Spectral Analysis of $χ$ Class Data of GRS 1915+105 Using TCAF Solution

Abstract: The class variable source GRS 1915+105 exhibits a wide range of time variabilities in time scales 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, which were later sequenced in ascending order of Comptonization Efficiency (CE). The CE is the ratio of the phenomenological power-law and disk blackbody model fitted spectral photon fluxes. For a better understanding of the flow dynamics in one of the variability classes, namely $\chi$, we fitted spectra with the physical two-component advective flow (TCAF) model. In $\chi$ class, X-ray flux is steady with no significant variation, however, various $\chi$ subclasses are observed at different X-ray flux and variations of count rates across different $\chi$ subclasses must be linked to the variation of flow parameters such as the accretion rates, be it the Keplerian disc rate and/or the low angular momentum halo rate. We find that in the $\chi_{2,4}$ classes, which are reportedly devoid of significant outflows, the spectra could be fitted well using TCAF solution alone. In the $\chi_{1,3}$ class, 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 the information on the relative dominance of the outflow in the latter two classes. TCAF fit also provides 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 $\chi$ classes of GRS 1915+105 has been obtained.