Events leading up to the June 2015 outburst of V404 Cyg

TL;DR

This study documents the optical precursors and disk behavior leading up to the 2015 X-ray outburst of V404 Cyg, suggesting a viscous-thermal instability triggered near the disk's inner edge.

Contribution

It provides the first detailed optical monitoring of V404 Cyg's outburst precursors and proposes a new model for the outburst trigger involving disk instability and inner disk refilling.

Findings

Optical flux was brighter a week before the X-ray outburst.

Spectral lines indicated an accretion disk in a hot state prior to the outburst.

An X-ray delay of about a week aligns with disk refilling timescales.

Abstract

On 2015 June 15 the burst alert telescope (BAT) on board {\em Swift} detected an X-ray outburst from the black hole transient V404 Cyg. We monitored V404 Cyg for the last 10 years with the 2-m Faulkes Telescope North in three optical bands (V, R, and i$^{'}$). We found that, one week prior to this outburst, the optical flux was 0.1--0.3 mag brighter than the quiescent orbital modulation, implying an optical precursor to the X-ray outburst. There is also a hint of a gradual optical decay (years) followed by a rise lasting two months prior to the outburst. We fortuitously obtained an optical spectrum of V404 Cyg 13 hours before the BAT trigger. This too was brighter ($\sim1\rm\,mag$) than quiescence, and showed spectral lines typical of an accretion disk, with characteristic absorption features of the donor being much weaker. No He II emission was detected, which would have been expected had the X-ray flux been substantially brightening. This, combined with the presence of intense H$\alpha$ emission, about 7 times the quiescent level, suggests that the disk entered the hot, outburst state before the X-ray outburst began. We propose that the outburst is produced by a viscous-thermal instability triggered close to the inner edge of a truncated disk. An X-ray delay of a week is consistent with the time needed to refill the inner region and hence move the inner edge of the disk inwards, allowing matter to reach the central BH, finally turning on the X-ray emission.