Abdo, A. A. AA ; Ackermann, M. ; Ajello, M. ;
Aug - 2009
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 2
type: Article Journal

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 ± 0.002 in phase. The first peak lags the maximum of the 2 GHz radio pulse by 0.162 ± 0.004 ± 0.01 in phase. The integral gamma-ray photon flux above 100 MeV is (56 ± 3 ± 11) × 108 cm2 s1. The photon spectrum is well described by an exponentially cut-off power law of the form , where the energy E is expressed in GeV. The photon index is “ = 1.5 ± 0.1 ± 0.1 and the exponential cut-off is Ec = 2.4 ± 0.3 ± 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 RM = 524 ± 4 rad m2 but a poorly constrained magnetic geometry. Re-analysis of Chandra X-ray Observatory 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.

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