Spectral Variations of γ -Rays in Mrk 421

Abstract We present a comprehensive analysis of the 17 yr Fermi-LAT observational data of Mrk 421 to investigate the spectral variations in the γ -ray bands. The light curve of the source in the 0.1–1000 GeV band with a 14 day time bin exhibits significant variability at a confidence level exceeding 5 σ , which is accompanied by spectral variation, displaying a harder-when-brighter behavior. Moreover, its flux variation can reach up to 1 order of magnitude within 1 day, with a daily flux up to (1.19 ± 0.84) × 10 −8 erg cm −2 s −1 on MJD 56152. The 17 yr integrated spectrum of Mrk 421 necessitates a complex model for explanation, whereas its time-resolved spectra over 1 day or several-day time intervals can be well fitted by a power-law model. We propose that the complex spectral shape of the 17 yr integrated spectrum stems from the superposition of different spectral shapes in different flux states. By generating the GeV spectra that are simultaneously observed with the archived TeV observations and constructing the combined GeV–TeV spectra, we find that some combined GeV–TeV spectral shapes clearly imply different radiation origins for the GeV and TeV emissions, challenging the one-zone leptonic model. It is found that the flux follows a lognormal distribution, while the photon spectral index distributions can be well fitted by either a lognormal or a Gaussian function. The possible nature of the γ -ray variability in Mrk 421 is discussed.