Spectral properties of GX 339−4 in the intermediate state using AstroSat observation

Abstract

We present the results obtained from the spectral studies of black hole X-ray binary GX 339–4 using AstroSat observations during its 2021 outburst. AstroSat observed the source in the intermediate state for ∼600 ks. The combined spectra of SXT and LAXPC in the 0.7 − 25 keV energy range are studied with phenomenological and physical models. The spectral study reveals a receding disc and a contracting corona during the observation period. The outflow rate is found to be increased though the accretion rates did not vary during the observation period. The X-ray flux decreases as the disc recedes and the spectrum becomes hard. At the same time, the Comptonized flux decreases with increasing fraction of thermal emission. This could be plausible that episodic jet ejection modified the corona and reduced Comptonized flux. An iron emission line at 6.4 keV is observed in the spectra of all the orbits of observation. We find that the equivalent width of the iron emission line correlates with the photon index, indicating a decrease in the reflection strength as the spectrum becomes hard. We observe that the disc flux does not follow FDBB − T4 relation.