Detection of Small Flares from the Crab Nebula with Fermi-LAT

Gamma radiation from the Crab pulsar wind nebula (PWN) shows significant variability at similar to 100 MeV energies, recently revealed with spaceborne gamma-ray telescopes. Here we report the results of a systematic search for gamma-ray flares using a 7.4 yr data set acquired with the Fermi Large Area Telescope. Analyzing the off-pulse phases of the Crab pulsar, we found seven previously unreported low-intensity flares (small flares). The small flares originate from the variable synchrotron component of the Crab PWN and show clearly different features from the steady component of the Crab PWN emission. They are characterized by larger fluxes and harder photon indices, similar to previously reported flares. These flares show day-scale time variability and imply a strong magnetic field ofB(min) 1 mG at the site of the gamma-ray production. This result seems to be inconsistent with the typical values revealed with modeling of the nonthermal emission from the nebula. The detection of the small flares gives a hint of production of gamma-rays above 100 MeV in a part of the nebula with properties that are different from the main emitters, e.g., due to bulk relativistic motion.