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A Proper Motion for the Pulsar Wind Nebula G359.23-0.82, "the Mouse," Associated with the Energetic Radio Pulsar J1747-2958

The "Mouse" (PWN G359.23-0.82) is a spectacular bow shock pulsar wind nebula, powered by the radio pulsar J1747-2958. The pulsar and its nebula are presumed to have a high space velocity, but their proper motions have not been directly measured. Here we present 8.5 GHz interferometric observations of the Mouse nebula with the Very Large Array, spanning a time baseline of 12 yr. We measure eastward proper motion for PWN G359.23-0.82 (and hence indirectly for PSR J1747-2958) of 12.9+/-1.8 mas/yr, which at an assumed distance of 5 kpc corresponds to a transverse space velocity of 306+/-43 km/s. Considering pressure balance at the apex of the bow shock, we calculate an in situ hydrogen number density of approximately 1.0(-0.2)(+0.4) cm^(-3) for the interstellar medium through which the system is traveling. A lower age limit for PSR J1747-2958 of 163(-20)(+28) kyr is calculated by considering its potential birth site. The large discrepancy with the pulsar's spin-down age of 25 kyr is possibly explained by surface dipole magnetic field growth on a timescale ~15 kyr, suggesting possible future evolution of PSR J1747-2958 to a different class of neutron star. We also argue that the adjacent supernova remnant G359.1-0.5 is not physically associated with the Mouse system but is rather an unrelated object along the line of sight.