Spectral Transition and Torque Reversal in X-Ray Pulsar 4U 1626−67

The accretion-powered, X-ray pulsar 4U 1626-67 has recently shown an abrupttorque reversal accompanied by a dramatic spectral transition and a relativelysmall luminosity change. The time-averaged X-ray spectrum during spin-down isconsiderably harder than during spin-up. The observed torque reversal can beexplained by an accretion flow transition triggered by a gradual change in themass accretion rate. The sudden transition to spin-down is caused by a changein the accretion flow rotation from Keplerian to sub-Keplerian. 4U 1626-67 isestimated to be near spin equilibrium with a mass accretion rate Mdot~2x10**16g/s, Mdot decreasing at a rate ~6x10**14 g/s/yr, and a polar surface magneticfield of ~2b_p**{-1/2} 10^**12G where b_p is the magnetic pitch. Duringspin-up, the Keplerian flow remains geometrically thin and cool. Duringspin-down, the sub-Keplerian flow becomes geometrically thick and hot. Softphotons from near the stellar surface are Compton up-scattered by the hotaccretion flow during spin-down while during spin-up such scattering isunlikely due to the small scale-height and low temperature of the flow. Thismechanism accounts for the observed spectral hardening and small luminositychange. The scattering occurs in a hot radially falling column of material witha scattering depth ~0.3 and a temperature ~10^9K. The X-ray luminosity atenergies >5keV could be a poor indicator of the mass accretion rate. We brieflydiscuss the possible application of this mechanism to GX 1+4, although thereare indications that this system is significantly different from othertorque-reversal systems.Comment: 10 pages, 1 figure, ApJ