A unified model for the time variability of the Crab: the dispersion measure variation and the $\gamma$-ray flare of the nebula

Abstract

The Crab pulsar and its nebula are very luminous and variable. Since 1982, random fluctuations of the dispersion measure (DM) have been recorded. Also in 2011, Fermi-LAT telescope detected $\gamma$-ray flux variation of the Crab nebula. No correlation between these phenomena have considered although their fluctuating patterns look similar. We propose a model which connects the DM and the Crab nebula $\gamma$-ray flux fluctuation, motivated by recently reported correlation between the braking index and the DM variability. Considering electrons within the inner shock region of the nebula produced in breaking index change events, we can explain both the DM enhancement and the $\gamma$-ray flare in a simple way.