On the Origin and Evolution of Curvature of the Spectral Energy Distribution of Fermi Bright Blazars

Muhammad Anjum, Liang Chen, Minfeng Gu

Published 2020-06-04Version 1

The origin and evolution of spectral curvature in blazar spectral energy distribution (SED) is still unclear. Since the observed SED curvature is related to an intrinsic curvature in emitting electron energy distribution (EED), we study this question by employing a log-parabolic EED with a curvature parameter and peak energy to model the quasi-simultaneous broadband SEDs of selected blazars in Fermi LAT Bright AGN Sample (LBAS) using synchrotron and inverse Compton (IC) processes. We find that log-parabolic IC model can successfuly explain the emission in all blazars in our sample. On average, FSRQs have higher magnetic field, Doppler factor and curvature than BL Lacs. The BL Lacs show an anti-correlation between the curvature parameter of the EED and its peak energy, which is a signature of stochastic acceleration. FSRQs do not manifest such correlation and rather show a mild positive relationship between these parameters. This suggests that the evolution of spectral curvature in the BL Lacs is dominated by a strong stochastic acceleration component, whereas the curvature in FSRQs evolves in a cooling dominated regime due to an additional external Compton (EC) component. The strong cooling in FSRQs not only restricts the electron peak energy but also adds extra curvature to the high energy tail of emitting EED. Since the curvature decreases from FSRQs towards high peak BL Lacs (HBLs), opposite to peak energy, the curvature parameter can be considered a third parameter of blazar sequence in addition to peak frequency and luminosity.