The variability of blazars throughout the electromagnetic spectrum

TL;DR

This review synthesizes how blazars exhibit extreme flux, spectral, and polarization variability across the electromagnetic spectrum, driven by intrinsic particle acceleration processes and geometric effects such as jet bending and Doppler beaming. It highlights time-series methods and multiwavelength monitoring programs that uncover correlations, time delays, and rapid microvariability, and discusses one-zone versus multi-zone emission models, including energy-stratified and spine–sheath configurations. The paper also covers the connection between blazar variability and neutrino production, and examines the roles of shocks, magnetic reconnection, turbulence, and instabilities in shaping observed light curves and polarization. IXPE and other polarization studies increasingly favor complex, multi-zone jet structures, while microlensing, periodicity claims, and line variability add further layers of insight and complication, underscoring that a unified model remains elusive but within reach with upcoming surveys and observatories.

Abstract

With their jet pointing towards us, blazars are ideal tools to study the physics and structure of extragalactic jets. Their powerful jets are cosmic particle accelerators and are alleged to be one of the production sites of the high-energy neutrinos detected by the IceCube Observatory. Doppler beaming of the jet nonthermal radiation increases blazar brightness, blue-shifts their emission, and shortens their variability time scales, which are observed to range from years down to minutes. This review will focus on blazar flux, spectral, and polarization variability across the electromagnetic spectrum. Interpretation of blazar variability calls into question both intrinsic and extrinsic mechanisms. Shock waves, magnetic reconnection, and turbulence can accelerate particles inside the jet, while jet precession, rotation, and twisting can produce variations in Doppler beaming. Microvariability puts strong constraints on the size of the emitting regions, suggesting a multizone emitting jet. Twisting jets have been proposed to explain the long-term multiwavelength variability. They are supported by radio observations of bent or helical jets, and by results of relativistic magnetohydrodynamics simulations of plasma jets. Detection of (quasi)periodic behaviour has been ascribed to orbital motion in black hole binary systems, jet precession, kink instabilities developing inside the jet, or perturbations in the accretion disc. Gravitational microlensing has been suggested to explain blazar behaviour in some cases. Polarization provides information on the structure and behaviour of the magnetic field in the emission zones. Both the degree and angle of polarization can show strong and fast variability, which is sometimes correlated with flux. The interpretation of flux, spectral, and polarization variability within a consistent picture challenges current models of blazar variability.

Figures (15)

• Figure 1: Spectra of the FSRQ CTA 102 in different brightness states. When the flux is low the spectra show strong emission lines from the BLR and the "big blue bump" due to the contribution of thermal radiation from the accretion disc. As the jet continuum increases, the lines gradually disappear and in the highest flux levels the object displays a featureless spectrum, which is typical of BL Lac objects. Image reproduced with permission from raiteri2017_nature, copyright by Macmillan.
• Figure 2: Spectral energy distribution of different blazar types obtained with data acquired at different epochs. Top-left: 4C 71.07, a FSRQ with a prominent contribution from the big blue bump (bbb) in the optical--UV spectral range, and a strong Compton dominance. Top-right: BL Lacertae, an LBL characterised by strong variability at all wavelengths. Bottom-left: Mkn 501, an HBL with a prominent contribution from the host galaxy (host) in the near-IR to optical spectral range, a strong variability in the X-rays and at TeV energies, and a high-energy bump peak lower than the low-energy bump peak. Bottom-right: 1ES 0229+200, an EHBL with a strong host-galaxy contribution and a low peak of the high-energy bump. Data for the plots were obtained through the ASI-SSDC SED builder tool (https://tools.ssdc.asi.it/SED/), including multiwavelength data from various observing facilities. A few strong outliers have been removed.
• Figure 3: The blazar sequence by ghisellini2017 (labeled "NEW", left), which is based on bins in $\gamma$-ray luminosities, compared to the original blazar sequence by fossati1998 (labeled "OLD", right), which considers bins in radio luminosity. FSRQs present the largest luminosities and lowest synchrotron (and inverse-Compton) peak frequencies, together with high Compton dominance, while HBLs display the lowest luminosities and the highest $\nu_{\rm S}$ and $\nu_{\rm IC}$. LBLs are located at intermediate positions. Image reproduced with permission from ghisellini2017, copyright by the author(s).
• Figure 4: Multiwavelength behaviour of the HBL Mkn 421 in 2007--2009. From top to bottom: light curves i) at TeV energies from the MAGIC telescope, ii) in the hard X-rays from the BAT instrument onboard the Swift satellite, iii) in the soft X-rays from the ASM detector onboard the RXTE satellite, iv) in the optical $R$-band from the WEBT, and v) at 37 and 15 GHz radio frequencies from the Metsähovi Observatory and OVRO, respectively. Image reproduced with permission from ahnen2016, copyright by ESO
• Figure 5: Examples of short-term optical variability in blazars. Top: $R$-band light curve of BL Lacertae in 2000 obtained from ground observations by the WEBT Collaboration villata2002. The green boxes highlight some of the most significant periods. Bottom: Kepler-K2 (black symbols) light curve of OJ 287 in 2018; additional ground-based data are shown in blue and magenta wehrle2023.