Particle acceleration signatures in the time-dependent one-zone synchrotron self-Compton model of blazar flares

TL;DR

This work interrogates rapid blazar flares within a time-dependent one-zone SSC framework to map how different particle injection and acceleration processes imprint on multiwavelength light curves. Using the EMBLEM code, it systematically compares scenarios of simple injection, Fermi I, and Fermi II acceleration (including reacceleration and adiabatic expansion) under hard-sphere scattering, and derives characteristic light-curve signatures such as energy-dependent onsets, peak delays, plateau behavior, and hysteresis. The study demonstrates that these LC features differ across scenarios and that well-sampled, multi-band data are crucial to distinguish the underlying microphysics, with an illustrative application to a 2013 Mrk 421 flare highlighting both potential and limitations. The results provide concrete, testable predictions for current and next-generation facilities (e.g., CTAO) and point toward future enhancements including external photons and multi-zone modeling to capture the full diversity of blazar flares.

Abstract

The study of multiwavelength flux and spectral variations during rapid flares from blazars provides strong constraints on the physical parameters of the compact emission regions responsible for these still poorly understood events. Although a full description of the continuous and transient emission from blazars seems to require more sophisticated scenarios, particle acceleration and loss mechanisms can be approximately described within the simple leptonic one-zone framework, enabling a systematic study of their impact on the observable properties of multiwavelength flare light curves. Our goal is to identify characteristic signatures in these light curve profiles that permit one to discriminate between the main physical processes situated inside the relativistic jet and commonly invoked to explain blazar flares. The present study exclusively focuses on modeling rapid flares from BL Lac type objects, which can be described within the synchrotron self-Compton (SSC) emission scenario. Combinations of several commonly employed mechanisms to describe the gain and loss of energetic particles in onezone models during flaring events are studied in a systematic way: particle injection; diffusive shock and stochastic acceleration and reacceleration; particle escape; adiabatic losses; radiative losses through synchrotron and inverse-Compton radiation. The current study is limited to the case of "hard-sphere" scattering. A large variety of light curve shapes arises from the different scenarios under study. Characteristic signatures, in particular energy-dependent time delays and differences in the shapes of the rising part of the flare, should allow the distinction to be made between different injection and acceleration scenarios, given the availability of sufficiently high-quality multiwavelength data sets. This is illustrated with a simplified application to a flare event from the blazar Mrk 421.

Figures (21)

• Figure 1: Example of LC obtained for the case of simple particle injection in the optical band, with the determined times of rise, plateau, and decay.
• Figure 2: (V)HE LC of a flare resulting from additional particle injection with adiabatic expansion for different injection rates (scenario 1b).
• Figure 3: (V)HE LC comparison of flares resulting from DSA for different shock timescales (scenario 2a).
• Figure 4: (V)HE LC comparison of flares resulting from Fermi I reacceleration for different timescales $t_{F_I}$ (scenario 2b).
• Figure 5: (V)HE LC comparison of flares resulting from hard-sphere Fermi II acceleration for different acceleration timescales $t_{F_{II}}$ (scenario 3a).