Zooming in on a sleeping giant: milliarcsecond HSA imaging of the black hole binary V404 Cyg in quiescence

TL;DR

This study uses high-resolution radio imaging to investigate the quiescent black hole binary V404 Cyg, revealing non-thermal emission likely from a jet, with evidence of ongoing activity and short-term flares.

Contribution

First milliarcsecond-scale imaging of V404 Cyg in quiescence, constraining source size, brightness temperature, and confirming persistent activity and flaring behavior.

Findings

Source size less than 1.3 mas

Brightness temperature exceeds 7 million K

Detected short-term radio flare

Abstract

Observations of the black hole X-ray binary V404 Cyg with the very long baseline interferometer HSA (the High Sensitivity Array) have detected the source at a frequency of 8.4 GHz, providing a source position accurate to 0.3 mas relative to the calibrator source. The observations put an upper limit of 1.3 mas on the source size (5.2 AU at 4 kpc) and a lower limit of 7 x 10^6 K on its brightness temperature during the normal quiescent state, implying that the radio emission must be non-thermal, most probably synchrotron radiation, possibly from a jet. The radio lightcurves show a short flare, with a rise time of about 30 min, confirming that the source remains active in the quiescent state.