X-ray softening in the new X-ray transient XTE J1719-291 during its 2008 outburst decay

TL;DR

This study observed the X-ray spectral evolution of the transient XTE J1719-291 during its 2008 outburst, revealing spectral softening with decreasing luminosity and suggesting a neutron star accretor based on luminosity ratios.

Contribution

It provides the first detailed spectral analysis of XTE J1719-291 during outburst decay, highlighting spectral softening and proposing the possible nature of the compact object.

Findings

X-ray spectrum softened as luminosity decreased

Spectral fitting indicates a thermal plus power-law component

Tentative evidence suggests a neutron star as the accretor

Abstract

The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a simple one-component model, but it can be fitted with a thermal component (black body or disc black body) plus power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ~ 7.7E-13 M_sun/yr for a neutron star as compact object and ~ 3.7E10-13 M_sun/yr in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.