Flares, wind and nebulae: the 2015 December mini-outburst of V404 Cygni

Abstract

After more than 26 years in quiescence, the black hole transient V404 Cyg went into a luminous outburst in June 2015, and additional activity was detected in late December of the same year. Here, we present an optical spectroscopic follow-up of the December mini-outburst, together with X-ray, optical and radio monitoring that spanned more than a month. Strong flares with gradually increasing intensity are detected in the three spectral ranges during the ~10 days following the Swift trigger. Our optical spectra reveal the presence of a fast outflowing wind, as implied by the detection of a P-Cyg profile (He I - 5876 A) with a terminal velocity of ~2500 km/s. Nebular-like spectra -- with an H_alpha equivalent width of ~500 A -- are also observed. All these features are similar to those seen during the main June 2015 outburst. Thus, the fast optical wind simultaneous with the radio jet is most likely present in every V404 Cyg outburst. Finally, we report on the detection of a strong radio flare in late January 2016, when X-ray and optical monitoring had stopped due to Sun constraints.

To be published in MNRAS Letters

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• Repetitive Patterns in Rapid Optical Variations in the Nearby Black-hole Binary V404 Cygni
• A connection between accretion state and $\text{Fe K absorption}$ in an accreting neutron star: black hole-like soft state winds?
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• Accretion and Outflow in V404 Cyg
• Photoionization instability of the Fe K absorbing plasma in the neutron star transient AX J1745.6-2901
• Discovery of optical outflows and inflows in the black hole candidate GRS 1716-249
• NuSTAR + XMM-Newton monitoring of the neutron star transient AX J1745.6-2901
• Radio Variability from a Quiescent Stellar Mass Black Hole Jet
• Rapid Optical Variations Correlated with X-rays in the 2015 Second Outburst of V404 Cygni (GS 2023$+$338)
• Rapid spectral transition of the black hole binary V404 Cyg