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Swift XRT observations indicates the end of the V404 Cyg 2015 outburst in early August

ATel #7959; G. R. Sivakoff, A. Bahramian (Alberta), D. Altamirano (Southampton), A. P. Beardmore (U. Leicester), E. Kuulkers (ESA/ESAC), S. Motta (Oxford) on behalf of a larger collaboration
on 27 Aug 2015; 14:27 UT
Credential Certification: Gregory R Sivakoff (sivakoff@ualberta.ca)

Subjects: Radio, X-ray, Black Hole, Transient

Referred to by ATel #: 8455, 8462, 8475, 8499, 8510, 8512, 8515

On 2015 June 15 18:32 UT (MJD~57188.772), the black hole low mass X-ray binary V404 Cyg (GS 2023+338) was first found by Swift (GCN #17929) to start a new outburst. After highly variable multiwavelength activity, the peak flare occurred on June 26 and the source began to fade towards its quiescent accretion levels (although there are indications of a hard X-ray flare on June 27 that was less significant than the June 26 flare by a factor of ten). Swift X-ray Telescope (XRT) observations indicate that the source reached quiescence (in X-rays) between 2015 August 5 and August 21. In the ~50 days between the detection of the outburst and the source reaching quiescence, the astronomical community undertook one of the most extensive sets of multiwavelength campaigns on an X-ray binary outburst to date (ATel #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, #7715, #7716, #7717, #7718, #7720, #7721, #7722, #7725, #7726, #7727, #7728, #7729, #7730, #7731, #7733, #7734, #7735, #7736, #7737, #7738, #7740, #7742, #7745, #7751, #7752, #7755, #7758, #7760, #7761, #7763, #7772, #7773, #7788, #7797, #7842; GCN #17929, #17931, #17933, #17938, #17940, #17944, #17945, #17946, #17948, #17949, #17956, #17963, #17980, #17988).

In the table below we list the count rates of the last eight Swift XRT Photon Counting (PC) observations.

 
Date        MJD       Count-Rate 
2015-08-01  57235.05  0.044+/-0.005 
2015-08-01  57235.25  0.026+/-0.002 
2015-08-05  57239.17  0.068+/-0.004 
2015-08-09  57243.56  0.026+/-0.005 
2015-08-13  57247.08  0.006+/-0.003 
2015-08-17  57251.01  0.037+/-0.007 
2015-08-21  57255.13  0.010+/-0.004 
2015-08-25  57259.46  0.033+/-0.010 

Past Swift XRT PC observations of V404 Cyg in quiescence (Bernardini & Cackett 2014, MNRAS, 439, 2771) indicate an average 0.3 - 10 keV count rate in quiescence of 0.014 with an observation-to-observation standard deviation of 0.008 cnt/s. This large standard deviation is due to the relatively large intrinsic variability of the source in quiescence. From the Bernardini & Cackett (2014) data, we estimate there would only be a 1% chance of the source being in quiescence and producing two out of four consecutive epochs with a Swift XRT rate above 0.033 cnt/s (at 0.3 - 10 keV). Based on the data above, we estimate that V404 Cyg reached quiescence (in X-rays) between August 5 and August 21. The X-ray data after August 5 has a combined count rate of 0.0197+/-0.002 cnt/s. Since the average spectrum in quiescence by Bernardini & Cackett (2014) is well modeled with an absorbed power-law with a photon index of 2.3 and absorption column density of 8.9e21 cm^-2, the ~0.020 cnt/s rate we measure after August 5 corresponds to an X-ray luminosity (0.3 - 10 keV) of about 1.7e33 erg/s, assuming a distance of 2.39 kpc (Miller-Jones et al. 2009, ApJ, 706, L230).

We would like to thank not only the Swift team for their rapid response that acquired 90 Swift observations during this outburst requested by 12 different proposers, in addition to 6 automatic Swift observations, but also the entire set of observatories and astronomers that responded rapidly to this incredible transient opportunity.