Spectral Transition and Torque Reversal in X-ray Pulsar 4U 1626-67

Abstract

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 Mdot~2x10**16 g/s, Mdot decreasing at a rate ~6x10**14 g/s/yr, and a polar surface magnetic field of ~2b_p**{-1/2} 10^**12G where b_p 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 ~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.

10 pages, 1 figure, ApJL

References in corpus (8)

• Observations of Accreting Pulsars
• On the Correlation of Torque and Luminosity in GX 1+4
• On the Dramatic Spin-up/Spin-down Torque Reversals in Accreting Pulsars
• BeppoSAX observation of 4U1626-67: Discovery of an absorption cyclotron resonance feature
• Warped Disks as a Possible Origin of Torque Reversals in Accretion-Powered Pulsars
• Torque Reversal in Accretion-Powered X-ray Pulsars
• Sidebands Due to Quasi-periodic Oscillations in 4U 1626-67
• Constraints on Torque-Reversing Accretion-Powered X-ray Pulsars

Cited by in corpus (12)

• New Torque Reversal and Spin-Up of 4u 1626- 67 Observed by Fermi/GBM and Swift/BAT
• Study of the Long Term Stability of two Anomalous X-ray Pulsars 4U 0142+61 and 1E 1048.1-5937 with ASCA
• 4U 1626-67 as seen by Suzaku before and after the 2008 torque reversal
• Torque reversals and pulse profile of the pulsar 4U 1626-67
• Spectral and temporal changes associated with flux enhancement in 4U 1626-67
• Pulse phase dependence of emission lines in the X-ray pulsar 4U 1626-67
• On the torque reversals of 4U 1626--67
• 4U 1626-67 Returns to Spin-Down: Timing features toe the line
• Constraints on the disc-magnetosphere interaction in accreting pulsar 4U 1626--67
• Torque bistability in the interaction between a neutron star magnetosphere and a thin accretion disc
• Sidebands to mHz QPOs in 4U 1626$-$67 in the second spin-down state
• Angular Momentum Transfer in the Binary X-ray Pulsar GX 1+4