ar X iv : a st ro - p h / 99 05 21 4 v 1 1 7 M ay 1 99 9 Spectral Transition and Torque Reversal in X - ray Pulsar 4 U 1626 - 67
نویسنده
چکیده
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. 4U 1626-67 is estimated to be near spin equilibrium with a mass accretion rate Ṁ ∼ 2 × 10g/s, Ṁ decreasing at a rate ∼ −6× 10 g/s/yr, and a polar surface magnetic field of ∼ 2b p × 10G 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 due to 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 ∼ 10K. 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. Subject headings: accretion, accretion disks − pulsars: general − stars: magnetic fields − X-rays: stars
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ar X iv : a st ro - p h / 97 10 14 8 v 1 1 4 O ct 1 99 7 Constraints on Torque
The observed abrupt torque reversals in X-ray pulsars, 4U 1626-67, GX 1+4, and OAO 1657-415, can be explained by transition in accretion flow rotation from Keplerian to sub-Keplerian, which takes place at a critical accretion rate, ∼ 10 − 10g/s. When a pulsar system spins up near equilibrium spin before the transition, the system goes into spin-down after transition to sub-Keplerian. If a syste...
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