Changes in the distribution of circum-binary material around the HMXB GX 301-2 during a rapid spin-up episode of the neutron star

TL;DR

This study investigates the changes in the accretion environment of the HMXB GX 301-2 during an 80-day rapid spin-up episode in 2019, revealing significant variations in circum-binary material and reprocessing conditions.

Contribution

It provides the first detailed time-resolved spectroscopic analysis of the circum-binary material changes during a neutron star spin-up episode in GX 301-2.

Findings

Absorption column density doubled during the spin-up.

Iron line equivalent width decreased by more than half.

Luminosity enhancements occurred after pre-periastron flares.

Abstract

Some accretion powered X-ray pulsars with supergiant companion stars undergo occasional rapid spin-up episodes that last for weeks to a few months. We explore the changes in the accretion environment of the pulsar GX 301-2 during its latest 80 days long spin-up episode in 2019 when the spin frequency of the pulsar increased by ~2% over two orbits of the binary. By performing time-resolved spectroscopy with the MAXI/GSC spectra of the source, we estimated the equivalent hydrogen column density and equivalent width of the iron fluorescence line during the spin-up episode, and compared them with the long-term average values estimated by orbital-phase resolved spectroscopy. The measured absorption column density during the spin-up episode is about twice that of an average orbit, while the equivalent width of the iron line is less than half of an average orbit. Though the spin-up episode started immediately after a pre-periastron flare and lasted for the two consecutive orbits of the binary, the associated enhancement in luminosity started a few days after the pre-periastron flare and lasted only during the first orbit, and some enhancement was seen again during the pre-periastron passage of the second orbit. The absorption column density and iron line equivalent width vary throughout the spin-up episode and are distinct from an average orbit. These observations indicate a significant change in the accretion and reprocessing environment in GX 301-2 during the spin-up episode and may hold important clues for the phenomenon in this source and several other sources with supergiant companions.