DISCOVERY OF HIGH-ENERGY GAMMA-RAY PULSATIONS FROM PSR J2021+3651 WITH AGILE

Discovered after the end of the Compton Gamma-Ray Observatory mission, the radio pulsar PSR J2021 + 3651 was long considered a likely counterpart of the high-energy gamma-ray source 2CG 075 + 00 p 3EG J2021 + 3716 = GeV J2020 + 3658, but it could not be confirmed due to the lack of a contemporaneous radio pulsar ephemeris to fold the sparse, archival gamma-ray photons. Here, we report the discovery of gamma-ray pulsations from PSR J2021 + 3651 in the 100-1500 MeV range using data from the AGILE satellite gathered over 8 months, folded on a densely sampled, contemporaneous radio ephemeris obtained for this purpose at the Green Bank Telescope. The gamma-ray pulse consists of two sharp peaks separated by 0.47 +/- 0.01 cycles. The single radio pulse leads the first gamma-ray peak by 0.165 +/- 0.010 cycles. These properties are similar to those of other gamma-ray pulsars, and the phase relationship of the peaks can be interpreted in the context of the outer-gap accelerator model for gamma-ray emission. Pulse-phase-resolved images show that there is only one dominant source, AGL J2020.5 + 3653 = PSR J2021 + 3651, in the region previously containing confused sources 3EG J2021 + 3716 and 3EG J2016 + 3657.