12 hours spikes from the Crab Pevatron

The Crab featured a large γ-ray flare on September 18, 2010. To better understand the origin of this phenomenon, we analyze the INTEGRAL (20-500 keV) and FERMI (0.1-300 GeV) data collected almost simultaneously during the flare. We divide the available data into three different sets, corresponding to the pre-flare period, the flare and the subsequent quiescence. For each period, we perform timing and spectral analysis to disentangle the contribution from the pulsar and from the surrounding nebula to the γ-ray luminosity. No significant variations of the pulse profile and spectral characteristics are detected in the hard X-ray domain. On the contrary, we find three separated enhancements of the γ-ray flux lasting 6–12 hours and separated by an interval of about two days from each other. The spectral analysis shows that the flux enhancement, confined below ∼1GeV, can be modelled by a power-law with high energy exponential cut-off, where either the cut-off energy or the model normalization increased by a factor ∼ 5 with respect to the pre-flare emission. We also confirm that the γ-ray flare is not pulsed. The timing and spectral analysis indicate that the γ-ray flare is due to synchrotron emission from a very compact Pevatron located closer to the pulsar than the equatorial termination shock between the supersonic wind and the surrounding nebula. The spectral properties of the flare are interpreted in the framework of a relativistically moving emitter and/or an enlarged emitting electron population.