http://arxiv.org/abs/1504.03996
We have investigated the critical conditions required for an efficient steady propeller mechanism in the spin-down phases of magnetized neutron stars with optically thick accretion disks. We have shown through simple analytical calculations that: (1) the strength of the dipole field at the Alfven radius is not sufficient to sustain an efficient mass-outflow even when the magnetic dipole field lines rotate much faster than the escape speed, (2) in the spin-down phase, mass accretion onto the star could persist above a minimum disk mass-flow rate that is orders of magnitude lower than the rate corresponding to the transition between the spin-up and the spin-down states, (3) below this critical mass-flow rate, a steady propeller state could be established with a maximum inner disk radius about 25 times smaller than the Alfven radius. Our results indicate that only for spherical accretion, the inner disk radius is likely to approach the Alfven radius, and for all realistic cases, the accretion-propeller transition could take place at a mass-flow rate lower than the rate equating the Alfven and the co-rotation radii. Our results are consistent with the properties of the transitional millisecond pulsars which show transitions between the accretion powered X-ray pulsar and the rotational powered radio pulsar states, and emit X-ray pulses in the sub-luminous X-ray phases.
U. Ertan
Thu, 16 Apr 15
48/48
Comments: 20 pages, submitted to ApJ