Implications of a VLBA parallax for PSR B0656+14

Using the VLBA we have determined the parallax of the radio pulsar PSR B0656+14 to be π = 3.47±0.36 mas, yielding a distance of 288 _-27⁺³³ pc. This constrains existing models of thermal X-ray emission from the neutron star's photosphere yielding R_∞ ∼7-8.5 km for simple blackbody models, and radii between ∼13 and ∼20 km with more realistic fits to a magnetized hydrogen atmosphere. Astrometric and age constrains argue in favor of a definite association with the Monogem Ring SNR. The latter's distance and age are consistent with the predictions of a model for the primary cosmic ray energy spectrum in which the 'knee' feature is produced by a single dominant source.