Magnetically Accreting Isolated Old Neutron Stars

Previous work on the emission from isolated old neutron stars (IONSs) accreting from the interstellar medium (ISM) has focused on gravitational captureÈi.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 decreases with increasing neutron star velocity, in magnetic accretors (MAGACs) M0 Bondi increases with increasing neutron star velocity vs. MAGACs will M0 MAGAC (M0 BondiP v~3 M0 MAGACP v1@3). be produced among high-velocity km s~1), high magnetic Ðeld (B[ 1014 G) radio pulsarsÈthe (Z100 ““magnetars ÏÏÈafter they have evolved Ðrst through magnetic dipole spin-down, followed by a ““ propeller ÏÏ phase (during which the object sheds angular momentum on a timescale yr). The [1010 properties of MAGACs may be summarized thus : dipole magnetic Ðelds of G; minimum BZ 1014 velocities relative to the ISM of 25È100 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 1028È1031 ergs s~1 ; and e†ective temperatures keV if they accrete onto the magnetic kTeff \ 0.3È2.5 polar cap. We Ðnd 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 Ñux-limited X-ray catalogs than their gravitationally accreting counterparts. Subject headings : pulsars : general È stars : magnetic Ðelds È stars : neutron È X-rays : stars