V404 Cyg was in optical quiescence from mid October to mid December 2015

ATel #8515; F. Bernardini (NYU Abu Dhabi), D. M. Russell (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project and Astrophysics Research Institute, LJMU)
on 8 Jan 2016; 14:10 UT
Credential Certification: Federico Bernardini (bernardini@nyu.edu)

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The black hole transient low mass X-ray binary V404 Cyg after approximately 26 years spent in quiescence entered a new X-ray outburst state on 2015 June 15 (GCN #17929). The source returned to X-ray quiescence between 2015 August 5 and August 21 (Atel #7959). More recently, it become again X-ray active on 2015 December 23 (GCN #18716).

We have been continuing our optical monitoring of V404 Cyg with the Las Cumbres Observatory Global Telescope Network (LCOGT). We observed V404 Cyg with the 2-m Faulkes Telescope North (Haleakala on Maui, Hawaii), and also with the 1-m telescope (McDonald Observatory, Texas, USA), in V, R and i' optical bands (see ATel #7761 for more details on the photometry). We here report on the optical light curves collected since the source entered X-ray quiescence in August 2015.

The inspection of the optical light curves folded on the orbital period (6.4714 days) using the ephemeris of Casares & Charles (1994) reveals that V404 Cyg is significantly brighter (about 0.1 mag) than the average 2006-2015 quiescent modulation up to 2015 October 10 (MJD 57305.39), while it is fully consistent with pre-outburst quiescent levels from 2015 October 20 (MJD 57315.32) up to 2015 December 13 (MJD 57369.23), our last pointing before the new X-ray detection. During the quiescent period we observed the source multiple times (200 s exposure) and its average quiescent magnitude combined with that of a nearby contaminating star (see ATel #7761 for more details), was V=18.06+-0.11 (10 pointings), R=16.26+-0.09 (9 pointings), and i'=15.69+-0.07 (9 pointings). Our first detection of the new outburst was on 2015 December 25 (MJD 57381.05) when the magnitudes were V=17.14+-0.12, R=15.53+-0.03, i'=14.88+-0.02 and on 2015 December 30 (MJD 57386.05) when R=15.28+-0.01, i'=14.41+-0.01. The outburst magnitudes are consistent with the mostly modest level of elevated optical activity reported during this time (ATel #8501; see also GCN #18717, GCN #18718, GCN #18721, GCN #18767, GCN #18783, and ATel #8453, ATel #8455). Therefore, we conclude that V404 Cyg entered quiescence at optical wavelengths between 2015 October 10 and October 15, and brightened again between 2015 December 13 and December 23. This is also consistent with the reports that V404 Cyg was in quiescence in November, from an X-ray observation on November 27 (ATel #8466) and from optical data acquired on November 17 (ATel #8509).

A faint optical precursor was found about a week before the 2015 June 15 X-ray outburst (ATel #7761). The source spent around two months in full quiescence before the December 2015 X-ray outburst. Consequently, if there was an optical precursor also to the December outburst, it must have occurred less than 13 days before the first X-ray flare was detected (e.g. between 2015 October 10 and October 23, when no optical data are available). Figures showing the full campaign monitoring optical light curves and the phase-resolved V, R, and i'-band light curves of V404 Cyg are available at the link at the bottom of this ATel. We will continue our monitoring in March, when the source becomes visible again.

The Faulkes Telescope observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008). This work makes use of observations from the Las Cumbres Observatory Global Telescope Network (LCOGT).