SWIFT view of the 2015 outburst of GS 2023+338 (V404 Cyg): complex evolution of spectral and temporal characteristics

Abstract

We study the spectral and temporal characteristics of the source GS 2023$+$338 (V404 Cyg) during the initial phase of its June 2015 outburst, over the energy range of 0.5 - 150 keV. This is the first detailed study of the characteristics of this source based on {\it SWIFT} observations, being reported. Based on our analysis, we understand that the source existed in the hard, intermediate and soft spectral states. We find that the evolution of the spectral parameters, the hardness intensity diagram and the rms-intensity diagram are not similar to those observed for most of the outbursting black hole sources. We also observe presence of weak peaked components in the power density spectra during the intermediate state of the source. Dramatic changes in the spectral and temporal properties are also exhibited before the ejection of a radio jet suggesting it to be associated with the coronal mass ejection. It seems that may be due to evacuation of the inner part of the Keplerian disc for a short duration, the disc component is not observed after the huge radio flare. The absorption features observed in the low energy spectra suggest the presence of wind emission and the evolution of the characteristics of the variable Fe line emission during both hard and intermediate states, indicate its origin to be probably related to the wind/outflow.