Challenging the high-energy emission zone in FSRQs

TL;DR

This study challenges the traditional view that high-energy emission in FSRQs occurs inside the broad-line region, by analyzing simultaneous gamma-ray observations that suggest a more complex jet structure and emission zone location.

Contribution

It provides evidence that the gamma-ray emission region in FSRQs can be outside the broad-line region, indicating the need to reconsider existing models of blazar emission zones.

Findings

Gamma-ray spectrum fits a single power law from 3 GeV to 400 GeV.

Absence of spectral cutoff implies emission outside the broad-line region.

Rapid variability suggests a very compact emission region.

Abstract

The blazar zone in quasars is commonly assumed to be located inside the broad-line region at some hundreds of Schwartzschild radii from the central black hole. Now, the simultaneous Fermi/LAT and MAGIC observations of a strong flare in the FSRQ PKS 1222+21 (4C 21.35, z=0.432) on 2010 June 17 challenge this picture. The spectrum can be described by a single power law with photon index 2.72+/-0.34 between 3 GeV and 400 GeV, and this is consistent with emission from a single component in the jet. The absence of a spectral cutoff constrains the gamma-ray emission region to lie outside of the broad-line region, which would otherwise absorb the VHE gamma-rays. On the other hand, the MAGIC measurement of a doubling time of about 10 minutes indicates an extremely compact emission region, in conflict with the "far dissipation" scenario. This could be a hint for the importance of jet sub-structures, such as filaments, reconnection zones or shear layers for the occurrence of blazar flares.