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Optical (V-band) observations of V404 Cygni with the 0.3m telescope at Wheaton College Observatory

ATel #7721; John Scarpaci, Dipankar Maitra (Wheaton College, MA)
on 26 Jun 2015; 22:16 UT
Credential Certification: Dipankar Maitra (maitra@astro.yale.edu)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 7722, 7729, 7737, 7740, 7959

We report V-band observations of the transient X-ray binary system V404 Cygni using a 12" Meade LX600 telescope equipped with an SBIG STT-8300M CCD in Wheaton College Observatory (lat = 41.965631 N, lon = 71.183542 W) during the night of 2015 June 24-25 (local time). The source is currently in outburst (ATels #7646, #7647, #7650, #7655, #7658, #7659, #7661, #7662, #7663, #7665, #7666, #7667, #7669, #7671, #7674, #7677, #7678, #7681, #7686, #7688, #7693, #7694, #7695, #7696, #7699, #7701, #7702, #7706, #7708, #7709, #7710, #7714, #7716, #7717, #7718, #7720, and GCNs #17929, #17931, #17929, #17931, #17933, #17938, #17940, #17944, #17945, #17946, #17948, #17949, #17956), and observers at all wavelengths have reported strong variability and flaring activity.

Our observations spanned from 2015 June 25 03:33-07:55 UT (MJD 57198.15 - 57198.33). During this time we employed an automated observing sequence consisting of repeated blocks of 30s science exposures followed by a 5s gap for CCD readout. This was continued throughout the night with the exception of a few 1-2 minute interruptions to clear condensation on the telescope optics, resulting in 429 usable images. We used 3 field stars with AAVSO Unique Identifiers (AUID) 000-BCL-467, 000-BCL-468, and 000-BCL-455 whose V-band magnitudes are 12.815 ± 0.014, 13.164 ± 0.012, and 13.361 ± 0.015 (from AAVSO database), respectively, to obtain the differential magnitude of V404 Cyg. Our light curve can be seen here.

Preliminary results indicate that the average V-band brightness during our observations was 12.1 mag, with no clear increasing or decreasing trend in the baseline optical flux. This is consistent with results presented by Martin-Carrillo et al. (ATEL #7718) who noted that the flux has remained constant since June 20. As noted in previous optical reports of this outburst, we also see strong variability and flaring in few-minute timescales. While flux changes were relatively small (~0.3mag) during the first hour of our observations, the source then entered a state of rapid variability with large amplitudes. Especially, we note that during one such fluctuations, starting at around MJD 57198.310, the flux dropped from ~11.5 mag (close to the highest flux we observed during our observing run) to ~13.5 mag (the lowest flux we observed during our run), i.e. by a factor of about 6.3x in ~15 minutes and then recovering to the baseline level in another ~15 minutes.

V-band light curve of V404 Cyg obtained during 2015 June 25-26 using WCO 0.3m