Magnetically Accreting Isolated Old Neutron Stars

Previous work on the emission from isolated old neutron stars (IONS)accreting the inter-stellar medium (ISM) focussed on gravitational capture -Bondi accretion. We propose a new class of sources which accrete via magneticinteraction with the ISM. While for the Bondi mechanism, the accretion ratedecreases with increasing NS velocity, in magnetic accretors (MAGACs="magics")the accretion rate increases with increasing NS velocity. MAGACs will beproduced among high velocity (~> 100 km s-1) high magnetic field (B> 1e14 G)radio pulsars - the ``magnetars'' - after they have evolved first throughmagnetic dipole spin-down, followed by a ``propeller'' phase (when the objectsheds angular momentum on a timescale ~< 1e10 yr). The properties of MAGACS maybe summarized thus: dipole magnetic fields of B~>1e14 G; minimum velocitiesrelative to the ISM of >25-100 km s-1, depending on B, well below the median inthe observed radio-pulsar population; spin-periods of >days to years; accretionluminosities of 1e28- 1e31 ergs s-1 ; and effective temperatures kT=0.3 - 2.5keV if they accrete onto the magnetic polar cap. We find no examples of MAGACsamong previously observed source classes (anomalous X-ray pulsars,soft-gamma-ray repeaters or known IONS). However, MAGACs may be more prevelantin flux-limited X-ray catalogs than their gravitationally accretingcounterparts.Comment: ApJ, accepte