Probing the shot behaviour in Cygnus X-1 using simultaneous AstroSat-NICER observation

TL;DR

This study analyzes the aperiodic flaring features, or shots, in Cygnus X-1 across a broad energy range using simultaneous AstroSat and NICER observations, revealing new insights into the origin and behavior of these shots and the accretion disk dynamics.

Contribution

First simultaneous observation of shots in soft and hard X-rays in Cygnus X-1, providing new spectral and timing insights into accretion disk behavior.

Findings

Detected 49 simultaneous shots in soft and hard X-ray bands.

First shot profile observed in soft X-rays showing a spectral peak at ~2 keV.

Inner accretion disk truncated at 6.7±0.2 gravitational radii, with inner edge moving during shots.

Abstract

We analyse the aperiodic flaring features, also known as shots, observed in Cyg X-1 in the 0.1-80 keV energy band using a 6.39 ks simultaneous observation with AstroSat and NICER. We detect 49 simultaneous shots in the soft and hard X-ray bands with NICER and AstroSat-LAXPC, respectively. We observe the shot profile for the first time in soft X-rays (0.1-3 keV), which shows a spectral peak at $\sim$2 keV. Using time-averaged spectroscopy, we measured the truncation of the inner accretion disk at $6.7\pm0.2$ gravitational radii. The shot-phase resolved spectroscopy allowed us to identify the origin of some of the brightest aperiodic peaks in the soft X-rays. We find that the accretion rate is consistent with a constant during the shots while the inner edge of the accretion disk moves inwards/outwards as these shots rise/decay. We discuss the possible mechanisms causing the swing in the inner radius.