27 research outputs found
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 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 ) of
. Our measured flux density of microJy/beam at 5.5
GHz implies a radio luminosity () of 5.8e27 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 X-ray luminosity), 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 1e33 erg/s suggests the system may instead be
ultracompact, with an orbital period of order 25 minutes. 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.Comment: 15 pages, 6 figures, accepted for publication in MNRA
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