2020ApJ...889...58R

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^+1.0×10³⁰(d/2kpc)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_Edd ∼ 10^–9. In 15 hours of observations with the Karl G. Jansky Very Large Array, no radio continuum emission is detected to a 3σ limit of <2.8 µJy at 6 GHz. Including GS 2000+25, four quiescent stellar-mass black holes with L_X < 10³² 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 > 10³² 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.