期刊
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
卷 467, 期 2, 页码 2199-2216出版社
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx166
关键词
accretion, accretion discs; stars: Black holes; stars: neutron; globular clusters: individual: 47 Tuc; X-rays: binaries
资金
- Australian Research Council Future Fellowship [FT 140101082]
- NSF [AST-1308124]
- Packard Fellowship
- NSERC Discovery Grants
- Humboldt Fellowship
- NASA Chandra grant [GO4-15029A]
- NASA [NAS8-03060]
- Australian Government
- NASA
- Smithsonian Institution, through the AISRP [NNG05GC23G]
- Smithsonian Astrophysical Observatory [SV3-73016]
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1308124] Funding Source: National Science Foundation
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.
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