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 gamma-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 gamma-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716. PSR J2021+3651 has a rotation period of P similar to 104 ms and (P) over dot similar to 9.6 x 10(-14), implying a characteristic age of tau(c) similar to 17 kyr and a spin-down luminosity of (E) over dot similar to 3.4 x 10(36) ergs s(-1). The dispersion measure DM similar to pc cm(-3) is by far the highest of any observed pulsar in the Galactic longitude range 55degrees <l<80degrees. This DM suggests a distance dgreater than or similar to 10 kpc and a high gamma-ray efficiency of similar to15%, 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.