Abstract

Here, we report on γ-ray observations of the Crab Pulsar and Nebula using 8 months of survey data with the Fermi Large Area Telescope (LAT). The high quality light curve obtained using the ephemeris provided by the Nançay and Jodrell Bank radio telescopes shows two main peaks stable in phase with energy. The first γ-ray peak leads the radio main pulse by (281 ± 12 ± 21) μs, giving new constraints on the production site of non-thermal emission in pulsar magnetospheres. The first uncertainty is due to γ-ray statistics, and the second arises from the rotation parameters. The improved sensitivity and the unprecedented statistics afforded by the LAT enable precise measurement of the Crab Pulsar spectral parameters: cut-off energy at E _c = (5.8 ± 0.5 ± 1.2) GeV, spectral index of Γ = (1.97 ± 0.02 ± 0.06) and integral photon flux above 100 MeV of (2.09 ± 0.03 ± 0.18) × 10 ^–6 cm ^–2 s ^–1. The first errors represent the statistical error on the fit parameters, while the second ones are the systematic uncertainties. Pulsed γ-ray photons are observed up to ~ 20 GeV which precludes emission near the stellar surface, below altitudes of around 4-5more » stellar radii in phase intervals encompassing the two main peaks. We also performed a detailed phase-resolved spectral analysis : the hardest emission from the Crab Pulsar comes from the bridge region between the two γ-ray peaks while the softest comes from the falling edge of the second peak. Furthermore, the spectrum of the nebula in the energy range 100 MeV-300 GeV is well described by the sum of two power laws of indices Γ _sync = (3.99 ± 0.12 ± 0.08) and Γ _IC = (1.64 ± 0.05 ± 0.07), corresponding to the falling edge of the synchrotron and the rising edge of the inverse Compton (IC) components, respectively. This latter, which links up naturally with the spectral data points of Cherenkov experiments, is well reproduced via IC scattering from standard magnetohydrodynamic nebula models, and does not require any additional radiation mechanism.« less

Authors:

Abdo, A. A. ^[1]

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+ Show Author Affiliations

1. Naval Research Lab., Washington, D.C. (United States); National Academy of Science, Washington, D.C. (United States). et al.

Publication Date:

2009-12-21

Research Org.:

SLAC National Accelerator Lab., Menlo Park, CA (United States)

Sponsoring Org.:

USDOE

Contributing Org.:

Fermi LAT Collaboration

OSTI Identifier:

1357592

Grant/Contract Number:  

AC02-76SF00515

Resource Type:

Accepted Manuscript

Journal Name:

Astrophysical Journal

Additional Journal Information:

Journal Volume: 708; Journal Issue: 2; Journal ID: ISSN 0004-637X

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; gamma rays: observations; ISM: individual (Crab Nebula); pulsars: individual (Crab); supernova remnants