Optical monitoring campaign of V404 Cyg with the Watcher Robotic Telescope
ATel #7718; A. Martin-Carrillo (UCD), D. Murphy (UCD), L. Hanlon (UCD), M. Topinka (CTU), H. J. van Heerden (UFS), B. van Soelen (UFS) and P. J. Meintjes (UFS)
on 26 Jun 2015; 20:25 UT
Credential Certification: Antonio Martin-Carrillo (firstname.lastname@example.org)
Subjects: Optical, Binary, Black Hole, Transient
V404 Cyg is a low-mass X-ray binary currently in outburst (GCN #17929, ATEL #7646) showing strong variable emission at all wavelengths from radio to gamma-rays (e.g. ATEL #7666, #7693, #7695, #7702, #7708, #7714, and #7716).
We report on the optical monitoring of this source with the 0.4m Watcher robotic telescope at Boyden Observatory (South Africa). Observations started on June 15 23:15 UT and currently span 11 days up to June 26 02:34 UT, resulting in 9537 observations in the SDSS r' band with exposures ranging between 0.5 sec and 120 sec (1.6 sec readout time). Observations of this source were not carried out on the night of June 16th due to a GRB override. Since June 20 22:42 UT the source has also been monitored in the SDSS g' and i' bands. The photometry has been carried out using 4 nearby reference stars from the APASS catalogue. All magnitudes quoted here refer to the r' filter.
As reported by other optical observatories (ATEL #7650, #7659, #7677, #7681 #7686, #7688, #7710, and #7717) the source is highly variable on all timescales. In a night-to-night comparison, the source was at ~14th mag on June 15 23:15 UT, June 18 00:09 UT and June 19 02:32 UT. The source brightened over the two following nights (June 19 22:43 UT and June 20 22:42 UT), reaching magnitude ~11. The source has remained in this bright state since then (see light curve here). The maximum and minimum measurement for each night is shown by bars. The length of each observation is represented by the width of the bars. All results reported here should be regarded as preliminary.
Below we provide more details on the observed nightly behaviour. A light curve of the observation from June 25 22:19 UT - June 26 02:34 UT can be found here.
> June 15 23:15 UT - June 16 01:36 UT
The source was seen to go from mag ~15.5 to ~13th mag in 1 hour, decaying back to ~15.5 mag in 1.5 hours.
> June 18 00:09 UT - June 18 00:48 UT
During the 40 mins of observation, the source showed some short timescale flaring activity, with magnitudes ranging between 13 and 14.
> June 19 02:32 UT - June 19 03:05 UT
The source is seen to steadily increase in magnitude from ~14.5 to ~14 over the course of 30 minutes.
> June 19 22:43 UT - June 20 02:58 UT
Variability on timescales of several minutes with complex structure.
> June 20 22:42 UT - June 21 02:53 UT
Smoother light curve with some short time variability but clearly dominated by an envelope of variability of the order of hours. The source reached 11th mag.
> June 21 22:36 UT - June 22 00:38 UT
Major prominent feature increasing in brightness by ~0.5 mag in ~20 min, peaking at ~10.6 mag.
> June 22 22:23 UT - June 23 02:44 UT
Variability on timescales from 5-30 min, with sudden changes in brightness of up to 1 mag in ~30min. The source faded from an initial ~11th mag down to ~13.5 by the end of the observation.
> June 23 22:29 UT - June 24 02:41 UT
Smooth emission for the most part, with a change of brightness in 30 min from ~12 to ~11.
> June 24 22:24 UT - June 25 02:38 UT
Again, a rather featureless light curve (compared to previous nights). The source remained at a magnitude of ~12.5 throughout the observation.