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OAO 1657-415 is an atypical supergiant X-ray binary among wind-fed anddisk-fed systems, showing alternate spin-up/spin-down intervals lasting on theorder of tens of days. We study different torque states of OAO 1657-415 basedon the spin history monitored by {\it Fermi}/GBM, together with fluxes from{\it Swift}/BAT and {\it MAXI}/GSC. Its spin frequency derivatives are wellcorrelated 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 isreduced by a factor of 2 around orbital phases of 0.2 and 0.8 compared to thatof spin-up episodes. The orbital hardness ratio profile of spin-down episodesis also lower than that of spin-up episodes around phases close to themid-eclipse, implying that there is more material between the neutron star andthe observer for spin-down episodes than for spin-up episodes around thesephases. These results indicate that the torque state of the neutron star isconnected with the material flow on orbital scale and support theretrograde/prograde disk accretion scenario for spin-down/spin-up torquereversal.

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