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Rapid Optical Photometry of V404 Cyg

ATel #7677; R. I. Hynes (Louisiana State University), E. L. Robinson (University of Texas at Austin), J. Morales (University of Texas at El Paso)
on 20 Jun 2015; 18:08 UT
Credential Certification: Robert Hynes (rih@phys.lsu.edu)

Subjects: Optical, Black Hole, Transient

Referred to by ATel #: 7681, 7686, 7688, 7702, 7708, 7710, 7714, 7718, 7721, 7722, 7725, 7734, 7740, 7959

We observed the outbursting black hole X-ray transient V404 Cyg (GCN #17929, #17931, #17933, #17938, #17940, #17944, #17945, #17946, #17948; ATel #7646, #7647, #7650, #7655, #7658, #7659, #7661, #7662, #7663, #7665, #7666, #7667, #7669, #7671, #7674) for about four hours on 2015 June 18.26-18.44 UT using the Argos photometer on the 2.1m Otto Struve Telescope at McDonald Observatory. We used a Sloan r filter and 2 second exposures with negligible deadtime. V404 Cyg was the brightest target in the field so we did not perform differential photometry but did monitor the next brightest star in the field, IPHAS2 J202403.00+335129.3 (r=15.98; Drew et al. 2005, MNRAS, 362, 753; Barentsen et al. 2014, MNRAS, 444, 3230), to check for transparency variations and establish a photometric zero-point.

As previously reported (ATEL #7650, #7659, #7662, #7666, #7669) the optical counterpart is extremely variable. We find large amplitude variations on all timescales, with the source ranging from r magnitude 12.1 to 15.2 during our observation. A plot of our lightcurve is available here.

The source spent about two hours between magnitude 13.2 and 15.7, showing both slow flickering variations and higher frequency noise on timescales as short as the limit of our resolution. The high frequency variability gradually increased and then abruptly vanished about two hours into our observation in an apparent state change. The source then brightened to magnitude 12.1 before fading again in an hour-long flare. Throughout the flare the lightcurve was much smoother than seen earlier with no variability measurable on timescales less than about 20-30 seconds. Towards the end of the flare the high frequency noise gradually returned and the source then stayed between magnitudes 13.0 and 14.7 for the remainder of the observation.

We find no periodicities or quasi-periodic oscillations in the lightcurve. The power spectrum is dominated by steep red noise.

This paper makes use of data obtained as part of the INT Photometric H-alpha Survey of the Northern Galactic Plane (IPHAS, www.iphas.org) carried out at the Isaac Newton Telescope (INT). The INT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. All IPHAS data are processed by the Cambridge Astronomical Survey Unit, at the Institute of Astronomy in Cambridge. The bandmerged DR2 catalogue was assembled at the Centre for Astrophysics Research, University of Hertfordshire, supported by STFC grant ST/J001333/1.