We report the detection of pulsed gamma-rays from the young , spin-powered radio pulsar PSR J2021+3651 using data acquired with the Large Area Telescope ( LAT ) on the Fermi Gamma-ray Space Telescope ( formerly GLAST ) .
The light curve consists of two narrow peaks of similar amplitude separated by 0.468 \pm 0.002 in phase .
The first peak lags the maximum of the 2 GHz radio pulse by 0.162 \pm 0.004 \pm 0.01 in phase .
The integral gamma-ray photon flux above 100 MeV is ( 56 \pm 3 \pm 11 ) \times 10 ^ { -8 } cm ^ { -2 } s ^ { -1 } .
The photon spectrum is well-described by an exponentially cut-off power law of the form { dF \over dE } = kE ^ { - \Gamma } e ^ { ( - E / E _ { c } ) } where the energy E is expressed in GeV .
The photon index is \Gamma = 1.5 \pm 0.1 \pm 0.1 and the exponential cut-off is E _ { c } = 2.4 \pm 0.3 \pm 0.5 GeV .
The first uncertainty is statistical and the second is systematic .
The integral photon flux of the bridge is approximately 10 % of the pulsed emission , and the upper limit on off-pulse gamma-ray emission from a putative pulsar wind nebula is < 10 \% of the pulsed emission at the 95 % confidence level .
Radio polarization measurements yield a rotation measure of \mbox { RM } = 524 \pm 4 rad m ^ { -2 } but a poorly constrained magnetic geometry .
Re-analysis of Chandra data enhanced the significance of the weak X-ray pulsations , and the first peak is roughly phase-aligned with the first gamma-ray peak .
We discuss the emission region and beaming geometry based on the shape and spectrum of the gamma-ray light curve combined with radio and X-ray measurements , and the implications for the pulsar distance .
Gamma-ray emission from the polar cap region seems unlikely for this pulsar .