AstroSat view of the neutron star low-mass X-ray binary GX 340+0

TL;DR

This study analyzes AstroSat data of the neutron star low-mass X-ray binary GX 340+0, revealing how spectral and timing properties evolve along the Z-source's horizontal and normal branches, shedding light on accretion processes near the neutron star.

Contribution

It provides a detailed spectral and timing analysis of GX 340+0's Z-source branches, highlighting the role of the corona and accretion disk in spectral evolution and QPO behavior.

Findings

Corona is replenished and energized on the horizontal branch.

Spectral modeling includes accretion disk, corona, and neutron star surface.

QPOs are strongly linked to coronal properties.

Abstract

Understanding the spectral evolution along the `Z'-shaped track in the hardness-intensity diagram of Z-sources, which are a class of luminous neutron star low-mass X-ray binaries, is crucial to probe accretion processes close to the neutron star. Here, we study the horizontal branch (HB) and the normal branch (NB) of the Z source GX 340+0 using $AstroSat$ data. We find that the HB and the NB appear as two different types of X-ray intensity dips, which can appear in any sequence and with various depths. Our $0.8-25$ ~keV spectra of dips and the hard apex can be modeled by the emissions from an accretion disk, a Comptonizing corona covering the inner disk, and the neutron star surface. We find, as the source moves onto the HB the corona is replenished and energized by the disk and a reduced amount of disk matter reaches the neutron star surface. We also conclude that quasi-periodic oscillations during HB/NB are strongly associated with the corona, and explain the evolution of strength and hard-lag of this timing feature using the estimated coronal optical depth evolution.