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

We report on a deep search for radio pulsations toward five unidentified ASCA X-ray sources coincident with EGRETg-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-energyg-ray source 2CG 075 00, also known as GeV J2020 3658 or 3EG J2021 3716. PSR J2021 3651 has a rotation period of ms P ∼ 104 and , implying a characteristic age of kyr and a spin-down luminosity of 14 ̇ ̇ P ∼ 9.6# 10 t ∼ 17 E ∼ 3.4# c 10 ergs s . The dispersion measure D pc cm 3 is by far the highest of any observed pulsar in the Galactic M ∼ 371 longitude range . This DM suggests a distance kpc and a high g-ray efficiency of∼15%, but 55 ! l ! 80 d 10 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: gamma rays: observations — pulsars: general — pulsars: individual (PSR J2021 365 ) — stars: neutron — X-rays: individual (AX J2021.1 3651)