X-ray observations of the very-faint X-ray transient XMMSL1 J171900.4--353217: A new candidate neutron star low-mass X-ray binary

TL;DR

This paper reports on X-ray observations of the very-faint transient XMMSL1 J171900.4-353217, suggesting it is a neutron star low-mass X-ray binary, and discusses its spectral properties and potential for future pulsation searches.

Contribution

The study provides detailed spectral analysis of a new candidate neutron star low-mass X-ray binary using Swift XRT data, proposing its nature based on spectral evolution.

Findings

Spectral indices range from 1.8 to 2.7.

Luminosity fluctuates between 10^{35} and 10^{36} erg/s.

Likely a neutron star low-mass X-ray binary at several kpc.

Abstract

XMMSL1 J171900.4-353217 is a very-faint X-ray transient that was discovered in 2010 March when it exhibited an outburst. We report on 7 observations, obtained with the X-Ray Telescope (XRT) aboard the Neil Gehrels {\it{Swift}} Observatory between 2010 May to October. By fitting a single absorbed power-law model to the XRT spectra, we infer power-law indices of $\Gamma = 1.8-2.7$ and an absorption column density of $\mathrm{N}_{\mathrm{H}}=(4.6-7.9)\times 10^{22}~\mathrm{cm}^{-2}$. The inferred $0.5-10$~keV luminosity fluctuated irregularly and peaked at $\mathrm{L}_\mathrm{X}\simeq 10^{35}-10^{36} ~\mathrm{erg~s^{-1}}$ for a distance of $4-12$~kpc. Based on the evolution of the power-law index with varying luminosity, we propose that the source most likely is a transient neutron star low-mass X-ray binary located at several kpc. If true, it would be a good candidate to search for coherent millisecond pulsations when it enters a new accretion outburst.