The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9

Creators: Bahramian, Arash; Heinke, Craig O.; Tudor, Vlad; Miller-Jones, James C. A.; Bogdanov, Slavko; Maccarone, Thomas J.; Knigge, Christian; Sivakoff, Gregory R.; Chomiuk, Laura; Strader, Jay; García, Javier A.; Kallman, Timothy R.

Abstract

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. 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 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 an ∼6.8-d modulation in the X-ray brightness by a factor of 10, in archival Chandra, Swiftand 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.

Additional Information

© 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2017 January 18. Received 2017 January 16; in original form 2016 August 2. Published: 24 February 2017. AB thanks E. W. Koch for help with computational aspects of this project, K. A. Arnaud for helpful discussion on spectral analysis and B. E. Tetarenko for help with the radio/X-ray luminosity plot. The authors thank F. A. Harrison for granting NuSTAR director's discretionary time for these observations and J. Tomsick for assisting in coordination of NuSTAR observations. JCAM-J is the recipient of an Australian Research Council Future Fellowship (FT 140101082). JS acknowledges support of NSF grant AST-1308124 and a Packard Fellowship. COH and GRS acknowledge support from NSERC Discovery Grants, and COH also acknowledges support from a Humboldt Fellowship. This work was funded in part by NASA Chandra grant GO4-15029A awarded through Columbia University and issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. The scientific results reported in this article are based on observations made by the Chandra X-ray Observatory, NuSTAR observatory and Australia Telescope Compact Array, and archival data obtained from Chandra and Swift data archives. The Australia Telescope Compact Array is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. We acknowledge use of the following packages/softwares for analysis: the NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (Caltech, USA); CIAO, provided by the Chandra X-ray Center (CXC); HEASOFT, provided by the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory's High Energy Astrophysics Division; PVM_XSTAR, developed at the MIT Kavli Institute for Astrophysics and funded in part by NASA and the Smithsonian Institution, through the AISRP grant NNG05GC23G and Smithsonian Astrophysical Observatory contract SV3-73016; ASTROPY (Astropy Collaboration et al. 2013); ASTROML (Vanderplas et al. 2012); GATSPY (Vanderplas 2015); and MATPLOTLIB (Hunter 2007). Our simulations were performed using Cybera cloud computing resources (http://www.cybera.ca/). We acknowledge extensive use of NASA's Astrophysics Data System and Arxiv.

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