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 relatively small luminosity change. The time-averaged X-ray spectrum during spin-down is considerably harder than during spin-up. The observed torque reversal 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 caused by a change in the accretion flow rotation from Keplerian to sub-Keplerian. The X-ray pulsar 4U 1626-67 is estimated to be near spin equilibrium with a mass accretion rate M ∼ 2 × 10¹⁶ g s^-1, M decreasing at a rate ∼-6 × 10¹⁴ g s^-1 yr^-1, and a polar surface magnetic field of ∼ 2^-1/2p × 10¹²G, where bp is the magnetic pitch. During spin-up, the Keplerian flow remains geometrically thin and cool. During spin-down, the sub-Keplerian flow becomes geometrically thick and hot. Soft photons from near the stellar surface are Compton up-scattered by the hot accretion flow during spin-down, while during spin-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 radially falling column of material with a scattering depth ∼0.3 and a temperature ∼10⁹ 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 there are indications that this system is significantly different from other torque-reversal systems.