Comptonization efficiencies of the variability classes of GRS 1915+105

TL;DR

This study introduces the Comptonizing efficiency (CE) as a metric to sequence and understand the variability classes of GRS 1915+105, linking CE changes to the physical properties of the Compton cloud.

Contribution

The paper proposes a novel CE-based method to predict class transitions and relate them to the optical depth variations in the Compton cloud.

Findings

CE sequence matches observed class transitions

CE correlates with optical depth changes

Harder classes have higher CE

Abstract

The Galactic microquasar GRS 1915+105 exhibits at least seventeen types of variability classes. Intra and inter class transitions are reported to be observed within seconds to hours. Since the observation was not continuous, these classes appeared to be exhibited in a random order. Our goal is to predict a sequence of these classes. In this paper, we compute the ratio of the photon counts obtained from the power-law component and the blackbody component of each class and call this ratio as the `Comptonizing efficiency' (CE) of that class. We sequence the classes in the ascending order of CE and find that this sequence matches with a few class transitions observed by RXTE satellite and IXAE instruments on board IRS-P3. A change in CE corresponds to a change in the optical depth of the Compton cloud. Our result implies that the optical depth of the Compton cloud gradually rises as the variability class becomes harder.