A hadronic minute-scale GeV flare from quasar 3C 279?

TL;DR

This paper investigates whether a hadronic proton synchrotron model can explain the rapid GeV gamma-ray flare observed in quasar 3C 279, considering electromagnetic cascade effects and jet environment constraints.

Contribution

It evaluates the viability of a hadronic proton synchrotron scenario for the flare, highlighting the challenges posed by electromagnetic cascades and jet conditions.

Findings

Electromagnetic cascades redistribute gamma-ray luminosity to softer bands.

Proton synchrotron origin is disfavored due to environmental constraints.

High Doppler factors and magnetic fields are required for alternative scenarios.

Abstract

The flat spectrum radio quasar 3C 279 is a known $\gamma$-ray variable source that has recently exhibited minute-scale variability at energies $>100$ MeV. One-zone leptonic models for blazar emission are severely constrained by the short timescale variability that implies a very compact emission region at a distance of hundreds of Schwarzschild radii from the central black hole. Here, we investigate a hadronic scenario where GeV $\gamma$-rays are produced via proton synchrotron radiation. We also take into account the effects of the hadronically initiated electromagnetic cascades (EMC). For a $\gamma$-ray emitting region in rough equipartition between particles and kG magnetic fields, located within the broad-line region (BLR), the development of EMC redistributes the $\gamma$-ray luminosity to softer energy bands and eventually leads to broad-band spectra that differ from the observed ones. Suppression of EMC and energy equipartition are still possible, if the $\gamma$-ray emitting region is located beyond the BLR, is fast moving with Doppler factor ($>70$), and contains strong magnetic fields ($>100$ G). Yet, these conditions cannot be easily met in parsec-scale jets, thus disfavouring a proton synchrotron origin of the Fermi-LAT flare.