期刊
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
卷 453, 期 4, 页码 3918-3931出版社
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stv1869
关键词
stars: black holes; globular clusters: individual: 47 Tuc; radio continuum: general; X-rays: binaries
资金
- Commonwealth of Australia
- NSF [AST-1308124]
- Australian Research Council [FT140101082, DP120102393]
- NSERC
- Ingenuity New Faculty Award
- Alexander von Humboldt Fellowship
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1350389, 1308124] Funding Source: National Science Foundation
- Australian Research Council [FT140101082] Funding Source: Australian Research Council
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 S-v alpha v(alpha)) of a = -0.4 +/- 0.4. Our measured flux density of 42 +/- 4 mu Jy beam(-1) at 5.5 GHz implies a radio luminosity (vL(v)) of 5.8 x 10(27) erg s(-1), 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 L-X), 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 10(33) erg s(-1) 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.
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