Spectral Evolution of the September 2010 gamma-ray flare from the Crab Nebula (V2)

TL;DR

This paper analyzes the spectral evolution of the September 2010 gamma-ray flare from the Crab Nebula, proposing a rapid acceleration and synchrotron cooling model that explains the observed spectral data and challenges previous cutoff predictions.

Contribution

It introduces a model of rapid particle acceleration and cooling that accounts for the spectral evolution during the flare, providing new insights into the acceleration mechanism and magnetic environment.

Findings

Successful explanation of spectral data with rapid acceleration and cooling

Constraints on acceleration timescale and magnetic field properties

Contradicts previous exponential cutoff predictions at >100 MeV

Abstract

Strong gamma-ray flares from the Crab Nebula have been recently discovered by AGILE and confirmed by Fermi-LAT. We study here the spectral evolution in the gamma-ray energy range above 50 MeV of the September 2010 flare that was simultaneously detected by AGILE and Fermi-LAT. We revisit the AGILE spectral data, and present an emission model based on rapid (within 1 day) acceleration followed by synchrotron cooling. We show that this model successfully explains both the published AGILE and Fermi-LAT spectral data showing a rapid rise and a decay within 2-3 days. Our analysis constrains the acceleration timescale and mechanism, the properties of the particle distribution function, and the local magnetic field. The combination of very rapid acceleration, emission well above 100 MeV, and the spectral evolution consistent with synchrotron cooling contradicts the idealized scenario predicting an exponential cutoff at photon energies above 100 MeV. We also consider a variation of our model based on even shorter acceleration and decay timescales which can be consistent with the published averaged properties.