PSR J2021+3651: A Young Radio Pulsar Coincident with an Unidentified EGRET γ-Ray Source

We report on a deep search for radio pulsations toward five unidentified ASCA X-ray sources coincident with EGRET γ-ray sources. This search has led to the discovery of a young and energetic pulsar using data obtained with the new Wideband Arecibo Pulsar Processor. PSR J2021+3651 is likely associated with the X-ray source AX J2021.1+3651, which in turn is likely associated with the COS B high-energy γ-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716. PSR J2021+3651 has a rotation period of P ~ 104 ms and ~ 9.6 × 10-14, implying a characteristic age of τc ~ 17 kyr and a spin-down luminosity of ~ 3.4 ×1036 ergs s-1. The dispersion measure DM ~ 371 pc cm-3 is by far the highest of any observed pulsar in the Galactic longitude range 55° < l < 80°. This DM suggests a distance d ≳ 10 kpc and a high γ-ray efficiency of ~15%, but the true distance may be closer if there is a significant contribution to the DM from excess gas in the Cygnus region. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA.