On the origin of the gamma-ray emission from the flaring blazar PKS 1222+216

TL;DR

This paper investigates the gamma-ray emission mechanisms of the blazar PKS 1222+216 during a flaring event, proposing models involving multiple emission regions outside the BLR to explain rapid variability and spectral features.

Contribution

It introduces multi-zone emission models for PKS 1222+216, challenging previous one-zone assumptions and suggesting jet recollimation at large distances from the black hole.

Findings

Rapid gamma-ray variability implies a compact emission region far beyond the BLR.

Spectral modeling favors inverse Compton scattering of IR radiation from the dusty torus.

Results disfavor models with emission from reconnection events or jet 'needles'.

Abstract

The flat spectrum radio quasar PKS 1222+216 was detected in the very high energy gamma-ray band by MAGIC during a highly active gamma-ray phase following an alert by the LAT onboard Fermi. Its relatively hard spectrum without a cut off, together with the observed variability on timescale of ~10 min challenges standard emission models. If the emission originates in a portion of the relativistic jet located inside the BLR, severe absorption of gamma rays above few tens of GeV is expected due to the pair production process. These observations imply the existence of a very compact (R_b ~5 x 10^{14} cm) and very fast blob located far beyond the BLR radius, responsible for the rapidly varying high energy flux. However the long term (days-weeks) coherent evolution of the GeV flux recorded by LAT indicates that there could be also the substantial contribution from another larger emission region. We model the spectral energy distribution of PKS 1222+216 during the epoch of the MAGIC detection assuming three different scenarios,: (1) a one-zone model considering only the emission from a compact blob outside the BLR; (2) a two-zone model considering the compact blob plus an emitting region filling the whole jet cross-section located outside the BLR and (3) a two zone model with the jet emitting region inside the BLR. We find that in all cases the high-energy emission from the compact blob is dominated by the inverse Compton scattering of the IR thermal radiation of the dusty torus. Both regions are matter dominated, with a negligible Poynting flux. These results disfavor models in which the compact blob is the result of reconnection events inside the jet or "needles" of high-energy electrons. Instead, the observational framework and our radiative models could be compatible with scenarios in which the jet is re-collimated and focussed at large distances from the central BH.