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 ^ { +1.0 } _ { -0.7 } \times 10 ^ { 30 } ( d / 2 { 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 _ { 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 .