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Swift triggers on V404 Cyg

ATel #8455; A. P. Beardmore (U. Leicester), K. L. Page (U. Leicester), E. Kuulkers (ESAC/ESA)
on 23 Dec 2015; 17:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andy Beardmore (apb@star.le.ac.uk)

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

Referred to by ATel #: 8458, 8462, 8466, 8475, 8489, 8494, 8499, 8501, 8507, 8512, 8515, 8520, 8531

On 2015-Dec-23 at 05:19:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source consistent with V404 Cyg (trigger=668097; GCN Circ. 18716). As is typical with BAT image triggers, the real-time light curve did not show anything significant. However, this is the first automatic detection of V404 Cyg by BAT since the on-board trigger threshold level was reset back to the pre-2015 June outburst value for the source on 2015-Aug-03.

Swift slewed to the location of the BAT source at 06:43 UT. The X-ray Telescope (XRT) collected 125 s of Photon Counting (PC) mode data which revealed a single point source consistent with the position of V404 Cyg at an 0.3-10 keV count rate of 0.33 +0.05 -0.04 c/s. This compares with the average quiescent XRT count rate from 2012 of ~0.014 c/s (Bernardini and Cackett 2014 MNRAS 439 2771), though an excursion to 0.1 c/s was seen at this time. Also, at the onset of last outburst on 2015-Jun-15 the XRT recorded a rate of ~0.9 c/s (ATel #7647), which had fallen to ~0.03 c/s by 2015-Aug-25 when the source was judged to have returned to its quiescent flux level (ATel #7959). Hence, V404 Cyg is presently a factor of ~10 times brighter in X-rays than it was at the end of the last outburst.

The spectrum formed from the PC mode data can be fit (using the C-statistic for low counts per bin data) by an absorbed power law with a photon index of 1.6+/-1.3 and column density of (1.4 +1.8 -1.4) x 10^22 cm^-2, for an observed 0.3-10 keV flux of 4 x 10^-11 erg cm^-2 s^-1.

The Swift UV and Optical Telescope (UVOT) obtained white and v-band images of the field which showed the source at magnitudes of 17.79+/-0.06 and 16.23+/-0.25, respectively. The latter is consistent with the MASTER-Tunka observations taken 3.5 hours later (GCN Circ. 18718).

Further Swift observations have been approved in an attempt to determine whether the source is entering another active phase.