Extreme BL Lacs: probes for cosmology and UHECR candidates

TL;DR

This paper identifies and characterizes a new sample of extreme BL Lac objects, which are promising probes for cosmology and ultra-high-energy cosmic ray research, by analyzing their multi-wavelength data and modeling their high-energy emissions.

Contribution

The study expands the known population of extreme BL Lac objects by selecting candidates based on Fermi LAT non-detections and high X-ray to radio flux ratios, and models their potential for VHE gamma-ray detection.

Findings

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

Multi-wavelength data confirms the nature of the selected EHBL candidates.

The results support the use of EHBLs as probes for extragalactic background light and intergalactic magnetic fields.

Abstract

High-energy observations of extreme BL Lac objects, such as 1ES0229+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. Moreover, their enigmatic properties have been interpreted in a scenario in which their primary high- energy output is through a beam of high-energy hadrons. However, despite their possible important role in all these topics, 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 considering those undetected (or only barely detected) by the LAT onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, using available archival data of Swift, GALEX, and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected very high energy flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming CTA.