Spectral and Timing evolution of GX 340+0 along its Z-track

Abstract

We present the results from spectral and timing study of the Z source GX 340+0 using AstroSat’s SXT and LAXPC data. During the observation the source traced out the complete Z-track, allowing for the spectral evolution study of the Horizontal, Normal and Flaring branches (HB, NB and FB) as well as the hard and soft apexes (HA and SA). The spectra are better and more physically described by a blackbody component and a hot Comptonizing corona with a varying covering fraction, rather than one having a disc component. Along the track, the Comptonized flux (as well as the covering fraction) monotonically decreases. It is the blackbody component (both the temperature and radius) which varies non-monotonically and hence gives rise to the Z-track behaviour. Rapid timing study reveals a prominent Quasi-periodic Oscillation (QPO) at ∼50 Hz at the HB, HA and upper NB, while a QPO at ∼6 Hz is seen for the other branches. The fractional r.m.s of the QPOs increase with energy and exhibit soft lags in all branches except SA and FB.