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.