Changes in the pulse phase dependence of X-ray emission lines in 4U 1626-67 with a torque reversal

TL;DR

This study reveals that the pulse phase dependence of low energy emission lines in 4U 1626-67 changes with its torque reversal, indicating structural changes in the accretion disk between spin-up and spin-down phases.

Contribution

It provides the first detailed analysis of how emission line behavior and pulse profiles vary with torque state in 4U 1626-67, highlighting changes in accretion disk structures.

Findings

Emission line at 0.915 keV varies by a factor of 2 with pulse phase.

Behavior of low energy emission lines differs between spin-up and spin-down phases.

Pulse profile below 2 keV changes during spin-up phase.

Abstract

We report results from an observation with the XMM-Newton observatory of a unique X-ray pulsar 4U 1626-67. EPIC-pn data during the current spin-up phase of 4U 1626-67 have been used to study pulse phase dependence of low energy emission lines. We found strong variability of low energy emission line at 0.915 keV with the pulse phase, varying by a factor of 2, much stronger than the continuum variability. Another interesting observation is that behavior of one of the low energy emission lines across the pulse phase is quite different from that observed during the spin-down phase. This indicates that the structures in the accretion disk that produce pulse phase dependence of emission features have changed from spin-down to spin-up phase. This is well supported by the differences in the timing characteristics (like pulse profiles, QPOs etc) between spin-down and spin-up phases. We have also found that during the current spin-up phase of 4U 1626-67, the X-ray pulse profile below 2 keV is different compared to the spin-down phase. The X-ray light curve also shows flares which produce a feature around 3 mHz in power density spectrum of 4U 1626-67. Since flares are dominant at lower energies, the feature around 3 mHz is prominent at low energies.