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dc.contributor.authorMiller-Jones, James
dc.contributor.authorStrader, J.
dc.contributor.authorHeinke, C.
dc.contributor.authorMaccarone, T.
dc.contributor.authorvan den Berg, M.
dc.contributor.authorKnigge, C.
dc.contributor.authorChomiuk, L.
dc.contributor.authorNoyola, E.
dc.contributor.authorRussell, T.
dc.contributor.authorSeth, A.
dc.contributor.authorSivakoff, G.
dc.identifier.citationMiller-Jones, J. and Strader, J. and Heinke, C. and Maccarone, T. and van den Berg, M. and Knigge, C. and Chomiuk, L. et al. 2015. Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate. Monthly Notices of The Royal Astronomical Society. 453 (4): pp. 3918-3931.

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−1 at 5.5 GHz implies a radio luminosity (νLν) of 5.8 × 1027 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 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−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.

dc.titleDeep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate
dc.typeJournal Article

This article has been accepted for publication in Monthly Notices of The Royal Astronomical Society, © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

curtin.departmentDepartment of Physics and Astronomy
curtin.accessStatusOpen access

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