Particle-acceleration timescales in TeV blazar flares

TL;DR

This paper compares four particle acceleration mechanisms in TeV blazar flares, concluding that some are excluded based on timescale constraints, while others remain viable, with stochastic acceleration being a promising candidate.

Contribution

It systematically evaluates acceleration mechanisms considering timescales and losses, identifying which processes can explain rapid TeV flare observations.

Findings

Shear acceleration and neutron-based mechanisms are excluded due to timescale constraints.

First-order Fermi and converter mechanisms can be nearly instantaneous with sufficient turbulence.

Stochastic acceleration is a promising explanation for observed gamma-ray time delays.

Abstract

Observations of minute-scale flares in TeV Blazars place constraints on particle acceleration mechanisms in those objects. The implications for a variety of radiation mechanisms have been addressed in the literature; in this paper we compare four different acceleration mechanisms: diffusive shock acceleration, second-order Fermi, shear acceleration and the converter mechanism. When the acceleration timescales and radiative losses are taken into account, we can exclude shear acceleration and the neutron-based converted mechanism as possible acceleration processes in these systems. The first-order Fermi process and the converter mechanism working via SSC photons are still practically instantaneous, however, provided sufficient turbulence is generated on the timescale of seconds. We propose stochastic acceleration as a promising candidate for the energy-dependent time delays in recent gamma-ray flares of Markarian 501.