Torque Reversal and Spectral Transition in 4U 1626-67

Details Torque Reversal and Spectral Transition in 4U 1626-67 Torque Reversal and Spectral Transition in 4U 1626-67

Dec 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...193.9302y&link_type=abstract

American Astronomical Society, 193rd AAS Meeting, #93.02; Bulletin of the American Astronomical Society, Vol. 30, p.1387

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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 ~ 5x10(11) G, a mass accretion rate ~ 2x10(16) g/s, and the accretion rate change ~ -6x10(14) 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 ~ 10(9K) . 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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