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Swift/XRT discovery of a dust scattering halo in V404 Cyg.

ATel #7736; A. P. Beardmore (U. Leicester), D. Altamirano (U. Southampton), E. Kuulkers (ESA/ESAC), S. E. Motta (U. Oxford), J. P. Osborne (U. Leicester), K. L. Page (U. Leicester), G. R. Sivakoff (U. Alberta) & S. A. Vaughan (U. Leicester)
on 30 Jun 2015; 16:07 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: Black Hole, Transient

Referred to by ATel #: 7755, 7763, 7959, 8462, 8507

Swift/XRT observations of the black hole X-ray binary V404 Cyg (GS 2023+33), which started on 2015-06-15, have continued approximately 2-3 times daily.

As previously reported (e.g. ATel #7647, #7665, #7694, #7727), the X-ray emission from the source is highly variable, with 1D Windowed Timing (WT) mode 0.3-10 keV count rates varying from 10 to >10,000 c/s (pile-up corrected). Subsequent to the last reported observations, another intense flaring episode was seen in the Swift/XRT observation starting 2015-06-26 12:35 UT, in which the WT count rate peaked above 20,000 c/s.

The following XRT observation, taken on 2015-06-26 23:46 UT, revealed a 1D detector profile which had significantly extended wings, approximately 1.6 arcminute in radius. Subsequent WT observations show this extended emission dominates over the central source point spread function, while increasing in size and fading with time. By 2015-06-28 16:05 UT, the emission had extended in size to fill the WT mode 7.8-arcminute wide window. Re-analysis of an earlier WT observation taken on 2015-06-21 also reveals a broader than expected profile, suggesting a similar event.

Given these findings, a 1 ks Photon Counting (PC) mode observation was taken at 2015-06-30 10:52 UT. The image reveals 4 concentric rings at angular separations of 2.15, 3.86, 4.26 and 5.59 arcminute from the central source, caused by dust scattering along the line of sight to the source. The rings and diffuse structure could be associated with differing flaring episodes from the central binary system, though multiple dust clouds can also influence the ring structure.

Analysis is ongoing. Further Swift/XRT observations are planned in PC mode to study evolution of the rings.

We thank the Swift team for continued observations of this fascinating source.

A 3 colour image from the PC observation.