The observed relation between the X-ray and radio properties of low-luminosity accreting black holes has enabled the identification of multiple candidate black hole X-ray binaries ( BHXBs ) in globular clusters .
Here we report an identification of the radio source VLA J213002.08+120904 ( aka M15 S2 ) , recently reported in , as a BHXB candidate .
They showed that the parallax of this flat-spectrum variable radio source indicates a 2.2 ^ { +0.5 } _ { -0.3 } kpc distance , which identifies it as lying in the foreground of the globular cluster M15 .
We determine the radio characteristics of this source , and place a deep limit on the X-ray luminosity of \sim 4 \times 10 ^ { 29 } erg s ^ { -1 } .
Furthermore , we astrometrically identify a faint red stellar counterpart in archival Hubble images , with colors consistent with a foreground star ; at 2.2 kpc its inferred mass is 0.1-0.2 M _ { \odot } .
We rule out that this object is a pulsar , neutron star X-ray binary , cataclysmic variable , or planetary nebula , concluding that VLA J213002.08+120904 is the first accreting black hole X-ray binary candidate discovered in quiescence outside a globular cluster .
Given the relatively small area over which parallax studies of radio sources have been performed , this discovery suggests a much larger population of quiescent BHXBs in our Galaxy , 2.6 \times 10 ^ { 4 } -1.7 \times 10 ^ { 8 } BHXBs at 3 \sigma confidence , than has been previously estimated ( \sim 10 ^ { 2 } -10 ^ { 4 } ) through population synthesis .