The GeV-band variability of Mrk 421

The TV-detected blazar Markarian 421 is the brightest high-energy peaked BL Lac (HBL) source in the GeV energy range, detectable with 3 sigma significance on daily timescales in medium and higher brightness states with Large Area Telescope onboard Fermi. The 0.3–300 GeV flux showed a strong variability on diverse time scales from the flares lasting a few weeks down to intraday flux fluctuations detected mostly in the flaring states. The source also exhibited a GeV spectral variability. We mostly observe a low spectral curvature in this band due to the presence of the Inverse Compton peak at the energies mainly beyond about 100 GeV. The GeV flux generally showed a significantly stronger correlation with the radio–UV fluxes than with those in the X-ray and TeV bands that confirms the hypothesis about the possible stronger connection between radio–UV and GeV emissions due the effect of Klein-Nishina suppression on the IC process, leading to the effective up-scattering of radio–UV photons at the GeV energies ...