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

Abstract

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 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.