# NuSTAR observations of the wind-fed X-ray pulsar GX 301--2 during an unusual spin-up event

## TL;DR

This study uses NuSTAR observations to analyze a rare spin-up event in the wind-fed X-ray pulsar GX 301--2, revealing a positive correlation between cyclotron line energy and luminosity and suggesting that wind accretion dominates despite transient disk formation.

## Contribution

First detailed analysis of GX 301--2 during a spin-up event showing cyclotron line behavior and insights into accretion mechanisms compared to typical wind-fed states.

## Key findings

- Positive correlation between cyclotron line energy and luminosity.
- Minor spectral and temporal differences during spin-up.
- Evidence suggesting wind accretion remains dominant despite transient disk formation.

## Abstract

We report on \textit{NuSTAR} observations of the well-known wind-accreting X-ray pulsar \source\ during a strong spin-up episode that took place in January-March 2019. A high luminosity of the source in a most recent observation allowed us to detect a positive correlation of the cyclotron line energy with luminosity. Beyond that, only minor differences in spectral and temporal properties of the source during the spin-up, presumably associated with the formation of a transient accretion disk, and the normal wind-fed state could be detected. We finally discuss conditions for the formation of the disk and possible reasons for lack of any appreciable variations in most of the observed source properties induced by the change of the accretion mechanism, and conclude that the bulk of the observed X-ray emission is still likely powered by direct accretion from the wind.