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 P \cong 104 ms and \dot { P } \cong 9.6 \times 10 ^ { -14 } , implying a characteristic age \tau _ { c } \sim 17 kyr and a spin-down luminosity \dot { E } \sim 3.4 \times 10 ^ { 36 } ergs s ^ { -1 } .
The dispersion measure DM \simeq 371 pc cm ^ { -3 } is by far the highest of any observed pulsar in the Galactic longitude range 55 ^ { \circ } < l < 80 ^ { \circ } .
This DM suggests a distance d \gtrsim 10 kpc , and a high \gamma -ray efficiency of \sim 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 .