Revisiting the radio/X-ray flux correlation in the black hole V404 Cyg: from outburst to quiescence

摘要

We report results of Chandra X-ray and VLA radio observations of the Galacticaccreting black hole V404 Cyg (GS 2023+338) in its quiescent state. V404 Cyg isdetected at its faintest level of radio and X-ray emission with a 0.5-10 keVunabsorbed luminosity of 8.3 x 10^32 (d/3.5 kpc)^2 erg/s. The X-ray spectrumfit with an absorbed power-law model yields a photon index of 2.17 +/- 0.13.Contrary to previous findings, this clearly indicates that V404 Cyg undergoes -like most black holes in quiescence - a softening of its X-ray spectrum at verylow luminosity compared to the standard hard state. The quiescent radioemission is consistent with the presence of self-absorbed compact jets. We havealso reanalyzed archival data from the decay of the 1989 outburst of V404 Cygin order to quantify more precisely the correlation between radio and X-rayemission in the hard state of V404 Cyg. We show that this correlation extendsover five decades in X-ray flux and holds down to the quiescent state of V404Cyg. The index of this correlation (~0.5) may suggest that synchrotronself-Compton emission is the dominant physical process at high energy in V404Cyg. However, this index is also consistent with scale invariant jet modelscoupled to an inefficiently radiating accretion disc. We discuss the propertiesof the quiescent state of black holes and highlight the fact that some of theirproperties are different from the standard hard state.