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Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate

Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate
Deep radio imaging of 47 Tuc identifies the peculiar X-ray source X9 as a new black hole candidate
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??????) of ? = ?0.4 ± 0.4. Our measured flux density of 42 ± 4 ?Jy beam?1 at 5.5 GHz implies a radio luminosity (?L?) 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.
0035-8711
3918-3931
Miller-Jones, J.C.A.
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Strader, J.
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Heinke, C.O.
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Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50
van den Berg, M.
3a120dad-151d-4e84-8d19-d82fb8a411ef
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Chomiuk, L.
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Noyola, E.
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Russell, T.D.
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Seth, A.C.
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Sivakoff, G.R.
d3dd17f7-f61d-4139-acb4-e97e13c7d3b8
Miller-Jones, J.C.A.
de372f09-251a-4a6c-93d9-a30ec089473f
Strader, J.
65549124-aafe-4b6e-a5cb-2ca8b2624843
Heinke, C.O.
81e86af6-563d-4efc-ba4f-8e2902b2c9da
Maccarone, T.J.
5d8ee27f-a174-424c-b3b7-a6739bfeea50
van den Berg, M.
3a120dad-151d-4e84-8d19-d82fb8a411ef
Knigge, C.
ac320eec-631a-426e-b2db-717c8bf7857e
Chomiuk, L.
71fe2d37-8846-4d13-8bcd-b8459a57aa93
Noyola, E.
81b643ee-f7e5-418b-86a7-954a308bcf6e
Russell, T.D.
a3056d2c-1f5d-4dac-8903-aa77e4af2e9e
Seth, A.C.
9e61d04f-e896-428c-ad21-eff18a15fdf8
Sivakoff, G.R.
d3dd17f7-f61d-4139-acb4-e97e13c7d3b8

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. (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), 3918-3931. (doi:10.1093/mnras/stv1869).

Record type: Article

Abstract

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??????) of ? = ?0.4 ± 0.4. Our measured flux density of 42 ± 4 ?Jy beam?1 at 5.5 GHz implies a radio luminosity (?L?) 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.

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Accepted/In Press date: 11 August 2015
e-pub ahead of print date: 14 September 2015
Published date: 11 November 2015
Organisations: Astronomy Group

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Local EPrints ID: 396290
URI: http://eprints.soton.ac.uk/id/eprint/396290
ISSN: 0035-8711
PURE UUID: dd4aa678-fa3c-4f8b-b448-248692781fcf

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Date deposited: 07 Jun 2016 14:32
Last modified: 16 Dec 2019 19:53

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