Mon. Not. R. Astron. Soc., 453, 3918-3931 (2015/November-2)
Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate.
MILLER-JONES J.C.A., STRADER J., HEINKE C.O., MACCARONE T.J., VAN DEN BERG M., KNIGGE C., CHOMIUK L., NOYOLA E., RUSSELL T.D., SETH A.C. and SIVAKOFF G.R.
Abstract (from CDS):
We report the detection of steady radio emission from the known X-ray source X9 in the globular cluster 47 Tuc. With a double-peaked C iv emission line in its ultraviolet spectrum providing a clear signature of accretion, this source had been previously classified as a cataclysmic variable. In deep ATCA (Australia Telescope Compact Array) imaging from 2010 and 2013, we identified a steady radio source at both 5.5 and 9.0 GHz, with a radio spectral index (defined as Sν∝να) of α = -0.4±0.4. Our measured flux density of 42±4 µJy/beam at 5.5 GHz implies a radio luminosity (νLν) of 5.8x1027 erg/s, significantly higher than any previous radio detection of an accreting white dwarf. Transitional millisecond pulsars, which have the highest radio-to-X-ray flux ratios among accreting neutron stars (still a factor of a few below accreting black holes at the same LX), show distinctly different patterns of X-ray and radio variability than X9. When combined with archival X-ray measurements, our radio detection places 47 Tuc X9 very close to the radio/X-ray correlation for accreting black holes, and we explore the possibility that this source is instead a quiescent stellar-mass black hole X-ray binary. The nature of the donor star is uncertain; although the luminosity of the optical counterpart is consistent with a low-mass main-sequence donor star, the mass transfer rate required to produce the high quiescent X-ray luminosity of 1033 erg/s suggests the system may instead be ultracompact, with an orbital period of order 25 min. This is the fourth quiescent black hole candidate discovered to date in a Galactic globular cluster, and the only one with a confirmed accretion signature from its optical/ultraviolet spectrum.
© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society (2015)
stars: black holes - globular clusters: individual: 47 Tuc - radio continuum: general - X-rays: binaries
Table 2: [MSH2015] JHHMMSS.sss+DDMMSS.ss N=14.
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