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 abrupt torque reversal accompanied by a dramatic spectral transition and a relati vely small luminosity change. The time-averaged X-ray spectrum during spin- down is considerably harder than during spin-up. The observed torque revers al can be explained by an accretion flow transition triggered by a gradual change in the mass accretion rate. The sudden transition to spin-down is ca used by a change in the accretion flow rotation from Keplerian to sub-Keple rian. The X-ray pulsar 4U 1626-67 is estimated to be near spin equilibrium with a mass accretion rate (M) over dot similar to 2 x 10(16) g s(-1), (M) over dot decreasing at a rate similar to-6 x 10(14) g s(-1) yr(-1), and a p olar surface magnetic field of similar to 2b(p)(-1/2) x 10(12)G, where b(p) is the magnetic pitch. During spin-up, the Keplerian Bow remains geometric ally thin and cool. During spin-down, the sub-Keplerian flow becomes geomet rically thick and hot. Soft photons from near the stellar surface are Compt on up-scattered by the hot accretion flow during spin-down, while during sp in-up such scattering is unlikely because of the small scale height and low temperature of the flow. This mechanism accounts for the observed spectral hardening and small luminosity change. The scattering occurs in a hot radi ally falling column of material with a scattering depth similar to 0.3 and a temperature similar to 10(9) K. The X-ray luminosity at energies greater than 5 keV could be a poor indicator of the mass accretion rate. We briefly discuss the possible application of this mechanism to GX 1+4, although the re are indications that this system is significantly different from other t orque-reversal systems.