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

Abstract

ABSTRACT AstroSat spectra of the black hole system GRS 1915+105 during the heartbeat state (with a varying oscillation period from 150 to 100 s) were analysed using a truncated relativistic disc model along with a Comptonization component. Spectra were fitted for segments of length ∼24 s. The oscillation can be described as coordinated variations of the accretion rate, Comptonized flux, and the inner disc radius, with the latter ranging from 1.235 to 5 gravitational radii. Comparison with results from the χ 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 Comptonized photons were larger for these states than for the heartbeat one. The coronal efficiency $\eta \equiv L_{ac}/\dot{M} c^2$, where Lac 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 χ and intermediate states where the inner radii are larger. The implications of these results are discussed.