  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
successfully explain the emission in all blazars in our sample. On average,
FSRQs have higher magnetic field, Doppler factor, and curvature than BL Lac
objects. The BL Lac objects show an anticorrelation 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 Lac objects 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 toward high peak BL Lac objects (HBLs), opposite
to peak energy, the curvature parameter can be considered a third parameter of
the blazar sequence in addition to peak frequency and luminosity.

High Energy Astrophysical Phenomena