Extremely High energy peaked BL Lac (EHBL) objects are a special class of blazars with peculiar observational properties at X-ray and γ-ray energies. The observations of these sources indicate hard X-ray and γ-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 γ-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. We find that the spectral features of the blazar Mrk 501 indicate an EHBL nature of the source. Whereas, the temporal characteristics with fast variability during the high activity state of the source in X-ray and γ-ray energy bands are not compatible with the behaviour of EHBL type of blazars. However, Mrk 501 can be considered as an EHBL candidate in its low emission state. We also discuss the implications of identifying more EHBL objects using present and future ground-based γ-ray observatories.
- Pub Date:
- November 2019
- BL Lacertae objects: Individual Mrk 501;
- Radiation mechanisms: Non-thermal;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- 4 Figures, Accepted for Publication in New Astronomy, Abstract shortened