Magnetically accreting isolated old neutron stars

Previous work on the emission from isolated old neutron stars (IONSs) accre ting from the interstellar medium (ISM) has focused on gravitational captur e-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 accre tion rate (M)over dot(Bondi) decreases with increasing neutron star velocit y, in magnetic accretors (MAGACs) (M)over dot(MAGAC) increases with increas ing neutron star velocity ((M)over dot(Bondi) proportional to (-3) vs. (M)o ver dot(MAGAC) proportional to v(1/3)). MAGACs will be produced among high- velocity (greater than or similar to 100 km s(-1)), high magnetic field (B > 10(14) G) radio pulsars-the "magnetars"-after they have evolved first thr ough magnetic dipole spin-down, followed by a "propeller" phase (during whi ch the object sheds angular momentum on a timescale less than or similar to 10(10) yr). The properties of MAGACs may be summarized thus : dipole magne tic fields of B greater than or similar to 10(14) G; minimum velocities rel ative to the ISM of 25-100 km s(-1) or higher, depending on B, well below t he median in the observed radio pulsar population; spin periods of greater than days to years; accretion luminosities of 10(28)-10(31) ergs s(-1); and effective temperatures kT(eff) = 0.3-2.5 keV if they accrete onto the magn etic polar cap. We find no examples of MAGACs among previously observed sou rce classes (anomalous X-ray pulsars, soft gamma-ray repeaters, or known IO Ns). However, MAGACs may be more prevalent in flux-limited X-ray catalogs t han their gravitationally accreting counterparts.