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X-ray and optical/UV variability from V404 Cyg (GS 2023+338) observed by Swift (part 2 of 2)

ATel #7666; S. Motta (University of Oxford), A. Beardmore (University of Leicester), S. Oates ( IAA-CSIC/UCL-MSSL), N. P.M. Kuin (UCL/MSSL) A. Sanna (University of Cagliari), E. Kuulkers, J. Kajava, C. Sanchez-Fernanedz (ESAC)
on 19 Jun 2015; 08:54 UT
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

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

Referred to by ATel #: 7669, 7671, 7674, 7677, 7681, 7686, 7694, 7702, 7708, 7714, 7718, 7721, 7722, 7734, 7740, 7959

We also report the results from the Swift/UVOT. First observations of V404 Cyg by Swift/UVOT began promptly after Swift/BAT was triggered by this source on the 15th June 2015 (trigger = 643949, GCN Circ. 17929). Initial magnitudes are reported in (GCN Circ 17933). UVOT also observed this source during three subsequent BAT triggers on the 18th June (Observation 2,3,4 above). The source is detected in all 7 UVOT filters (white,v,b,u,uvw1,uvm2,uvw2), although the emission is weaker in the UV compared to the optical and white filters. The light curves in all filters show complex, highly variable behaviour, with rapid increases or decreases in flux over short time scales. During the first trigger on the 18th June (trigger 644520, GCN 17944, Observation 2 above) all filters show a rapid decrease in flux which is about 2 orders of magnitudes lower than those observed during the trigger on the 15th June (trigger 643949, GCN 17929). The light curve is available at: ftp://ftp.sciops.esa.int/pub/ekuulker/V404_cyg_lc_full.ps (Key: black = white, red = v, green = b, blue = u, light blue = uvw1, purple = uvm2, yellow = uvw2)

During the third trigger on the 18th June (Observation 4 above), UVOT observations of V404 Cyg recommence at magnitudes brighter than the trigger of 15th June and rapidly increase, reaching an order of magnitude brighter than those measured on the 15th. A detailed lightcurve for Observation 4 is available at: ftp://ftp.sciops.esa.int/pub/ekuulker/V404_cyg_lc_Obs4.ps (key: as above). Overall this source is observed to change in magnitude by almost 4 orders of magnitude during Swift/UVOT observations.

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.