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

TL;DR

This study systematically analyzed 7.4 years of Fermi-LAT data to discover seven low-intensity gamma-ray flares from the Crab Nebula, revealing new insights into nebular magnetic fields and emission mechanisms.

Contribution

It reports the first detection of small gamma-ray flares from the Crab Nebula's PWN, highlighting their distinct features and implications for nebular magnetic fields and emission processes.

Findings

Seven previously unreported small flares detected.

Flares show day-scale variability and harder spectra.

Implication of strong magnetic fields (~1 mG) at flare sites.

Abstract

Gamma radiation from the Crab pulsar wind nebula (PWN) shows significant variability at $\sim100$ 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-year 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 of $B_{\rm min}\approx 1~\mathrm{mG}$ at the site of the gamma-ray production. This result seems to be inconsistent with the typical values revealed with modeling of the non-thermal 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 which are different from the main emitters, e.g., due to bulk relativistic motion.