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 S _ { \nu } \propto \nu ^ { \alpha } ) of \alpha = -0.4 \pm 0.4 .
Our measured flux density of 42 \pm 4 \mu Jy beam ^ { -1 } at 5.5 GHz implies a radio luminosity ( \nu L _ { \nu } ) of 5.8 \times 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 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 .