An Investigation into the Spectral Properties of Bright Fermi Blazars

TL;DR

This study analyzes the spectral properties of Fermi-detected blazars, finding that a log-parabola model generally describes their spectra well and revealing insights into their spectral stability and variability during different flux states.

Contribution

It demonstrates that a log-parabola model is typically superior for describing blazar spectra in the Fermi-LAT energy range and clarifies the causes of broken power law appearances.

Findings

Log-parabola fits are generally better than power laws.

Spectral stability observed during quiescent states.

Spectral shape deviations during high flux states.

Abstract

We investigate the spectral properties of blazars detected with the Fermi-Large Area Telescope (LAT) in the high energy regime 100 MeV - 100 GeV. We find that over long timescales a log-parabola provides an adequate description of the spectrum in almost all objects and in most cases is significantly better than a simple power law or broken power law description. Broken power law descriptions appear to arise from two causes: confusion with nearby sources and as an artifact of older LAT instrument response functions. We create a light curve for 2FGLJ2253.9+1609 (3C 454.3), the brightest of the objects investigated. During the quiescent state we find the spectrum to be fairly stable and well-described by a log-parabola. There is some evidence that, on average, the peak energy of the inverse Compton emission is lower in the quiescent state than in the time-averaged state, suggesting that increases in flux are due to changing parameters within the jet as opposed to changes in an external photon field. However, no correlation between inverse Compton peak energy and flux is apparent. During high flux states, deviation of the spectral shape from a simple power law continues. In some cases a log-parabola provides a significantly better fit than a broken power law but in others the reverse is true.