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Continued optical/UV observations of V404 Cyg with Swift/UVOT

ATel #7734; S. R. Oates (IAA-CSIC, UCL-MSSL), S. Motta (University of Oxford), E. Kuulkers (ESAC/ESA), N. P.M. Kuin (UCL/MSSL), D. Altamirano, P. Gandhi, C. Knigge (University of Southampton) and J. A. Kennea (PSU)
on 30 Jun 2015; 12:29 UT
Credential Certification: Erik Kuulkers (Erik.Kuulkers@sciops.esa.int)

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

Referred to by ATel #: 7740, 7959

The black hole binary V404 Cyg (GS 2023+33) is currently undergoing a bright outburst and it is showing marked variability at many different wavelengths (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, #7708, #7709, #7710, #7714, #7715, #7716, #7717,#7727,#7729).

First observations of V404 Cyg by Swift/UVOT began immediately after Swift/BAT was triggered by this source on the 15th June 2015 (at 18:31:38 UT, trigger 643949, GCN #17929). The first few days of Swift/UVOT observations were reported in GCN #17933 and ATel #7666.

The optical/UV evolution has been monitored by Swift/UVOT since then. Most observations have been taken using the u filter, which therefore yields the best sampled light curve. Observations in the other filters (v, b, uvw1, uvm2, uvw2, white) have been performed more intermittently. V404 Cyg is detected in all 7 UVOT filters, being brighter in the redder filters. The u band light curve shows complex, highly variable behaviour. During the first 5 days, the average u-band brightness increased, but rapid increases and decreases in flux are clearly visible in the light curve with changes of 1 to 4 magnitudes. From the 20th June, the u band magnitude plateaus at ~13.5 mag with fluctuations of the order of ~0.5 mag. On 24th June (8.44 days after the first BAT trigger), a sudden sharp decrease in flux by ~3 magnitudes is observed between two exposures taken 1.5 hrs apart, followed by a return to the plateau level, which occurred within a day. Afterwards, the plateau phase appears to end, and a decline by 1.3 mag was observed for a further 2 days. Between 26th and 28th June (between 11.23 days and 12.9 days after the initial trigger), the u band flux dropped ~5 magnitudes to reach the lowest observed level since observations began (on the 15th June) as seen also in the optical and X-rays (ATels #7729, #7731). The most recent observations show a brief increase in flux (see also ATel #7730). The most recent UVOT light curve can be found at ftp://ftp.sciops.esa.int/pub/ekuulker/V404_30June_2015.ps. Key: red = v, green = b, blue = u, light blue = uvw1, purple = uvm2, yellow = uvw2.

The light curves of the other filters, while less well sampled, appear to show similar behaviour.

UVOT observations were downloaded from the quick look archive. Magnitudes were obtained with the standard UVOT software and the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373).

Swift will continue monitoring this source in the upcoming days. We thank the Swift PI, Neil Gehrels, for approving the observations and the Swift schedulers for the prompt insertion into the observing schedule.