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 P ∼= 104 ms and Ṗ ∼= 9.6 × 10, implying a characteristic age τc ∼ 17 kyr and a spin-down luminosity Ė ∼ 3.4 × 10 ergs s. The dispersion measure DM≃ 371 pc cm 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. Subject headings: pulsars: general — pulsars: individual (PSR J2021+3651) — stars: neutron — X-rays: individual (AX J2021.1+3651) — gamma-rays: individual (GeV 2020+3658)