A statistical model for the $\gamma$-ray variability of the Crab nebula

Abstract

A statistical scenario is proposed to explain the $\gamma$-ray variability and flares of the Crab nebula, which were observed recently by the Fermi/LAT. In this scenario electrons are accelerated in a series of knots, whose sizes follow a power-law distribution. These knots presumably move outwards from the pulsar and have a distribution in the Doppler boost factor. The maximal electron energy is assumed to be proportional to the size of the knot. Fluctuations at the highest energy end of the overall electron distribution will result in variable $\gamma$-ray emission via the synchrotron process in the $\sim 100$ MeV range. Since highly boosted larger knots are rarer than smaller knots, the model predicts that the variability of the synchrotron emission increases with the photon energy. We realize such a scenario with a Monte-Carlo simulation and find that the model can reproduce both the two $\gamma$-ray flares over a period of $\sim$ year and the monthly scale $\gamma$-ray flux fluctuations as observed by the Fermi/LAT. The observed $\gamma$-ray spectra in both the steady and flaring states are also well reproduced.

11 pages, 4 figures. Fig. 2 updated. Accepted for publication in The Astrophysical Journal Letters

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• High sigma model of pulsar wind nebulae

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• Gamma-ray flares in the Crab Nebula: A case of relativistic reconnection?
• Crab Nebula gamma-ray flares as relativistic reconnection minijets
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• Twinkling pulsar wind nebulae in the synchrotron cut-off regime and the gamma-ray flares in the Crab Nebula
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• Radiative Magnetic Reconnection in Astrophysics
• Characterization of the Inner Knot of the Crab: The Site of the Gamma-ray Flares?
• Flares in the Crab Nebula Driven by Untwisting Magnetic Fields
• Gamma-ray flare and absorption in Crab Nebula: Lovely TeV-PeV astrophysics
• Jitter radiation model of the Crab gamma ray flares
• A systematic study of $\gamma$-ray flares from the Crab Nebula with Fermi-LAT: I. flare detection