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
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.