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Further Rapid Optical Photometry of V404 Cyg

ATel #7710; R. I. Hynes (Louisiana State University), E. L. Robinson (University of Texas at Austin), J. Morales (University of Texas at El Paso)
on 25 Jun 2015; 02:59 UT
Credential Certification: Robert Hynes (rih@phys.lsu.edu)

Subjects: Optical, Black Hole, Transient

Referred to by ATel #: 7714, 7718, 7721, 7722, 7725, 7734, 7740, 7959

We obtained a second time-resolved observation of the outbursting black hole X-ray transient V404 Cyg (ATEL #7677 and references therein) for about two hours on 2015 June 19.35-19.44 UT using the Argos photometer on the 2.1m Otto Struve Telescope at McDonald Observatory. Our observation was performed as described in ATEL #7677 except that we used 1 second exposures.

The optical counterpart continued to be extremely variable, but consistent with the rising trend reported by ATEL #7696 and ATEL #7708 was brighter, ranging from r magnitude 11.0 to 13.0 during our observation. A plot of our lightcurve is available here.

Examining our lightcurves from the two nights we highlight three remarkable behaviors.

i) Rapid variability, on timescales faster than 20 seconds and with large amplitudes, can quite abruptly appear and disappear. On June 18 we observed this variability present most of the time, but then during the largest flare it disappeared. On June 19 we saw a mostly very smooth lightcurve (as also seen by ATEL #7686), but with a short period (about 1000 secs) when intense rapid variability was present. During this episode we observed up to 0.3-0.4 mag flares within single 1 second exposures, and so these are unresolved by our 1 second cadence. These variations occur on timescales more than an order of magnitude shorter than the light travel time across the accretion disk, and so are unlikely to be caused by the disk reprocessing X-ray emission. They may instead be associated with optical jet emission as suggested in other black hole binaries showing very rapid variability (see discussion and references in ATEL #7686)

ii) Twice on June 19, at the two brightest points in the lightcurve around r = 11.4, there were very short episodes of flaring lasting for less than a minute, which again had peak amplitudes around 0.3 mag. We have examined these images carefully and know of no artifacts that could explain them, so believe they are intrinsic to the source. It is unclear if these represent the same behavior as described in i) or a distinct very transient phenomenon associated with reaching a certain luminosity threshold.

iii) On two occasions (June 18, JD 2,457,191.87, June 19, JD 2,457,192.87), we see extended, smooth decays. A third event may happen at the end of the June 18 lightcurve, but interrupted by flares. The cleanest example on June 19 can be fitted well by an exponentially decaying component with an e-folding time of about 70 seconds.