An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?

TL;DR

This paper identifies and analyzes a new group of extreme BL Lac objects with potential for probing extragalactic background light and magnetic fields, expanding the known sample and assessing their detectability with current and future gamma-ray telescopes.

Contribution

The study introduces a method to select new EHBL candidates from existing samples and confirms their nature through multi-wavelength data, estimating their VHE flux for future observations.

Findings

Most candidates have VHE flux within reach of current Cherenkov telescopes.

The selected EHBLs exhibit properties consistent with extreme high-energy peaked BL Lacs.

The sample expansion aids in using EHBLs as probes for cosmic magnetic fields and background light.

Abstract

High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. However, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates starting from the BL Lac sample of Plotkin et al. (2011), considering those undetected (or only barely detected) by the Large Area Telescope onboard Fermi and characterized by a high X-ray vs. radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, profiting of publicly available archival observations performed by the Swift, Galex and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected VHE flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming Cherenkov Telescope Array (CTA).