The Crab pulsar - the remnant of the supernova that exploded in 1054 AD and was recorded by Chinese astronomers is now known to be a magnetized neutron star rotating 30 times a second. The pulsar is slowly braking and converting the spin-down power into an extremely broad range of photon energies from radio waves to gamma-rays with an amazingly high efficiency. The particular details of the power conversion mechanism are still under debate, but it is thought that the highly relativistic wind produced by the pulsar magnetosphere carries the spin-down power out to distances of about a few percent of a light year where it is radiated as the broad band spectrum of photons that shows up in the sky as the Crab Nebula. The relativistic pulsar wind is likely to be highly magnetized and its energy is converted via some unknown mechanism into electrons and positrons of energies up to petaelectronoVolts (1015 eV) regime - far beyond any man-made particle accelerators can achieve. Then the energetic particles produce gamma-rays as synchrotron radiation in magnetic fields and also by scattering up the cosmic microwave background photons. It is a challenging problem indeed to reveal how the macroscopic magnet of about 25 km diameter rotating at a frequency of about 30 Hz is radiating electromagnetic waves of frequencies up to 1025 Hz. Moreover, the emission of the Crab Nebula was found to be very stable in the X-ray and gamma-ray bands and it has been used for years to calibrate X-ray and gamma-ray detectors aboard space-borne and balloon-borne observatories.
All of a sudden, a high variability of the Crab Nebula, that appeared to produce from time to time strong flares of a few days duration along with shorter time scale flux variations in GeV regime gamma-rays, was recently discovered by AGILE (see [1]) and Fermi (see [2]) space missions. It has casted a number of questions both on the physical nature of the variability and on the reliability of the Crab Nebula as a calibration source. The variability was detected at the very end of the GeV regime spectrum (i.e. in the so called "spectral cut-off regime"). The GeV photons are likely to have the maximal energies produced by the synchrotron part of the spectrum in the fluctuating magnetic fields of the nebula.
The origin of the flares can be attributed either to some not yet known process that accelerates electrons and positrons beyond their energies in the quiescent periods or by the Doppler boosting of the observed photons if they were produced in the highly relativistic flows toward the observer before such flows are terminated. A completely new idea, which could lead to a very different interpretation of the gamma-ray flares in the Crab Nebula, has been developed by A.M. Bykov and collaborators and is published in Monthly Notices of the Royal Astronomical Society (see [3]).
The authors show that rare strong departures of the amplitude of magnetic fluctuations from their mean values may result in synchrotron photon flares just as they were observed in the Crab Nebula. The flares can be reproduced in the model that accounts for the effect of the long tail of the probability distribution of magnetic fluctuation amplitudes on the synchrotron radiation photon spectra in the spectral cut-off regime. The long tails of probability distribution of magnetic fluctuation magnitudes were directly measured in the Earth magnetosphere storm events by Cluster satellite experiment and that may provide a clue to understand the observed gamma-ray flares in the Crab Nebula where the driving power is by some seventeen orders of magnitude larger than that in the Earth magnetosphere storms disturbing the mobile connections. Recently the tails were detected by VOYAGER 1 magnetometers crossing the Heliosheath. The sudden reformations of the tails of the probability distribution of magnetic fluctuation magnitudes in the Crab Nebula may be connected to the appearance of dynamical wisps observed in the Crab Nebula with Hubble Space Telescope and Chandra X-ray Observatory - a hypothesis that may be tested with the ongoing multiwavelength observational campaigns. More generally, the effect of the long tail of the probability distribution of magnetic fluctuations may be important for modeling of synchrotron emission from relativistic outflows. The tails would make a number previously unseen and exciting gamma-ray sources observable in the GeV to TeV range with the next generation of gamma-ray telescopes based on Cherenkov air shower detectors.