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

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy
² Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15783 Athens, Greece
³ NASA Goddard Space Flight Center, Greenbelt, MD, USA

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 October 2025
Accepted: 12 December 2025

Abstract

Blazars, a subclass of radio-loud active galactic nuclei with relativistic jets aligned close to our line of sight, emit highly variable nonthermal 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, γ-ray spectral indices, and the positions of synchrotron and inverse Compton peaks, while variations in the 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.