The first low-mass black hole x-ray binary identified in quiescence outside of a globular cluster

The observed relation between the X-ray and radio properties of low-luminosity accreting black holes (BHs) has enabled the identification of multiple candidate black hole X-ray binaries (BHXBs) in globular clusters (GCs). Here, we report an identification of the radio source VLA J213002.08+120904 (aka M15 S2), recently reported in Kirsten et al., as a BHXB candidate. They showed that the parallax of this flat-spectrum variable radio source indicates a - + 2.2 0.30.5 kpc distance, which identifies it as lying in the foreground of the GC M15. We determine the radio characteristics of this source and place a deep limit on the X-ray luminosity of ~4 × 1029 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 0.1-0.2Me. 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 BHXB candidate discovered in quiescence outside of a GC. 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 ± 104-1.7 × 108 BHXBs at 3× confidence, than has been previously estimated (~102-104) through population synthesis