AstroSat view of ‘Clocked’ burster GS 1826-238: broad-band spectral nature of persistent and burst emission during soft state

Abstract

In this paper, we have carried out a detailed study of the ‘Clocked’ burster GS 1826 − 238 using ∼ 90 ks broad-band (0.7 - 60.0 keV) data obtained with AstroSat observatory. The source was observed during a soft spectral state and traced a ‘banana’ type track in the colour-colour diagram (CCD). We find that a combination of thermal component (multi-colour disc/bbodyrad) and Comptonized component is statistically good description for all the sections of the track in the CCD. The corona becomes optically thick (τ increases from ∼ 5 to 21) and cooler (kTe decreases from ∼ 4.8 to 2.2 keV) as the source moves up in the ‘banana’ branch. Probably cooling is caused by increase in the supply of soft-seed photons from the disc/boundary-layer. Reflection signature is observed at upper ‘banana’ branch of the source. Two type-I X-ray bursts are detected during the AstroSat observations. During the bursts, hard X-rays increased unlike previous observations where a reduction in hard X-rays is observed during the bursts. Decrease in the electron temperature and increase in the optical depth are observed during the bursts. The PSD (Power Spectral Density) of all the sections of the CCD can be represented by a pure power-law component. The strength of this component increases from ∼ 1 per cent to 4.5 per cent as the source moves up in the ‘banana’ track. Search for burst oscillations gave a null result. We discuss the implications of our results in the context of previous findings.

AstroSat/LAXPC view of GX 17+2: spectral evolution along the Z-track

“…Finally, we attempted a combination of MCD and nthComp (hereafter Model 3) components to describe the X-ray spectra of this source. Both models have been widely used to fit the spectra of the NS-LMXBs (Di Salvo et al 2002;Di Salvo et al 2001;Barret 2001;Agrawal, Nandi, and Katoch 2023;Agrawal and Sreekumar 2003). We also need an additional smeared edge component (at ∼ 7 -8 keV) to improve the fit.…”

Revealing the Spectral State Transition of the Clocked Burster, GS 1826-238, with NuSTAR StrayCats

“…Variations in the hard flux lasted until roughly MJD 57100 (2015 March), when the hard flux completely disappeared, while the soft flux showed significant variability over a multiyear period (Figure 2, bottom panel). The ASTROSAT observations show evidence for a classical banana soft state for the source similar to the 2014 soft episode, with the emission dominated by an optically thick corona with a low electron temperature (Agrawal et al 2022).…”

“…Other than short snapshots from NICER, one focused observation with NuSTAR, and a pair of observations using ASTROSAT (Agrawal et al 2022), until 2022 the source has not been intentionally observed since it started its transition. Fortunately, GS 1826-238 is nearby several sources with recurring outbursts (e.g., V4641 Sgr).…”

Spectro-polarimetric view of bright atoll source GX 9+9 using IXPE and AstroSat

“…X-ray spectra of the NS-LMXBs are complex and require multiple components to describe them. A combination of multi-temperature disc blackbody and Comptonized emission (Di Salvo et al 2000aAgrawal & Sreekumar 2003;Tarana et al 2008;Agrawal & Misra 2009;Agrawal et al 2023) has been widely used to describe the X-ray spectra of the atoll and Zsources. Here, the softer component from the disc is directly observed, and the Comptonized emission may come from the boundary layer (BL) or spreading layer (SL) near the surface of the neutron star ('eastern model').…”