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Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects
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Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (5 May 2026) | Viewed by 1696

Special Issue Editors

Dr. Bidzina Kapanadze

E-Mail Website
Guest Editor
1. Space Research Center, Department of Astronomy and Astrophysics, School of Natural Sciences and Medicine, Ilia State University, Colokashvili Av. 3/5, Tbilisi 0162, Georgia
2. E. Kharadze National Astrophysical Observatory, Mt. Kanobili, Abastumani 0803, Georgia
Interests: AGN physics and multiwavelength variability; relativistic jets; high-energy astronomy; Fermi acceleration; non-thermal emission
Special Issues, Collections and Topics in MDPI journals
Dr. Stefano Vercellone

E-Mail Website
Guest Editor
INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, LC, Italy
Interests: multi-wavelength properties of active galactic nuclei (especially in the optical, ultra-violet, X-ray, and gamma-ray energy bands); AGN observations via the NASA/Swift and the ASI/AGILE and NASA/Fermi satellites

Special Issue Information

Dear Colleagues,

High-Energy-Peaked BL Lacertae Objects (HBLs) form one of the most extreme blazar groups, according to the multiwavelength (MWL) observational features and underlying hypothetic physical processes. By representing the active galactic nuclei (AGN) of elliptical galaxies, these sources also should be powered by matter accretion onto central supermassive black holes and possess a pair of oppositely directed jets of magnetized, high-energy plasma moving outward with relativistic speeds and aligned closely to the line of sight. Individual particles should be accelerated to the highest energies among AGNs, since the peak of the synchrotron spectral energy distribution (SED) is generally located in X-rays and sometimes are observed at the energies beyond 10 keV during the strong X-ray flares. On the contrary, this peak can be observed at the UV frequencies during the low X-ray states. The origin of the higher-energy SED component, representing the gamma-ray range in HBLs and generally peaking beyond 100 GeV, is still controversial, and different emission models (leptonic and hadronic) are proposed. Similarly to other BL Lac groups, HBLs exhibit featureless optical spectra, strong MWL variability with timescales from years (radio to optical) down to a few hundred seconds (X-rays and TeV-band), high and variable polarization, and compact radio structure. Since one cannot directly resolve the HBL emission zone due to its extremely small angular size, an intense MWL variability study provides an efficient tool for drawing conclusions about the structure of the jet emission zone. Along with the detailed X-ray and gamma-ray spectral studies, the MWL observations of HBLs allow us to make progress in solving different problems associated with the jet particle content, their acceleration up to ultrarelativistic energies, and unstable processes triggering the flux and spectral variations on different timescales. The present Special Issue is devoted to summarizing the observational achievements in diverse spectral ranges by means of the various ground-based instruments and space missions, the results from different theoretical studies and simulations of HBL jet physics, and the current understanding of the underlying unstable processes.

Dr. Bidzina Kapanadze
Dr. Stefano Vercellone
Guest Editors

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Keywords

  • blazars
  • bl lacertae objects
  • blazar variability
  • multi-wavelength
  • acceleration mechanisms
  • emission models
  • jet instabilities

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Published Papers (5 papers)

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Review

56 pages, 2380 KB  
Review
Flux and Spectral Variability of High-Energy-Peaked BL Lacertae Objects in the 0.3–10 keV Band
by Bidzina Kapanadze
Galaxies 2026, 14(3), 57; https://doi.org/10.3390/galaxies14030057 - 25 May 2026
Abstract
BL Lacertae objects (BL Lacs) are active galactic nuclei notable for beamed emission generated in the relativistic jets, forming a small angle with respect to our line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). The group of [...] Read more.
BL Lacertae objects (BL Lacs) are active galactic nuclei notable for beamed emission generated in the relativistic jets, forming a small angle with respect to our line-of-sight. The broadband spectra of BL Lacs show a two-component spectral energy distribution (SED). The group of high-energy-peaked BL Lacs (HBLs) exhibit their lower-energy SED peak at the UV to X-ray frequencies. Consequently, these objects are generally bright in the 0.3–10 keV band (compared to other blazar subclasses) and allow us to carry out intense timing/spectral studies on the wide range of timescales (from years down to a few minutes). Although X-ray emission of HBLs is widely accepted to have a synchrotron origin (along with the occasional presence of the inverse-Compton component), many problems associated with the jet particle content, their acceleration up to ultra-relativistic energies and unstable mechanisms responsible for the extreme flux/spectral variability still remain to be solved. This review highlights the basic timing and polarimetric and spectral results obtained in the framework of the numerous studies of HBLs in the 0.3–10 keV band, which was covered by the X-ray instruments operating onboard the different space missions. Moreover, the plausible physical processes responsible for the observed HBL features (relativistic shocks, magnetic reconnection, turbulence etc.) are also addressed. Full article
(This article belongs to the Special Issue Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects)
29 pages, 2053 KB  
Review
Optical Variability of HBLs on Diverse Timescales
by Ileana Andruchow, Ezequiel J. Marchesini and Florencia L. Vieyro
Galaxies 2026, 14(3), 54; https://doi.org/10.3390/galaxies14030054 - 20 May 2026
Viewed by 276
Abstract
Since their discovery almost 60 years ago, BL Lac objects have been defined by their strong optical variability and their classification in the spectral energy distribution scheme. High-synchrotron-peaked BL Lacs (HBLs) are those whose synchrotron component peaks at frequencies higher than UV/X-rays. Historically, [...] Read more.
Since their discovery almost 60 years ago, BL Lac objects have been defined by their strong optical variability and their classification in the spectral energy distribution scheme. High-synchrotron-peaked BL Lacs (HBLs) are those whose synchrotron component peaks at frequencies higher than UV/X-rays. Historically, optical variability studies have focused mostly on their counterparts, low-synchrotron-peaked BL Lacs (LBLs), since HBLs have shown weaker optical variability. However, a population-wide study of HBL optical variability is still lacking, and it remains unclear whether HBLs are intrinsically less optically variable as a class or whether this reflects observational biases. Only a handful of HBL sources have been studied extensively due to their strong variability and reported periodicity. These sources have motivated several theoretical models, often conflicting, to explain the optical variability when present. Nevertheless, understanding the connection between the apparent weaker optical variability and the emission processes of HBLs remains a challenge. In this work, we review the current state of knowledge on this topic, with the expectation that upcoming optical monitoring observatories, such as the Vera C. Rubin, will provide new insights into the optical emission (and variability) mechanisms in HBLs. Full article
(This article belongs to the Special Issue Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects)
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41 pages, 770 KB  
Review
TeV-Band Properties of Nearby HBLs
by Bidzina Kapanadze and Stefano Vercellone
Galaxies 2026, 14(3), 45; https://doi.org/10.3390/galaxies14030045 - 13 May 2026
Viewed by 258
Abstract
Nearby (z<0.1) TeV-detected, high-energy-peaked BL Lacertae objects (HBLs) are among the most prominent extragalactic sources of the highest-energy photons, sometimes detected at energies of ∼10 TeV or beyond. These objects show a strong and complex flux variability, with strong [...] Read more.
Nearby (z<0.1) TeV-detected, high-energy-peaked BL Lacertae objects (HBLs) are among the most prominent extragalactic sources of the highest-energy photons, sometimes detected at energies of ∼10 TeV or beyond. These objects show a strong and complex flux variability, with strong flares and exceptional outbursts, as well as very rapid and large-amplitude TeV-band variations on timescales down to a few minutes during such instances. The higher-energy component of broadband spectral energy distribution (SED) is stretched over the MeV–TeV domain and, generally peaking beyond 100 GeV, has a controversial origin, and different emission scenarios (one- or multi-zone synchrotron self-Compton, hadronic cascades, etc.) are proposed. This paper presents a review of the TeV-band timing and spectral results obtained in the framework of different observational campaigns for nearby HBLs, their implications for different emission scenarios, and basic results from the corresponding SED modelings. Finally, the prospect of filling the observational gaps above some threshold energy by means of the planned projects for the dedicated γ-ray observations and, consequently, solving the different persisting problems related to the innermost structure, particle acceleration, and emission mechanisms are also presented. Full article
(This article belongs to the Special Issue Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects)
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63 pages, 3517 KB  
Review
High-Synchrotron-Peaked BL Lacs as Multi-Messenger Sources: Connecting Ultra-High-Energy Cosmic Rays and Neutrinos
by Luiz Augusto Stuani Pereira and Rita C. Anjos
Galaxies 2026, 14(3), 40; https://doi.org/10.3390/galaxies14030040 - 30 Apr 2026
Viewed by 248
Abstract
High-synchrotron-peaked (HSP) BL Lac objects are extreme particle accelerators whose synchrotron emission peaks at high frequencies, typically in the UV-to-X-ray band (νpeak>1015 Hz; νpeak≥1017 for EHSPs), implying electron Lorentz factors of order 105 [...] Read more.
High-synchrotron-peaked (HSP) BL Lac objects are extreme particle accelerators whose synchrotron emission peaks at high frequencies, typically in the UV-to-X-ray band (νpeak>1015 Hz; νpeak≥1017 for EHSPs), implying electron Lorentz factors of order 105–106. Their relative proximity (z≲0.5), clean radiation environments, and favorable Hillas parameters make them prime candidates for ultra-high-energy cosmic ray (UHECR) acceleration beyond 1019 eV and for neutrino production above 100 TeV. The 2017 association of IceCube-170922A with the flaring blazar TXS 0506+056 provided compelling evidence for blazars as neutrino sources, while an archival neutrino flare from 2014–2015 with no clear electromagnetic counterpart (13 events) revealed additional complexity in the emission mechanism. This review examines HSP physical properties, identifies them through WISE-based infrared selection (the 2WHSP and 3HSP catalogs, ∼2000 sources), and contrasts leptonic synchrotron self-Compton models with hadronic alternatives. We assess the observational evidence linking HSPs to high-energy neutrinos and UHECRs, finding that extreme baryonic loading (Lp/Le∼103–105) strains energetic budgets, Auger composition measurements favor heavy nuclei over proton-dominated scenarios, and the near-isotropy of UHECR arrival directions is difficult to reconcile with rare beamed sources. Potential resolutions involving magnetic reconnection, structured jets, and duty cycle effects are discussed. Next-generation facilities, including IceCube-Gen2, KM3NeT, CTAO, IXPE, and AugerPrime/TA × 4, will probe key observables to either establish HSP BL Lacs as sources of the highest-energy cosmic particles or redirect the search toward alternative accelerator classes. Full article
(This article belongs to the Special Issue Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects)
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11 pages, 14049 KB  
Review
VLBI Observations of Nearby HBLs: Physical Implications
by Svetlana G. Jorstad, Alan P. Marscher and José L. Gómez
Galaxies 2026, 14(2), 36; https://doi.org/10.3390/galaxies14020036 - 21 Apr 2026
Viewed by 299
Abstract
We review the jet kinematics of HBL blazars based on results of the MOJAVE survey, which obtained images of active galactic nuclei with the Very Long Baseline Array (VLBA) at 15 GHz, and the VLBA-BU-BLAZAR program as well as its successor BEAM-ME program, [...] Read more.
We review the jet kinematics of HBL blazars based on results of the MOJAVE survey, which obtained images of active galactic nuclei with the Very Long Baseline Array (VLBA) at 15 GHz, and the VLBA-BU-BLAZAR program as well as its successor BEAM-ME program, which have observed γ-ray blazars at 43 GHz. We present and discuss recent kinematic behavior and polarization properties of the parsec-scale jets of three typical HBL sources over the 2020–2025 period. We outline current physical implications for reconciling the high-energy characteristics of HBL sources with their parsec-scale jet properties. Full article
(This article belongs to the Special Issue Multiwavelength Variability and Unstable Processes in High-Energy-Peaked BL Lacertae Objects)
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