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Significant decrease in the average optical emission from V404 Cyg

ATel #7729; A. Martin-Carrillo (University College Dublin, 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 29 Jun 2015; 16:00 UT
Credential Certification: Antonio Martin-Carrillo (antonio.martin-carrillo@ucd.ie)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 7731, 7733, 7734, 7737, 7738, 7740, 7760, 7763, 7959

We continue to monitor the LMXB V404 Cyg, in outburst since June 15th (GCN #17929, ATEL #7646), with the 0.4 m UCD Watcher robotic telescope at Boyden Observatory (South Africa). In the observations reported in ATEL #7718, we reported that the source had been in a bright state since June 20th with an average magnitude of ~11.4 in the SDSS r' filter (see also ATEL #7721).

In observations from June 27 and June 28 we have noticed a significant decrease of the average magnitude from the initial 11.4 mag observed up to the night of June 25 (June 25 22:19 UT - June 26 02:33 UT) down to 14.8 mag observed on June 27 and 28. Due to bad weather, no observations were made on the night of June 26. An updated plot of the evolution of the average nightly magnitude can be found here. The maximum and minimum brightness on each night are also indicated in the plot.

During the observing window between June 27 22:10 UT and June 28 01:36 UT, the source's r'-band magnitude decreased from 12.5 to 14.5 during one 12 minute interval. There was another significant flare on the night of June 28 22:11 UT - June 29 02:19 UT (see light curve here), which lasted about 11 min, with a Δmag of ~1.3.

We plan to continue monitoring V404 Cyg, but due to the declining brightness of the source, we will increase exposure times to 60 sec from June 29-30.