Optically thick jet base and explanation of edge brightening in AGN jets

TL;DR

This paper explains the edge brightening observed in AGN jets as a result of an angular-dependent Lorentz factor in optically thick jet bases, supported by analytical and Monte Carlo simulations.

Contribution

It introduces a model where jet Lorentz factor varies with angle, leading to edge brightening, and demonstrates this with analytical and numerical methods.

Findings

Edge brightening explained by angular Lorentz factor dependence

Monte Carlo simulations confirm the analytical model

Provides a new way to probe jet structure and parameters

Abstract

The jet cores in blazars are resolved and found to harbour an edge brightened structure where the jet base appears extended at sides compared to its propagation axis. This peculiar phenomenon invites various explanations. We show that the photosphere of an optically thick jet base in Active Galactic Nuclei (AGNs) is observed edge brightened if the jet Lorentz factor harbours an angular dependence. The jet assumes a higher Lorentz factor along the jet axis and decreases following a power law along its polar angle. For an observer near the jet axis, the jet has a lower optical depth along its propagation axis compared to off axis regions. Higher optical depths at the outer region makes the jet photosphere appear to extend to larger radii compared to a deeper photosphere along its propagation axis. We tackle the problem both analytically and numerically, confirming the edge brightening through Monte Carlo simulations. Other than the edge brightening, the outcomes are significant as they provide a unique tool to determine the jet structure and associated parameters by their resolved observed cores. The study paves way to explore the spectral properties of optically thick cores with structured Lorentz factors in the future.