Flux Variation of the Crab Pulsar Observed Using Fermi LAT and NuSTAR

The flux of pulsed emission from the Crab pulsar, just like most of other pulsars, is considered to be steady over time scales much larger than the pulsation period, except for a smooth, secular decrease related to the aging of pulsars. Such a steady assumption prevails in all theoretical investigations of pulsar emission models. However, here we report the discovery of stochastic fluctuation in the flux and photon index of the Crab pulsar gamma-ray and X-ray emissions utilizing archival data of Fermi LAT (0.1 -- 100~GeV, spanning 17 years) and of NuSTAR (3.0 -- 79.0~keV, spanning 13 years) observations. The origin of such fluctuation is intriguing. It might be related to possible configuration adjustment of magnetic fields in neutron stars' superconducting and superfluid interiors.