Blazar jets launched with similar energy per baryon, independently of their power

TL;DR

This paper proposes that all blazar jets are launched with similar energy per baryon, explaining the observed diversity in their emission properties and the blazar sequence through differences in jet acceleration and magnetization.

Contribution

It introduces a simple model where blazar jet properties depend on a universal energy per baryon, accounting for the entire blazar population and their spectral differences.

Findings

Simulations reproduce the blazar sequence.

Jets with similar energy per baryon explain observed spectra.

Model constrains jet composition across blazar types.

Abstract

The most extreme active galactic nuclei (AGN) are the radio active ones whose relativistic jet propagates close to our line of sight. These objects were first classified according to their emission line features into flat-spectrum radio quasars (FSRQs) and BL Lacertae objects (BL Lacs). More recently, observations revealed a trend between these objects known as the \emph{blazar sequence}, along with an anti-correlation between the observed power and the frequency of the synchrotron peak. In the present work, we propose a fairly simple idea that could account for the whole blazar population: all jets are launched with similar energy per baryon, independently of their power. In the case of FSRQs, the most powerful jets, manage to accelerate to high bulk Lorentz factors, as observed in the radio. As a result, they have a rather modest magnetization in the emission region, resulting in magnetic reconnection injecting a steep particle energy distribution and, consequently, steep emission spectra in the $\gamma$-rays. For the weaker jets, namely BL Lacs, the opposite holds true; i.e., the jet does not achieve a very high bulk Lorentz factor, leading to more magnetic energy available for non-thermal particle acceleration, and harder emission spectra at frequencies $\gtrsim$ GeV. In this scenario, we recover all observable properties of blazars with our simulations, including the \emph{blazar sequence} for models with mild baryon loading ($50 \lesssim \mu \lesssim 80$). This interpretation of the blazar population, therefore, tightly constrains the energy per baryon of blazar jets regardless of their accretion rate.