Michigan State Univ. East Lansing, MI, United States)
Garcia, Javier A.
(Harvard-Smithsonian Center for Astrophysics Cambridge, MA, United States)
Kallman, Timothy
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
October 11, 2017
Publication Date
February 24, 2017
Publication Information
Publication: Monthly Notices of the Royal Astronomical Society
Publisher: Oxford University Press
Volume: 467
Issue: 2
ISSN: 0035-8711
e-ISSN: 1365-8711
Subject Category
Report/Patent Number
Funding Number(s)
Distribution Limits
stars: Black hole
accretion discs

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