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A Faint X-ray Dust Scattering Echo from V404 Cyg in Response to Recent Flares

ATel #8507; Sebastian Heinz (UW-Madison, USA), Andrew Beardmore (U Leicester, UK), Peter Jonker (SRON, NL), Erik Kuulkers (ESA/ESAAC, ES), Kim L. Page (U Leicester, UK), Sara Motta (U Oxford, UK), Lia Corrales (MIT, USA), Niel Brandt (PSU, USA)
on 5 Jan 2016; 15:41 UT
Credential Certification: Sebastian Heinz (heinzs@astro.wisc.edu)

Subjects: X-ray, Gamma Ray, Black Hole, Transient

In June of 2015, V404 Cyg displayed a spectacular light echo in the form of several bright rings in response to a series of bright flares, detected by Swift and first reported by Beardmore et al. in ATel #7736. After recent renewed X-ray and gamma-ray flaring activity (ATel #8455 #8457, #8458, #8462, #8475, #8489, #8494, #8500), Swift has been monitoring the source for potential echoes.

Here, we report the detection of a faint dust scattering echo around V404 Cyg in response to the X-ray flaring activity on UT 2015 Dec 31 reported by Swift, MAXI, and INTEGRAL in ATel #8489, #8494, #8500. Swift observed the source on five occasions in the days following the flare on Dec 31. Imaging observations presented here took place on UT 2016 Jan 01 17:33 (00031403134), UT 2016 Jan 02 04:28 (0003140318), UT 2016 Jan 02 12:41 (00031403135), UT 2016 Jan 03 00:03 (0003140319), and UT 2016 Jan 03 13:57 (00031403140).

The observations show extended emission on arcminute scales with decreasing surface brightness consistent with the echo from a flare on 2015 Dec 31. Radial profiles show a dust scattering halo that is increasing in size, with evidence for structure that could be interpreted as rings, shown on the left panel of the plot in the link below. The angular scale of the surface brightness peaks increases with time, allowing a rough estimate of the time of the flare to be placed around 2015 Dec 31.75, consistent with the time of the flares observed by MAXI and INTEGRAL. Re-projecting the profiles onto a uniform angular scale corrected for the increase in size (assuming the time of the flare to be 2015 Dec. 31T18:30) shows several peaks that line up relatively well (shown in the right panel of the plot), as expected for the rings of a light echo.

Because the soft fluence of the flares in December was much lower than in June, the echo is also correspondingly weaker. Further observations and analysis are forthcoming. The spatially and temporally varying echo may complicate background subtraction for spectral analysis of the fading point source.

We would once again like to thank the Swift team for the prompt scheduling of observations during the holiday season.

Jan 2016 Intensity Profiles ov V404 Light Echo