The Outburst Observations of Black Hole Binary System V404 Cyg by Swift/XRT in 2015

After a quiescence of 26 years, the black-hole binary system V404 Cyg underwent another outburst in June 2015. During the later phase of this outburst, Swift/XRT (X-ray Telescope) detected for the first time a peculiar structure of a series of concentric rings under the photon counting (PC) mode. In this paper, by using the public Swift/XRT data, we analyzed the energy spectrum and time variation of this ring structure. Our results show that the energy spectrum of the ring structure is highly softened with respect to that of the central source, and the spectral intensity varies as the rings move outward. These results were explained with the model of a cloud which is located between the source and the observer at a distance of roughly 6842 light years (about 2.1 kpc). And some constraints for the relationship between the characteristics of the rings and their burst sources were presented.