GS 2000+25: The Least Luminous Black Hole X-ray Binary

Abstract

Little is known about the properties of the accretion flows and jets of the lowest-luminosity quiescent black holes. We report new, strictly simultaneous radio and X-ray observations of the nearby stellar-mass black hole X-ray binary GS 2000+25 in its quiescent state. In deep Chandra observations we detect the system at a faint X-ray luminosity of $L_X=1.1_{-0.7}^{+1.0}\times 10^{30}(d/2\mathrm{kpc}{)}^2$ erg s${}^{-1}$ (1-10 keV). This is the lowest X-ray luminosity yet observed for a quiescent black hole X-ray binary, corresponding to an Eddington ratio $L_X/L_{\mathrm{Edd}}\sim 10^{-9}$. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a $3\sigma$ limit of $<2.8\mu$Jy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with $L_X<10^{32}$ erg s${}^{-1}$ have deep simultaneous radio and X-ray observations and known distances. These sources all have radio to X-ray luminosity ratios generally consistent with, but slightly lower than, the low state radio/X-ray correlation for stellar-mass black holes with $L_X>10^{32}$ erg s${}^{-1}$. Observations of these sources tax the limits of our current X-ray and radio facilities, and new routes to black hole discovery are needed to study the lowest-luminosity black holes.

arXiv comment7 pages, 2 figures, accepted for publication in ApJ

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