SSC radiation in BL Lac sources, the end of the tether

TL;DR

This paper investigates the SSC radiation process in BL Lac objects, deriving spectral relations and variability predictions, and identifies the model's limitations during rapid TeV flares, suggesting the need for additional components.

Contribution

It provides a detailed analysis of SSC spectral relations and variability in BL Lacs, highlighting the model's applicability and limitations during extreme flaring events.

Findings

The SSC model explains spectral correlations in many LBL sources.

Thomson regime suffices for many sources, but not for rapid TeV flares.

Fast flares imply the presence of a second emission component.

Abstract

The synchrotron-self Compton (SSC) radiation process is widely held to provide a close representation of the double peaked spectral energy distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal beamed radiations, highly variable on timescales of days or less. Their outbursts in the gamma ray relative to the optical/X rays might be surmised to be enhanced in BL Lacs as these photons are upscattered via the inverse Compton (IC) process. From the observed correlations among the spectral parameters during optical/X-ray variations we aim at predicting corresponding correlations in the gamma-ray band, and the actual relations between the gamma-ray and the X-ray variability consistent with the SSC emission process. We start from the homogeneous single-zone SSC source model, with log-parabolic energies distributions of emitting electron as required by the X-ray data of many sources. We find relations among spectral parameters of the IC radiation in both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly for High energy BL Lacs) and we compute how variability is driven by a smooth increase of key source parameters, primarily the root mean square electron energy. The single component SSC source model in the Thomson regime turns out to be adequate for many LBL sources. However, the simple model meets its limits with the fast/strong flares recently reported for a few sources in the TeV range; these require sudden accelerations of emitting electrons in a second source component.