Disk vs wind accretion in X-ray pulsar GX 301-2

TL;DR

This study compares disk and wind accretion in the X-ray pulsar GX 301-2 using Insight-HXMT and Fermi/GBM data, revealing distinct pulse profile behaviors and absorption characteristics during different accretion states.

Contribution

It provides the first detailed comparison of disk versus wind accretion effects in GX 301-2 through multi-instrument observations, highlighting differences in pulse profiles and absorption.

Findings

Pulse profiles differ between spin-up and wind-fed states.

Absorption column density is lower during spin-up, indicating disk formation.

Main peak in pulse profile increases with luminosity during spin-up.

Abstract

GX 301-2 provides a rare opportunity to study both disk and wind accretion in a same target. We report Insight-HXMT observations of the spin-up event of GX 301-2 happened in 2019 and compare with those of wind-fed state. The pulse profiles of the initial rapid spin-up period are dominated by one main peak, while those of the later slow spin-up period are composed of two similar peaks, as those of wind-fed state. These behaviors are confirmed by Fermi/GBM data, which also show that during the rapid spin-up period, the main peak increases with luminosity up to $8\times10^{37}$ erg s$^{-1}$, but the faint peak keeps almost constant. The absorption column densities during the spin-up period are $\sim1.5\times10^{23}$ cm$^{-2}$, much less than those of wind-fed state at similar luminosity ($\sim9\times10^{23}$ cm$^{-2}$), supporting the scenario that most of material is condensed into a disk during the spin-up period. We discuss possible differences between disk and wind accretion that may explain the observed different trend of pulse profiles.