From Equipartition to Curvature: The Spectral Evolution of 4FGL Blazars

We investigate the evolution of spectral energy distribution (SED) and underlying electron energy distribution (EED) by modeling the nearly simultaneous broadband spectra of selected bright 4FGL blazars, in the context of a combined cooling and stochastic acceleration scenario. We find that a one-zone leptonic model with log-parabolic EED can successfully fit the GeV–TeV emission of blazars. The synchrotron frequency ν s of blazars mainly evolves due to the variation of electron peak energy γ 3 p . BL Lac (BL Lacs) objects show a negative trend in the ν s – ν s L s SED plane, known as the blazar sequence, that does not seem to be an artifact of Doppler boosting, but is driven by the equipartition constraints. A positive correlation is found between the derived magnetic field B and electron density n e , whereas n e and γ 3 p negatively relate, as expected in an equipartition scenario. The flat-spectrum radio quasars (FSRQs) deviate significantly from such a scenario, indicating their jet parameters should vary independently. The synchrotron peak frequency ν s and its spectral curvature b s negatively correlate for all blazars, confirming the stochastic particle acceleration in blazar jets. However, blazars do not show the signature of hard-sphere acceleration, indicating that magnetic turbulence in the jets might be soft and physical conditions might be near steady state, consistent with equipartition. Furthermore, for BL Lacs, the SED curvature b s and the EED curvature r nearly meet the theoretical relationship of r = 5 b s , whereas the FSRQs show a large deviation due to a poor constraint on b s due to the presence of a thermal component.