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 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 Kirsten et al. 2014, 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 $\sim4\times10^{29}$ 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, $2.6\times10^4-1.7\times10^8$ BHXBs at $3\sigma$ confidence, than has been previously estimated ($\sim10^2-10^4$) through population synthesis. 
B.E. Tetarenko, A. Bahramian, R.M. Arnason, J.C.A. Miller-Jones, S.   Repetto, C.O. Heinke, T.J. Maccarone, L. Chomiuk, G.R....
astro-ph.HE