AstroSat observation of rapid Type-I thermonuclear burst from the low mass X-ray binary GX 3+1

TL;DR

This paper reports the first AstroSat observation of a rapid Type-I thermonuclear burst from GX 3+1, revealing a photospheric radius expansion event, providing new distance estimates, and comparing findings with previous studies.

Contribution

First AstroSat detection of a Type-I burst from GX 3+1, including detailed time-resolved spectroscopy and a new distance estimate based on burst parameters.

Findings

Detected a 15 s PRE burst from GX 3+1

Estimated source distance as approximately 9.3 kpc

Confirmed burst characteristics with time-resolved spectral analysis

Abstract

We report the results of an observation of low mass X-ray binary GX 3+1 with {\it AstroSat}'s Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments on-board for the first time. We have detected one Type-1 thermonuclear burst ($\sim$ 15 s) present in the LAXPC 20 light curve, with a double peak feature at higher energies and our study of the hardness-intensity diagram reveals that the source was in a soft banana state. The pre-burst emission could be described well by a thermally Comptonised model component. The burst spectra is modelled adopting a time-resolved spectroscopic method using a single color blackbody model added to the pre-burst model, to monitor the parametric changes as the burst decays. Based on our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be approximately constant at an averaged value $\sim$ 2.56 in $10^{-8}$ ergs s$^{-1}$ cm$^{-2}$ units. On the basis of literature survey, we infer that \textit{AstroSat}/LAXPC 20 has detected a burst from GX 3+1 after more than a decade which is also a PRE one. Utilising the burst parameters obtained, we provide a new estimation to the source distance, which is $\sim$ 9.3 $\pm$ 0.4 kpc, calculated for an isotropic burst emission. Finally, we discuss and compare our findings with the published literature reports.