X-ray softening during the 2008 outburst of XTE J1810-189

TL;DR

This study analyzes the 2008 outburst of XTE J1810-189, revealing X-ray spectral softening driven by changes in the non-thermal emission component, and estimates the source distance using a photospheric radius expansion burst.

Contribution

First to determine the distance to XTE J1810-189 using photospheric radius expansion burst analysis and to link spectral softening to non-thermal emission evolution during its outburst.

Findings

Distance estimated between 3.5 and 8.7 kpc.

X-ray spectrum softens as luminosity decreases.

Softening is due to non-thermal component evolution.

Abstract

XTE J1810-189 underwent an outburst in 2008, and was observed over $\sim 100$ d by RXTE. Performing a time-resolved spectral analysis on the photospheric radius expansion burst detected on 2008 May 4, we obtain the source distance in the range of 3.5--8.7 kpc for the first time. During its outburst, XTE J1810-189 did not enter into the high/soft state, and both the soft and hard colours decreased with decreasing flux. The fractional rms remained at high values ($\sim 30$ per cent). The RXTE/PCA spectra for 3-25 keV can be described by an absorbed power-law component with an additional Gaussian component, and the derived photon index $\Gamma$ increased from $1.84\pm0.01$ to $2.25\pm0.04$ when the unabsorbed X-ray luminosity in 3-25 keV dropped from $4\times10^{36}$ ergs s$^{-1}$ to $6\times10^{35}$ ergs s$^{-1}$. The relatively high flux, dense observations and broadband spectra allow us to provide strong evidence that the softening behaviour detected in the outburst of XTE J1810-189 originates from the evolution of non-thermal component rather than the thermal component (i.e. neutron star surface emission).