Torque reversal and orbital profile of X-ray pulsar OAO 1657-415

TL;DR

This study analyzes the torque states of the X-ray pulsar OAO 1657-415, revealing correlations between spin changes and flux, and suggesting disk accretion as the cause of torque reversals.

Contribution

It provides the first detailed correlation analysis of spin behavior, flux, and orbital profiles, supporting the retrograde/prograde disk accretion model for torque reversals.

Findings

Spin frequency derivatives correlate with flux during rapid spin-up.

Anti-correlated with flux during rapid spin-down.

Orbital profile and hardness ratio vary with torque state.

Abstract

OAO 1657-415 is an atypical supergiant X-ray binary among wind-fed and disk-fed systems, showing alternate spin-up/spin-down intervals lasting on the order of tens of days. We study different torque states of OAO 1657-415 based on the spin history monitored by {\it Fermi}/GBM, together with fluxes from {\it Swift}/BAT and {\it MAXI}/GSC. Its spin frequency derivatives are well correlated with {\it Swift}/BAT fluxes during rapid spin-up episodes, anti-correlated with {\it Swift}/BAT fluxes during rapid spin-down episodes, and not correlated in between. The orbital profile of spin-down episodes is reduced by a factor of 2 around orbital phases of 0.2 and 0.8 compared to that of spin-up episodes. The orbital hardness ratio profile of spin-down episodes is also lower than that of spin-up episodes around phases close to the mid-eclipse, implying that there is more material between the neutron star and the observer for spin-down episodes than for spin-up episodes around these phases. These results indicate that the torque state of the neutron star is connected with the material flow on orbital scale and support the retrograde/prograde disk accretion scenario for spin-down/spin-up torque reversal.