# Extremely High energy peaked BL Lac nature of the TeV blazar Mrk 501

## TL;DR

This paper investigates whether the blazar Mrk 501 exhibits the characteristics of an Extremely High energy peaked BL Lac (EHBL), using long-term observational data to compare its properties with known EHBLs.

## Contribution

The study provides a detailed analysis of Mrk 501's long-term data to assess its EHBL nature, contributing to the identification of potential new EHBL objects.

## Key findings

- Mrk 501 shows features similar to EHBL blazars during certain activity states.
- Comparison with known EHBLs suggests possible classification of Mrk 501 as EHBL.
- Results support the idea that Mrk 501 can exhibit EHBL-like behavior under specific conditions.

## Abstract

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 and absence of rapid flux variations in the multi-wavelength light curves. These observational features challenge the leptonic models for blazars due to unusually hard particle spectrum in the emission region of the blazar jet and provide a strong motivation for exploring alternative scenarios to interpret the broad-band emission from blazars. At present, only few TeV blazars have been observed as EHBL objects in the extragalactic Universe. Due to their hard $\gamma$--ray spectra and long term variability, the observations of EHBL type of blazars at different redshifts help in probing the cosmic magnetic field and extragalactic background light in the Universe. Such objects also provide astrophysical sites to explore the particle acceleration mechanisms like magnetic reconnection and second order Fermi acceleration. Therefore, it has become important to identify more objects as EHBL using the observations available in the literature. Recent studies on the blazar Mrk 501 indicate that this source may exhibit an EHBL behaviour. In this paper, we use long term observations of Mrk 501 to explore its nature. Two sets of data, related to low and high/flaring activity states of Mrk 501, have been presented and compared with the observed features of a few well known EHBL type of blazars.