The role of jet properties in the spectral evolution of the powerful blazars

Abstract

In the work, we explore the role of jet properties in the spectral evolution for a sample of Fermi-LAT bright blazars composed primarily by flat spectrum radio quasars (FSRQs). We introduce a near-equipartition log-parabola (NELP) model to fit the quasi-simultaneous multi-waveband spectral energy distributions (SEDs). The Markov Chain Monte Carlo (MCMC) method is employed to determine the best values of spectral parameters and its errors. The correlations of synchrotron peak luminosity $L_{pk}^*$ and its peak frequency $\nu_{pk}^*$, SSC-dominant factor $\xi_s$ and $\nu_{pk}^*$, and their implications as to the spectral evolution were studied. The statistical analysis and the comparison with theoretical approximation indicate that: (1)the spectral evolution in $L_{pk}^*-\nu_{pk}^*$ plane may not only ascribed to variation of the characteristic energy $\gamma_{br}^\prime$ of the employed electron energy distribution (EED), but that other quantities must be varying as well.(2) for the spectral evolution in $\xi_s-\nu_{pk}^*$ plane the magnetic field $B^\prime$ may play the dominant role, and $\gamma_{br}^\prime$ may be considered to be the subdominant role. (3) Variation of the beaming factor $\delta_b$ may be mainly responsible for the absence of a possible correlation, since $B^\prime$ is strongly anti-correlated with the size of radiation zone $R_b^\prime$. The relation is agreement with that derived from the analytical theory, and should be self-consistent with the underling acceleration mechanism of the employed EED. By assuming a conical jet with opening angle $\theta_{op}\sim1/\Gamma_b$, we find that the $\gamma$-ray emission site being located at a distance $\sim10^{17}-10^{19}$ cm.