Interpretation of blazar SEDs based on broad band quasi-simultaneous observations

TL;DR

This study analyzes quasi-simultaneous broad-band SEDs of 48 blazars to characterize their spectral features, derive empirical relations, and assess the adequacy of simple emission models, revealing the need for more complex scenarios.

Contribution

It provides a comprehensive analysis of blazar SEDs using multi-wavelength data and introduces empirical relations to estimate spectral peak positions from broad-band colors and gamma-ray spectral index.

Findings

Synchrotron peak frequencies range from 10^12.5 to 10^14.5 Hz in FSRQs.

Gamma-ray spectral slope correlates with synchrotron peak energy.

Simple SSC models are insufficient to explain most SEDs, especially for FSRQs and LBL BL Lacs.

Abstract

We study the quasi-simultaneous Spectral Energy Distributions (SED) of 48 LBAS blazars, collected within three months of the Fermi LAT Bright AGN Sample (LBAS) data taking period, combining Fermi and Swift data with radio NIR-Optical and hard-X/gamma-ray data. We have used these SEDs to characterize the peak position and intensity of both the low and the high-energy features of blazar spectra. The results have been used to derive empirical relationships that estimate the position of the two peaks from the broad-band colors (i.e. the radio to optical, alpha_ro, and optical to X-ray, alpha_ox, spectral slopes) and from the gamma-ray spectral index. Our data show that the synchrotron peak frequency is positioned between 10^(12.5) and 10^(14.5) Hz in broad-lined FSRQs and between 10^(13) and 10^(17) Hz in featureless BL Lacertae objects. We find that the gamma-ray spectral slope is strongly correlated with the synchrotron peak energy and with the X-ray spectral index, as expected at first order in synchrotron - inverse Compton scenarios. However, simple homogeneous, one-zone, Synchrotron Self Compton (SSC) models cannot explain most of our SEDs, especially in the case of FSRQs and low energy peaked (LBL) BL Lacs. More complex models involving External Compton Radiation or multiple SSC components are required to reproduce the overall SEDs and the observed spectral variability.