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

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

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Galaxies is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

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