Magnetically Accreting Isolated Old Neutron Stars

Abstract

Previous work on the emission from isolated old neutron stars (IONSs) accreting from the interstellar medium (ISM) has focused on gravitational capture-i.e., Bondi accretion. We propose a new class of sources that accrete via magnetic interaction with the ISM. While for the Bondi mechanism the accretion rate MBondi decreases with increasing neutron star velocity, in magnetic accretors (MAGACs) MMAGAC increases with increasing neutron star velocity (MBondi~v-3 vs. MMAGAC~v1/3). MAGACs will be produced among high-velocity (>~100 km s-1), high magnetic field (B>1014 G) radio pulsars-the ``magnetars''-after they have evolved first through magnetic dipole spin-down, followed by a ``propeller'' phase (during which the object sheds angular momentum on a timescale ~1014 G; minimum velocities relative to the ISM of 25-100 km s-1 or higher, depending on B, well below the median in the observed radio pulsar population; spin periods of greater than days to years; accretion luminosities of 1028-1031 ergs s-1; and effective temperatures kTeff=0.3-2.5 keV if they accrete onto the magnetic polar cap. We find no examples of MAGACs among previously observed source classes (anomalous X-ray pulsars, soft gamma-ray repeaters, or known IONs). However, MAGACs may be more prevalent in flux-limited X-ray catalogs than their gravitationally accreting counterparts.