Time-resolved spectroscopy on the heartbeat state of GRS 1915+105 using AstroSat

TL;DR

This study uses AstroSat to analyze the spectral variations of GRS 1915+105 during its heartbeat state, revealing coordinated changes in accretion rate, inner disc radius, and coronal efficiency, with implications for accretion physics.

Contribution

It provides the first detailed spectral analysis of GRS 1915+105's heartbeat state using AstroSat data, highlighting the relationship between accretion parameters and spectral states.

Findings

Inner disc radius varies from 1.235 to 5 gravitational radii.

Coronal efficiency decreases with increasing accretion rate.

Spectral parameters are similar across different states except for the inner radius.

Abstract

AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 secs) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ~ 24 secs. The oscillation can be described as coordinated variations of the accretion rate, Comptonised flux, and the inner disc radius, with the latter ranging from 1.235-5 gravitational radii. Comparison with results from the $\chi$ and Intermediate states shows that while the accretion rate and the high energy photon index were similar, the inner disc radius and the fraction of Comptonised photons were larger for these states than for the heartbeat one. The coronal efficiency $\eta \equiv L_{ac}/\dot M c^2$, where $L_{ac}$ is the radiative luminosity generated in the corona is found to be approximately $\propto \dot M^{-2/3}$ for all the observations. The efficiency decreases with inner radii for the heartbeat state but has similar values for the $\chi$ and Intermediate states where the inner radii is larger. The implications of these results are discussed.