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In December 2015 the black hole binary V404 Cyg underwent a secondaryoutburst after the main June 2015 event. We monitored this re-brightening withthe INTEGRAL and Swift satellites, and in this paper we report the results ofthe time-resolved spectral analysis of these data. The December outburst sharedsimilar characteristics with the June one. The well sampled INTEGRAL lightcurve shows up to 10 Crab flares, which are separated by relatively weaknon-flaring emission phases when compared to the June outburst. The spectra arewell described by absorbed Comptonization models, with hard photon indices,$\Gamma \lesssim 2$, and significant detections of a high-energy cut-off onlyduring the bright flares. This is in contrast to the June outburst, where theComptonization models gave electron temperatures mostly in the 30--50 keVrange, while some spectra were soft ($\Gamma \sim 2.5$) without signs of anyspectral cut-off. Similarly to the June outburst, we see clear sings of avariable local absorber in the soft energy band covered by Swift/XRT andINTEGRAL/JEM-X, which causes rapid spectral variations observed during theflares. During one flare, both Swift and INTEGRAL captured V404 Cyg in a statewhere the absorber was nearly Compton thick, $N_\textrm{H} \approx10^{24}\,\textrm{cm}^{-2}$, and the broad band spectrum was similar to obscuredAGN spectra, as seen during the "plateaus" in the June outburst. We concludethat the spectral behaviour of V404 Cyg during the December outburst wasanalogous with the first few days of the June outburst, both having hard X-rayflares that were intermittently influenced by obscuration due to nearlyCompton-thick outflows launched from the accretion disc.

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