A one-parameter two-zone leptonic model for the blazar sequence

TL;DR

This paper introduces a self-consistent two-zone leptonic model that explains the blazar sequence by linking emission characteristics to the accretion rate onto the black hole, unifying diverse blazar behaviors.

Contribution

It presents a novel two-zone leptonic model that reproduces the blazar sequence by varying only the accretion rate, explaining different blazar types and their spectral features.

Findings

Model reproduces blazar sequence features

External Compton dominates in FSRQs

Synchrotron and SSC dominate in BL Lacs

Abstract

Blazars, a subclass of radio-loud active galactic nuclei with relativistic jets aligned close to our line of sight, emit highly variable non-thermal radiation across the electromagnetic spectrum. The physical origin of their emission and the blazar sequence remain open questions. We present a self-consistent two-zone leptonic model in which relativistic electrons accelerate in a compact region, losing energy via synchrotron and inverse Compton processes, and escape into a larger zone permeated by an external photon field associated with magnetohydrodynamic winds from the accretion disk. By varying only the mass accretion rate onto the central black hole, the model naturally reproduces the blazar sequence, including Compton Dominance, $\gamma$-ray spectral indices, and the positions of synchrotron and inverse Compton peaks, while variations in secondary parameters account for the observed spread in the data. Flat Spectrum Radio Quasars exhibit strong external Compton emission from the extended zone, whereas BL Lac objects are dominated by synchrotron and synchrotron self-Compton emission from the compact acceleration region. This framework highlights the key role of accretion rate and spatially structured emission zones in shaping blazar spectra and provides a unified interpretation of their diverse phenomenology.