The Crab's Gamma-Ray Flare

As noted on this blog last month, the Crab Nebula was seen by the Fermi telescope to flare in the gamma ray band. The flare itself is pictured to the right and was the subject of yesterday's APOD. The flare itself lasted for 3 days and saw a 30 fold increase in emission from the Crab.

Immediately after this occurred astronomers from all over the world looked with different instruments to see if there was increased emission from the Crab Nebula in other bands. No increase was detected though.

This is not the first time the Crab has done this, but it is confusing to astronomers trying to explain the flare. Since the flare lasts for only 3 days we know that the region of emission can't be that large. Since it was seen only in the gamma-rays seen by Fermi, we know that it is probably due to fluctuations in the synchrotron spectrum of the nebula. At these high energies the synchrotron light that is being seen is coming from the highest energy electrons, so fluctuations at these energies indicates fluctuations either in the magnetic field or the population of the highest energy electrons. No one is quite sure which, and while several theories have been tossed about, there is no favored model.

The variability in the gamma-ray is also a little concerning for observers in those bands as the Crab Nebula is used as a standard calibration source since it's emission is thought to be relatively steady. This is still likely true in the normal state, so astronomers just have to be aware if they are observing during a flaring state or not. Also most of the experiments using the Crab as a standard work in the TeV gamma-ray which is much higher energy than Fermi and do not see any fluctuations in the Crab's emission.