NuSTAR discovers a long type-I X-ray burst from the clocked burster GS 1826-24

TL;DR

This paper reports the detection and detailed spectroscopic analysis of a long type-I X-ray burst from the neutron star low mass X-ray binary GS 1826-24 using NuSTAR, revealing insights into burst dynamics and persistent emission behavior.

Contribution

First detailed spectroscopic analysis of a long X-ray burst from GS 1826-24, showing complex spectral evolution and persistent emission enhancement during the burst.

Findings

Burst lasted approximately 600 seconds.

Peak blackbody temperature reached 2.10 keV.

Persistent emission increased during the burst.

Abstract

The source GS~1826-24 is a neutron star low mass X-ray binary known as the 'clocked burster' because of its extremely regular bursting behavior. We report on the detection of a long type-I X-ray burst from this source. We perform a detailed spectroscopic analysis of the long X-ray burst, lasting for $\sim 600$ s, seen in the NuSTAR observation carried out on 2022 September. The persistent emission is well described by an absorbed thermal Comptonization model (nthcomp), and the source exhibits a soft spectral state during this observation. The observed burst exhibits a rise time of $\sim 25$ s and a decay time of $\sim 282$ s. The time-resolved spectroscopy of the burst shows a significant departure from a pure thermal spectrum and is described with a model consisting of a varying-temperature blackbody plus an evolving persistent emission component. We observe a significant enhancement in the persistent emission during the burst. The enhancement of the pre-burst persistent flux is possibly due to Poynting-Robertson drag or coronal reprocessing. At the peak of the burst, the blackbody temperature and the blackbody emitting radius reached a maximum of $2.10\pm 0.07$ keV and $5.5\pm 2.1$ km, respectively. The peak flux ($F_{peak}$) during the burst is $\approx 2.4\times 10^{-8}$ ergs cm$^{-2}$ s$^{-1}$, which corresponds to a luminosity of $\approx 9.7\times 10^{37}$ ergs s$^{-1}$.