The December 2015 re-brightening of V404 Cyg: Variable absorption from the accretion disc outflow

TL;DR

This study analyzes the December 2015 re-brightening of V404 Cyg, revealing variable absorption from accretion disc outflows that cause spectral changes similar to the June outburst, with evidence of nearly Compton-thick obscuration during flares.

Contribution

It provides the first detailed time-resolved spectral analysis of the December 2015 outburst, highlighting the role of variable absorption and comparing it to the June event.

Findings

Spectral analysis shows variable local absorption causing rapid spectral changes.

Flares sometimes occur with nearly Compton-thick absorption, similar to AGN spectra.

Spectral characteristics during December resemble early June outburst phases.

Abstract

In December 2015 the black hole binary V404 Cyg underwent a secondary outburst after the main June 2015 event. We monitored this re-brightening with the INTEGRAL and Swift satellites, and in this paper we report the results of the time-resolved spectral analysis of these data. The December outburst shared similar characteristics with the June one. The well sampled INTEGRAL light curve shows up to 10 Crab flares, which are separated by relatively weak non-flaring emission phases when compared to the June outburst. The spectra are well described by absorbed Comptonization models, with hard photon indices, $\Gamma \lesssim 2$, and significant detections of a high-energy cut-off only during the bright flares. This is in contrast to the June outburst, where the Comptonization models gave electron temperatures mostly in the 30--50 keV range, while some spectra were soft ($\Gamma \sim 2.5$) without signs of any spectral cut-off. Similarly to the June outburst, we see clear sings of a variable local absorber in the soft energy band covered by Swift/XRT and INTEGRAL/JEM-X, which causes rapid spectral variations observed during the flares. During one flare, both Swift and INTEGRAL captured V404 Cyg in a state where the absorber was nearly Compton thick, $N_\textrm{H} \approx 10^{24}\,\textrm{cm}^{-2}$, and the broad band spectrum was similar to obscured AGN spectra, as seen during the "plateaus" in the June outburst. We conclude that the spectral behaviour of V404 Cyg during the December outburst was analogous with the first few days of the June outburst, both having hard X-ray flares that were intermittently influenced by obscuration due to nearly Compton-thick outflows launched from the accretion disc.