A comprehensive comparison of spin-up and spin-down episodes of 4U 1538-522 observed with NuSTAR

TL;DR

This study compares the temporal and spectral properties of 4U 1538-522 during spin-up and spin-down episodes, finding no significant luminosity or profile changes, and suggests accretion mode transition as a possible cause for torque reversal.

Contribution

It provides a detailed analysis of the spin state transition in 4U 1538-522 using NuSTAR data, proposing a new explanation involving accretion mode changes.

Findings

No abrupt luminosity change during torque reversal

Pulse profile and spectral shape remained stable

Accretion mode transition may explain torque reversal

Abstract

4U 1538-522 is a persistent high mass X-ray binary which exhibits secular spin evolution. In 2019, it underwent a torque reversal from spinning up to spinning down. We performed an extensive study using four NuSATR observations to compare temporal and spectral properties during different states. We observed no abrupt change in luminosity associated with the torque reversal. In addition, the pulse profile, the spectral shape and the power spectrum remained unchanged before and after the torque reversal. The orbital and super-orbital modulation profiles also showed no significant changes. We discuss possible mechanisms for the torque reversal and conclude that it is unlikely to be caused by interactions between the accretion disk and the magnetosphere. Instead, the transition of accretion modes in spherical accretion may be a plausible explanation.