Optical follow-up of V404 Cyg during the current enhanced activity.

ATel #7650; K. Gazeas (UOA: University of Athens), G. Vasilopoulos (MPE), M. Petropoulou (Purdue University), K. Sapountzis (UOA)
on 17 Jun 2015; 15:12 UT
Credential Certification: Georgios Vasilopoulos (gevas@mpe.mpg.de)

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V404 Cyg is a known black hole Low mass X-ray binary with a late G-type companion, having a ~6.5 d orbital period. On June 15 18:32 UT Swift Burst Alert Telescope (BAT) was triggered due to the high X-ray activity of the system (Barthelmy et al. GCN #17929) The X-ray activity of the system was later confirmed by MAXI/CSC (ATel #7646). X-ray spectral analysis of Swift/XRT data taken within the last 2 days yields a strongly variable absorption (ATel #7647).

We report on the optical follow-up of the system at its current bright state, presenting the R-band light-curve (LC) of the system for the first two nights. On June 15 18:58 UT, the 0.40 m f/8 robotic telescope at the University of Athens (Greece) performed follow-up photometric observations in optical R-band. Data collection has started on (UT) 2015-06-15.29 and finished on (UT) 2015-06-15.35 (mid-exposure time). A sum of 30 exposures of 180 sec each was collected. On the second night, data collection has started on (UT) 2015-06-16.44 and finished on (UT) 2015-06-16.59. A sum of 72 exposures of 180 sec each was collected.
Aperture photometry was performed with a 3 pixel (approximately 4 arcsec) radius aperture. Differential photometry and magnitude calibration was performed utilizing the nearby stars, namely USNO 1200-15039207 (Rmag=12.9) and USNO 1200-15046396 (Rmag=12.7), located 140 arcsec west and 96 arcsec southeast of the source, respectively. No further absolute photometric calibration has been applied on these data.

The LC in the first hour of the first day revealed a fading trend of ~2.7 mag/hour, which was then followed by a more shallow decay (~0.2 mag/hour). The LC during the second day revealed a periodic behaviour. We performed timing analysis to the second day data by using the Lomb-Scargle algorithm. The mean magnitude during the second night was ~14.1 with a decline trend of ~0.1 mag/hour. A ~1.68 hour periodic modulation was detected. The amplitude of the sinusoidal fit to the LC was 0.47 mag.

We will continue monitoring the system during the next days. Additional observations are required to confirm this periodic behaviour.

The extracted photometric light curve can be found:
1st and 2nd day LC
2nd day LC