Spectral analysis of χ class data of GRS 1915+105 using TCAF solution

Abstract

Abstract The class variable source GRS 1915+105 exhibits a wide range of time variabilities on timescales of a few seconds to a few days. Depending on the count rates in different energy bands and the nature of the conventional color-color diagram, the variabilities were classified into sixteen classes that were later sequenced in ascending order of Comptonization Efficiency (CE), which is the ratio of power-law and blackbody photons. However, CE estimation is based on an empirical model which does not provide us with a comprehensive picture regarding accretion flow dynamics around the central source. In reality, the accretion flow is comprised of two components: the high angular momentumKeplerian flow in the form of a radiatively efficient disk and a low angular momentumradiatively inefficient sub-Keplerian halo enveloping the disk. These two components contribute differently to the overall flux due to the differences in their radiative efficiencies. Therefore, it is necessary to analyze the spectral behaviors and time variabilities in terms of accretion rates. In χ class, X-ray flux is steady with no significant variation, however various χ subclasses are observed at different X-ray fluxes and variations of count rates across different χ subclasses must be linked to the variation of flow parameters such as the accretion rates, be it the Keplerian disk rate and/or the low angular momentum halo rate. This motivated us to analyze the spectra of the χ class data implementing the physical Two Component Advective Flow (TCAF) solution which directly extracts these two rates from spectral fits. We find that in the χ 2,4 classes, which are reportedly devoid of significant outflows, the spectra could be fitted well applying the TCAF solution alone. In the χ 1,3 classes, which are always linked with outflows, a cutoff power-law model is needed in addition to the TCAF solution. At the same time, the normalization required by this model along with the variation of photon index and exponential roll-off factor provides us with information on the relative dominance of the outflow in the latter two classes. TCAF fit also supplies us with the size and location of the Compton cloud along with its optical depth. Thus by fitting with TCAF, a physical understanding of the flow geometry in different χ classes of GRS 1915+105 has been obtained.