Spectral evolution of GX 17+2 using AstroSat and NICER observations

TL;DR

This study analyzes the spectral evolution of GX 17+2 using AstroSat and NICER data, revealing how accretion disk and corona properties change across different spectral states, with implications for neutron star accretion physics.

Contribution

It provides detailed spectral analysis of GX 17+2 across different branches, highlighting geometric changes and luminosity variations with new insights into neutron star accretion behavior.

Findings

In the NB, flux remains constant with changing disk and corona parameters.

In the FB, luminosity varies significantly while accretion efficiency stays constant.

Inner disk radii and corona fraction vary, indicating geometric changes.

Abstract

We study the spectral evolution of the Z-track source GX 17+2 using AstroSat and NICER observations taken between 2016 and 2020. The AstroSat observations cover the period when the source is in the normal branch (NB) and the flaring branch (FB), while for the NICER ones the variability can be associated with the FB branch. The source spectra at different regions of the branches are well described by accretion disk emission, blackbody surface emission and a thermal Comptonization component. In the NB, the total bolometric unabsorbed flux remains constant and the variation is due to changes in the Comptonization, disk fluxes. In particular, the inferred luminosity ($L_{\rm T}$) and accretion rate ($\dot M$) remain constant, while there is significant variation in the inner disk radii and fraction of disk photons entering the corona, indicating changes in the geometry of the system. On the other hand, in the FB, there is significant variation in luminosity from $\sim 4.0$ to $\sim 7.0 \times 10^{38}$ ergs s$^{-1}$. Despite this significant variation in luminosity and in the inner disk radii, the accretion efficiency defined as $\eta = L_{\rm T}/{\dot M} c^2$, remains nearly constant at $\sim 0.20$ throughout the evolution of the source, as expected for a neutron star system.