GX 301-2 pre-periastron and apastron flares with MAXI

TL;DR

This study analyzes the spectral and timing properties of GX 301-2's pre-periastron and apastron flares using MAXI data, revealing evidence of a transient accretion disc and variable hot spots on the neutron star surface.

Contribution

It provides the first detailed spectral analysis of GX 301-2's orbit-related flares and suggests the presence of a transient accretion disc during apastron passages.

Findings

Flares are best described by a blackbody continuum with Fe K-edge absorption.

Some apastron flares are as bright as pre-periastron flares.

Evidence of a transient accretion disc around the neutron star.

Abstract

The bright high-mass X-ray binary GX 301-2 exhibits two periodic flare episodes along its orbit which are produced when the neutron star is close to the apastron and periastron passages. Time-resolved spectra were extracted and several models applied to describe all of them. The best description was obtained with a blackbody continuum modified by the Fe K-shell absorption edge, absorbed by a large column density on the order of $10^{23}$ cm$^{-2}$ and, if present, a Fe K$\alpha$ fluorescent emission line. Three of the nine apastron flares were as bright as the pre-periastron flare and two of them coincided with spin-up episodes of the neutron star. This fact points to the presence of a transient disc around the neutron star as it passes through the apastron that increases the accretion process. The size of emitting region on the neutron star surface showed some variability but quite consistent with a hot spot.