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 \times 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 \sim 30 min , confirming that the source remains active in the quiescent state .