Zooming in on a sleeping giant: milliarcsecond HSA imaging of the black hole binary V404 Cyg in quiescence
J.C.A. Miller-Jones (1), E. Gallo (2), M.P. Rupen (1), A.J., Mioduszewski (1), W. Brisken (1), R.P. Fender (3,4), P.G. Jonker (5,6,7), and, T.J. Maccarone (3) ((1) NRAO, (2) UCSB, (3) U. Southampton, (4) U. Amsterdam,, (5) SRON, (6) U. Utrecht, (7) CfA)

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.
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