Lord of the Rings – Return of the King:Swift-XRT observations of dust scattering rings around V404 Cygni

Heinz et al. (2016) recently published an independent analysis of\udthe X-ray dust scattering rings seen around V404 Cyg, using both\udChandra and Swift data. They derive a dust distribution along the\udline of sight to the source similar to the one found here.On 2015 June 15, the black hole X-ray binary V404 Cygni went into outburst, exhibiting extreme X-ray variability which culminated in a final flare on June 26. Over the following days, the Swift-X-ray Telescope detected a series of bright rings, comprising five main components that expanded and faded with time, caused by X-rays scattered from the otherwise unobservable dust layers in the interstellar medium in the direction of the source. Simple geometrical modelling of the rings' angular evolution reveals that they have a common temporal origin, coincident with the final, brightest flare seen by INTEGRAL's JEM X-1, which reached a 3-10 keV flux of ~25 Crab. The high quality of the data allows the dust properties and density distribution along the line of sight to the source to be estimated. Using the Rayleigh-Gans approximation for the dust scattering cross-section and a power-law distribution of grain sizes a, ∝a-q, the average dust emission is well modelled by q = 3.90-0.08+0.09 and maximum grain size of a+ = 0.147-0.004+0.024 μm, though significant variations in q are seen between the rings. The recovered dust density distribution shows five peaks associated with the dense sheets responsible for the rings at distances ranging from 1.19 to 2.13 kpc, with thicknesses of ~40-80 pc and a maximum density occurring at the location of the nearest sheet. We find a dust column density of Ndust ≈ (2.0-2.5) × 1011 cm-2, consistent with the optical extinction to the source. Comparison of the inner rings' azimuthal X-ray evolution with archival Wide-field Infrared Survey Explorer mid-IR data suggests that the second most distant ring follows the general IR emission trend, which increases in brightness towards the Galactic north side of the source.We thank the Swift and INTEGRAL teams for graciously granting,\udthen performing the observations, as well as the numerous ToO\udrequesters who asked for observing time. APB, JPO and KLP acknowledge\udsupport from the UK Space Agency. GRS acknowledges\udsupport from an NSERC Discovery Grant. APB thanks the authors\udof the excellent PYTHON, NUMPY, SCIPY, MATPLOTLIB, NLOPT and ASTROPY\udsoftware, which were used during this work. We thank the\udreferee whose comments helped improve the manuscript.\udThis research has made use of the NASA/IPAC Infrared Science\udArchive, which is operated by the Jet Propulsion Laboratory, California\udInstitute of Technology, under contract with the National\udAeronautics and Space Administration.Peer-reviewedPublisher Versio