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

TL;DR

This study reports the lowest X-ray luminosity detection of a quiescent black hole X-ray binary, GS 2000+25, with simultaneous radio and X-ray observations revealing extremely faint emissions and challenging existing models of black hole accretion at very low luminosities.

Contribution

First simultaneous radio and X-ray observations of GS 2000+25 at unprecedented low luminosity, providing new insights into black hole accretion and jet behavior in quiescence.

Findings

Detected X-ray luminosity of 1.1 x 10^{30} erg/s, the lowest for a quiescent black hole.

No radio emission detected, setting a new upper limit at 6 GHz.

Radio to X-ray luminosity ratios are slightly below the established low state correlation.

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 $L_X = 1.1^{+1.0}_{-0.7} \times 10^{30}\,(d/2 {\rm \,\, kpc})^2$ 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 $L_X/L_{\rm Edd} \sim 10^{-9}$. 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.