Study of Accretion Flow Dynamics of V404 Cygni during Its 2015 Outburst

Abstract

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after $26$ years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using {\it Swift}/XRT and {\it Swift}/BAT data in the energy range of $0.5-150$ keV. We have done spectral analysis with the two-component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field $B$ for the outburst and found that the highest $B \sim 900$~Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the mismatch of the cooling timescale and infall timescale of the post-shock matter.