Synchrotron Emission from Pair Cascades in AGN Environments

TL;DR

This paper explores how synchrotron emission from pair cascades in AGN environments affects gamma-ray observations, revealing that magnetic field strength cannot be determined from gamma-ray spectra alone without observing cascade synchrotron emission.

Contribution

It demonstrates the importance of cascade synchrotron emission in constraining magnetic fields and explains spectral features in blazars related to pair cascades.

Findings

Cascade synchrotron emission can mimic the big blue bump in blazar spectra.

Magnetic field strength cannot be uniquely determined from gamma-ray spectra alone.

Observing synchrotron emission from cascades is essential to break degeneracies.

Abstract

Recent detections of very-high-energy (VHE, E > 100 GeV) gamma-ray blazars which do not belong to the high frequency peaked BL Lac (HBL) class, suggest that gamma-gamma absorption and pair cascading might occur in those objects. In the presence of even weak magnetic fields, these Compton-supported pair cascades will be deflected and contribute to the Fermi gamma-ray flux of radio galaxies. We demonstrate that, in this scenario, the magnetic field can not be determined from a fit of the cascade emission to the gamma-ray spectrum alone, and the degeneracy can only be lifted if the synchrotron emission from the cascades is observed as well. We illustrate this fact with the example of NGC 1275. We point out that the cascade synchrotron emission may produce spectral features reminiscent of the big blue bump observed in the spectral energy distributions of several blazars, and illustrate this idea for 3C 279.