Mon. Not. R. Astron. Soc., 467, 2199-2216 (2017/May-2)
The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9.
BAHRAMIAN A., HEINKE C.O., TUDOR V., MILLER-JONES J.C.A., BOGDANOV S., MACCARONE T.J., KNIGGE C., SIVAKOFF G.R., CHOMIUK L., STRADER J., GARCIA J.A. and KALLMAN T.
Abstract (from CDS):
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.
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
accretion, accretion discs - stars: Black holes - stars: neutron - globular clusters: individual: 47 Tuc - X-rays: binaries - X-rays: binaries
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