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

TL;DR

This paper presents a statistical model explaining the gamma-ray variability and flares of the Crab nebula, attributing them to electron acceleration in knots with a power-law size distribution and Doppler boosting, successfully reproducing observed flux fluctuations and spectra.

Contribution

The paper introduces a novel Monte Carlo simulation model linking knot size distribution and Doppler boosting to gamma-ray variability in the Crab nebula, explaining flares and flux fluctuations.

Findings

Model reproduces observed gamma-ray flares and flux variations.

Variability increases with photon energy due to knot size distribution.

Simulated spectra match observed steady and flaring states.

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.