Synchrotron and inverse-Compton emission from blazar jets I: a uniform conical jet model

TL;DR

This paper models synchrotron and inverse-Compton emission from a conical blazar jet, fitting multi-wavelength data for BL Lac without fine-tuning or re-acceleration, providing insights into jet physics.

Contribution

It presents the first simultaneous fit of synchrotron and SSC emission for a blazar using a uniform conical jet model with dynamic electron evolution.

Findings

Conical jet model reproduces flat radio spectrum without fine-tuning.

No re-acceleration needed to match BL Lac spectrum.

Model incorporates Klein-Nishina effects and detailed opacity calculations.

Abstract

In the first of a series of papers investigating emission from blazar jets from radio to high-energy {\gamma}-rays, we revisit the class of models where the jet has a uniform conical ballistic structure. We argue that by using simple developments of these models, in the context of new multi-frequency data extending to gamma-ray energies, valuable insights may be obtained into the properties that fully realistic models must ultimately have. In this paper we consider the synchrotron and synchrotron-self-Compton emission from the jet, modelling the recent simultaneous multi-wavelength observations of BL Lac. This is the first time these components have been fitted simultaneously for a blazar using a conical jet model. In the model we evolve the electron population dynamically along the jet taking into account the synchrotron and inverse-Compton losses. The inverse-Compton emission is calculated using the Klein-Nishina cross section and a relativistic transformation into the jet frame, and we explicitly show the seed photon population. We integrate synchrotron opacity along the line of sight through the jet plasma, taking into account the emission and opacity of each section of the jet. In agreement with previous studies of radio emission, we find that a conical jet model which conserves magnetic energy produces the characteristic blazar flat radio spectrum, however, we do not require any fine-tuning of the model to achieve this. Of particular note, in our model fit to BL Lac--which at ~10^37W is a relatively low jet-power source--we find no requirement for significant re-acceleration within the jet to explain the observed spectrum.