Broadband time-resolved spectroscopy of thermonuclear X-ray bursts from 4U 1636-536 using AstroSat

TL;DR

This study presents broadband, time-resolved spectroscopy of 15 thermonuclear X-ray bursts from 4U 1636-536 using AstroSat, revealing spectral excesses near burst peaks and exploring their possible origins to understand burst-accretion interactions.

Contribution

First broadband simultaneous analysis of Type-I X-ray bursts with AstroSat's LAXPC and SXT, providing new insights into burst spectra and accretion environment interactions.

Findings

Detected spectral excess near burst peaks.

Observed modest spectral evolution during the island state.

Explored possible origins of excess related to accretion disk and neutron star atmosphere.

Abstract

Broadband spectral studies of Type-I X-ray bursts can put strong constraints on the physics of burst spectra as well as their interaction with the environment. We present the results obtained from the broadband time-resolved spectroscopy of 15 thermonuclear bursts detected simultaneously from the neutron star atoll source 4U 1636-536 using LAXPC and SXT onboard AstroSat. During the observations with AstroSat, the Low mass X-ray binary (LMXB) 4U 1636-536 is observed to show a modest spectral evolution within the island state. The broadband burst spectra are observed to show an excess in addition to the thermal emission from the neutron star surface near the peak of the bursts. We investigate the interpretation of the excess observed near the peak of the burst as re-emission/reprocessing of the photons by the accretion disk/corona or scattering of the photons in the neutron star atmosphere or the enhanced persistent emission due to Poynting-Robertson drag. This is the first reported broadband simultaneous study of Type-I bursts using LAXPC and SXT onboard AstroSat. This kind of study may provide a better understanding of the burst-accretion interaction and how the bursts influence the overall accretion process contributed by the accretion disk as well as the corona.