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

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.

Burst–Disk Interaction in 4U 1636–536 as Observed by NICER

“…They fit the X-ray spectra extracted from LAXPC and the Soft X-Ray Telescope either with two blackbody functions with a constant background assumption or with a model that includes a scaling factor for the preburst emission and another blackbody model for the burst emission. Although the data presented in Kashyap et al (2022) were not sensitive enough to differentiate among these models, they statistically demonstrated the requirement of the existence of a second component to describe the burst spectra near the peaks of the bursts. As with other examples, the excess can be interpreted as the reemission/reprocessing of the photons by the accretion disk/ corona, the scattering of the photons in the neutron star atmosphere, or enhanced persistent emission due to the PR drag (in't Zand et al 2013;Degenaar et al 2013;Worpel et al 2013;Keek et al 2017Keek et al , 2018a.…”

“…4U 1636-536 was one of the first X-ray binaries where evidence of deviation from pure blackbody emission during the X-ray bursts was reported (Worpel et al 2013). More recently, Kashyap et al (2022) reported the detection of such excess near the peaks of the bursts in broadband AstroSat observations. They fit the X-ray spectra extracted from LAXPC and the Soft X-Ray Telescope either with two blackbody functions with a constant background assumption or with a model that includes a scaling factor for the preburst emission and another blackbody model for the burst emission.…”

NICER observations of the evidence of Poynting-Robertson drag and disk reflection during type I X-ray bursts from 4U 1636–536

“…Thereafter, the distance of 6.0 kpc was adopted. The enhanced persistent emissions during X-ray bursts have also been observed in 4U 1636-536 (Worpel et al 2013(Worpel et al , 2015Roy et al 2021;Kashyap et al 2022). However, the reflection features accompanying with the enhanced persistent emissions have only been detected during its superburst (Keek et al 2014a(Keek et al , 2018.…”

The Thermonuclear X-Ray Bursts of 4U 1730–22

“…The detection of excess burst emission has become increasingly common in recent years, either through the increased sensitivity afforded by NICER (Keek et al 2018a(Keek et al , 2018bBult et al 2019;Jaisawal et al 2019;Güver et al 2022aGüver et al , 2022b or through broadband X-ray coverage with AstroSat (Bhattacharyya et al 2018;Roy et al 2021;Kashyap et al 2022) or Insight-HXMT (Chen et al 2019). While the emission of a neutron star atmosphere is expected to deviate from Planck's law (London et al 1986;Madej et al 2004;Suleimanov et al 2011), the observed deviations from a blackbody spectrum are well in excess of what an atmosphere model can explain (in't Zand et al 2017).…”