The COS B high energy \gamma -ray source 2CG 075+00 , also known as GeV J2020+3658 or 3EG J2021+3716 , has avoided identification with a low energy counterpart for over twenty years .
We present a likely identification with the discovery and subsequent timing of a young and energetic pulsar , PSR J2021+3651 , with the Wideband Arecibo Pulsar Processor at the Arecibo Observatory .
PSR J2021+3651 has a rotation period P \sim 104 ms and \dot { P } \sim 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 } erg s ^ { -1 } .
The pulsar is also coincident with the ASCA source AX J2021.1+3651 .
The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA .
The pulsar ’ s unexpectedly high dispersion measure ( DM \sim 371 pc cm ^ { -3 } ) and the d \geq 10 kpc DM distance pose a new question : is PSR J2021+3651 an extremely efficient \gamma -ray pulsar at the edge of the Galaxy ?
This is a question for AGILE and GLAST to answer .