AAS Meeting #193 - Austin, Texas, January 1999
Session 93. Pulsars I
Oral, Friday, January 8, 1999, 2:00-3:30pm, Room 10 (B)

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[93.02] Torque Reversal and Spectral Transition in 4U 1626-67

I. Yi (CAIS-EWHA and KIAS), E. T. Vishniac (Johns Hopkins Univ.)

The accretion-powered, X-ray pulsar 4U 1626-67 has recently shown an abrupt torque reversal accompanied by a dramatic spectral transition and a small luminosity change. The time-averaged X-ray spectrum during spin-down is considerably harder than during spin-up. The sudden transition to spin-down is caused by a change in the accretion flow rotation from Keplerian to sub-Keplerian, which is triggered by a gradual change in accretion rate. 4U 1626-67 is estimated to be near spin equilibrium with a surface magnetic field of ~5x1011G, a mass accretion rate ~2x1016 g/s, and the accretion rate change ~-6x1014 g/s/yr. 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 due to 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 material with a scattering depth ~0.3 and a temperature ~109K. The X-ray luminosity at energies >5keV 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.


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