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Swift/UVOT observations of V404 Cyg during the recent X-ray outburst

ATel #8523; K. L. Li (Michigan State University) and A. K. H. Kong (National Tsing Hua University, Taiwan)
on 9 Jan 2016; 18:34 UT
Credential Certification: K. L. Li (lilirayhk@gmail.com)

Subjects: Ultra-Violet, Binary, Black Hole, Transient

We report the Swift/UVOT photometric data of V404 Cyg taken from 2015-12-23 (MJD 57379) to 2016-01-08 (MJD 57395) for the recent X-ray outburst detected on 2015 December 23 (MJD 57379) by Swift (GCN #18716). During the period, Swift/UVOT has observed V404 Cyg with its four UV filters 28 times, including nineteen u-band, five uw1-band, two um2-band, and two w2-band images. V404 Cyg was only detected in the u and uw1 bands with mean magnitudes (in Vega system) as shown in the following,

u (26956s): 18.26 +/- 0.04 (stat) +/- 0.02 (sys)
uw1 (7174s): 20.75 +/- 0.14 (stat) +/- 0.03 (sys)
um2 (2932s): > 21.55 (3-sigma upper-limit)
uw2 (2932s): > 21.53 (3-sigma upper-limit).

From the u-band lightcurve, a few optical flares are clearly seen from MJD 57384 to 57389. The last and also the strongest flare was detected from MJD 57386 to 57389, with a peak magnitude of u = 15.51 +/- 0.05 mag at MJD 57388.8. We conclude that this UVOT major flare was likely unresolved and in fact built-up by several short-timescale flares, corresponding to the ones detected by INTEGRAL/OMC on revolutions 1627 and 1628 (ATel #8516). After the major flare, the u-band light dropped suddenly by 4 magnitudes in half a day. Since then, no significant flare was detected and V404 Cyg stayed faint with a mean magnitude of u = 20.18 +/- 0.07 mag. We note that, although V404 Cyg is relatively faint and stable after the u-band peak, it is probably still in an active state in optical as the current magnitude is still brighter than the quiescent magnitude in 2002 (i.e., u’ = 22.6 +/- 0.7 mag; Shahbaz et al. 2003).

For the uw1-band images, V404 Cyg was undetected in the first and the last observations, but marginally detected in the three images between them with magnitudes shown as below,

2015-12-23 (MJD 57379.7): > 21.03 (3-sigma upper-limit)
2015-12-27 (MJD 57383.4): 20.78 +/- 0.28 (stat) +/- 0.03 (sys)
2015-12-27 (MJD 57383.6): 20.18 +/- 0.16 (stat) +/- 0.03 (sys)
2016-01-08 (MJD 57395.2): 20.52 +/- 0.30 (stat) +/- 0.03 (sys).
2016-01-08 (MJD 57395.7): > 20.72 (3-sigma upper-limit)

Due to the bad S/N ratios, no significant variability has been seen among the three uw1-band detections. However, the flux raise at the beginning and drop at the end of the observations are statistically significant to show daily timescale variabilities of V404 Cyg. V404 Cyg was detected with a uw1-band magnitude of 22.80 +/- 0.15 on a stacked image taken during quiescence from 2012-07-03 to 2012-09-16. Therefore, V404 Cyg could be still active in the uw1-band.

We acknowledge the use of public data from the Swift data archive.

The UVOT u-band lightcurve of V404 Cyg