The 2015 Decay of the Black Hole X-Ray Binary V404 Cygni: Robust Disk-jet Coupling and a Sharp Transition into Quiescence
Abstract
We present simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cygni at the end of its 2015 outburst. From 2015 July 11-August 5, we monitored V404 Cygni with Chandra, Swift, and NuSTAR in the X-ray, and with the Karl G. Jansky Very Large Array and the Very Long Baseline Array in the radio, spanning a range of luminosities that were poorly covered during its previous outburst in 1989 (our 2015 campaign covers 2× {10}33≲ {L}{{X}}≲ {10}34 {erg} {{{s}}}-1). During our 2015 campaign, the X-ray spectrum evolved rapidly from a hard photon index of {{Γ }}≈ 1.6 (at {L}{{X}}≈ {10}34 {erg} {{{s}}}-1) to a softer {{Γ }}≈ 2 (at {L}{{X}}≈ 3× {10}33 {erg} {{{s}}}-1). We argue that V404 Cygni reaching {{Γ }}≈ 2 marks the beginning of the quiescent spectral state, which occurs at a factor of ≈3-4 higher X-ray luminosity than the average pre-outburst luminosity of ≈ 8× {10}32 {erg} {{{s}}}-1. V404 Cygni falls along the same radio/X-ray luminosity correlation that it followed during its previous outburst in 1989, implying a robust disk-jet coupling. We exclude the possibility that a synchrotron-cooled jet dominates the X-ray emission in quiescence, leaving synchrotron self-Compton from either a hot accretion flow or from a radiatively cooled jet as the most likely sources of X-ray radiation, and/or particle acceleration along the jet becoming less efficient in quiescence. Finally, we present the first indications of correlated radio and X-ray variability on minute timescales in quiescence, tentatively measuring the radio emission to lag the X-ray by 15+/- 4 minute, suggestive of X-ray variations propagating down a jet with a length of <3.0 au.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2017
- DOI:
- 10.3847/1538-4357/834/2/104
- arXiv:
- arXiv:1611.02810
- Bibcode:
- 2017ApJ...834..104P
- Keywords:
-
- accretion;
- accretion disks;
- stars: black holes;
- stars: individual: V404 Cygni;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 24 pages, 11 figures, 5 tables. Accepted for publication in ApJ