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

Abstract

ABSTRACT OAO 1657−415 is an atypical supergiant X-ray binary among wind- and disc-fed systems, showing alternate spin-up/spin-down intervals lasting of the order of tens of days. We study different torque states of OAO 1657−415 based on the spin history monitored by Fermi/Gamma-Ray Burst Monitor, together with fluxes from Swift/Burst Alert Telescope (BAT) and Monitor of All-sky X-ray Image/Gas Slit Camera. Its spin frequency derivatives are well correlated with Swift/BAT fluxes during rapid spin-up episodes, anticorrelated with 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 disc accretion scenario for spin-down/spin-up torque reversal.