The TANAMI Multiwavelength Program: Dynamic SEDs of Southern Blazars

TL;DR

This study analyzes the spectral evolution of 22 blazars during different activity states using multiwavelength data, revealing correlations and inconsistencies with existing models, and providing new insights into blazar emission mechanisms.

Contribution

It presents the first broadband spectral studies for many sources, employing Bayesian analysis to characterize spectral states and challenging assumptions about thermal excess origins.

Findings

Low states follow the blazar sequence with lower peak frequencies at higher luminosities.

Gamma-ray photon index correlates with synchrotron peak frequency in low/intermediate states.

Thermal excess in FSRQs can be modeled by blackbody emission, not accretion disks.

Abstract

Simultaneous broadband spectral and temporal studies of blazars are an important tool for investigating active galactic nuclei (AGN) jet physics. We study the spectral evolution between quiescent and flaring periods of 22 radio-loud AGN through multi-epoch, quasi-simultaneous broadband spectra. For many of these sources these are the first broadband studies. We use a Bayesian block analysis of \Fermi/LAT light curves in order to determine time ranges of constant flux for constructing quasi-simultaneous SEDs. The shapes of the resulting 81 SEDs are described by two logarithmic parabolas and a blackbody spectrum where needed. For low states the peak frequencies and luminosities agree well with the blazar sequence, higher luminosity implying lower peak frequencies. This is not true for sources in a high state. The $\gamma$-ray photon index in Fermi/LAT correlates with the synchrotron peak frequency in low and intermediate states. No correlation is present in high states. The black hole mass cannot be determined from the SEDs. Surprisingly, the thermal excess often found in FSRQs at optical/UV wavelengths can be described by blackbody emission and not an accretion disk spectrum. The "harder-when-brighter" trend, typically seen in X-ray spectra of flaring blazars, is visible in the blazar sequence. Our results for low and intermediate states, as well as the Compton dominance, are in agreement with previous results. Black hole mass estimates using the parameters from Bonchi (2013) are in agreement with some of the more direct measurements. For two sources, estimates disagree by more than four orders of magnitude, possibly due to boosting effects. The shapes of the thermal excess seen predominantly in flat spectrum radio quasars are inconsistent with a direct accretion disk origin.