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Extremely high-peaked BL Lac objects (or extreme blazars) are unique extragalactic laboratories where particle acceleration processes are pushed at their physical limits. In these blazars, synchrotron emission peaking above keV energies is…

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. In blazars, the spectral energy distribution (SED) is dominated by the non-thermal emission of the relativistic jet,…

Blazars are very broadband cosmic sources with spectra spanning over twenty orders of magnitude in frequency, down to the 100 MHz regime in the radio range, up to VHE at several tens of TeV. The modelling of their spectral energy…

BL Lac objects are the best candidates to study the jet properties since their spectral energy distributions (SEDs) are less contaminated by the emission from the accretion disk and external Compton processes. We compile the broadband SEDs…

The one-zone synchrotron-self-Compton (SSC) model aims to describe the spectral energy distribution (SED) of BL Lac objects via synchrotron emission by a non-thermal population of electrons and positrons in a single homogeneous emission…

Extreme TeV blazars (ETBs) are active galactic nuclei with jets presumably pointing towards the observer having their intrinsic spectral energy distributions (SEDs) peaked at an energy in excess of 1 TeV. These sources typically reveal…

Extremely High energy peaked BL Lac (EHBL) objects are a special class of blazars with peculiar observational properties at X-ray and $\gamma$--ray energies. The observations of these sources indicate hard X-ray and $\gamma$--ray spectra…

The very hard $\gamma$-ray spectrum from distant blazars challenges the traditional synchrotron self-Compton (SSC) model, which may indicate that there is the contribution of an additional high-energy component beyond the SSC emission. In…

Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the spectral energy distribution (SED) plot with the…

In this paper, we compile the very-high-energy and high-energy spectral indices of 43 BL Lac objects from the literature. Based on a simple math model, $\Delta\Gamma_{obs}=\alpha {\rm{z}}+\beta $, we present evidence for the origin of an…

We study the requirement on the jet power in the conventional p-$\gamma$ models (photopion production and Bethe-Heitler pair production) for TeV BL Lac objects. We select a sample of TeV BL Lac objects whose SEDs are difficult to be…

The blazar 1ES 0229+200 is a high frequency peaked BL Lac object with a hard TeV spectrum extending to 10 TeV. Its unusual spectral characteristics make it a frequently used probe for intergalactic radiation and magnetic fields. With new,…

We focus our analysis on 55 BL Lac objects with a hard Fermi gamma-ray spectrum, and for which a redshift or a lower limit to it has been determined by a previous study of ours. We extrapolate the spectral fits given by the 4FGL catalogue…

1ES 1959+650 is a bright TeV high-frequency-peaked BL Lac object exhibiting interesting features like "orphan" TeV flares and a broad emission in the high-energy regime, that are difficult to interpret using conventional one-zone…

(Abridged) Very high-energy (VHE, $E>100$ GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone…

Very high energy (~ TeV) $\gamma$-rays from blazars are attenuated by photons from the extragalactic background light (EBL). Observations of blazars can therefore provide an ideal opportunity for determining the EBL intensity if their…

Most modeling attempts of blazars use a small emission zone located close to the central black hole in order to explain the broad-band spectral energy distribution. Here we present a case where additionally to the small region a…

Blazars have traditionally been separated into two broad categories based upon their optical emission characteristics; BL Lacs, with faint or no emission lines, and flat spectrum radio quasars (FSRQs) with prominent, broad emission lines.…

Observations of very-high-energy (VHE, E > 250 GeV) gamma-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR).…

Most of the extragalactic sources detected at TeV energies are BL Lac objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs) exhibiting spectral energy distributions with a lower energy peak in the X-ray band; this…