Constraining the synchrotron peak and estimating the very high energy brightness of a sample of extreme high synchrotron peak blazars

¹ Dipartimento di Fisica e Astronomia (DIFA) Augusto Righi, Università di Bologna, Via Gobetti 93/2, I-40129, Bologna, Italy
² INAF-Osservatorio di Astrofisica e Scienza dello Spazio (OAS), Via Gobetti 93/3, I-40129, Bologna, Italy
³ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC, 29634, USA
⁴ INFN Sezione di Padova and Università degli Studi di Padova, Via Marzolo 8, I-35131, Padova, Italy
⁵ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738, Zeuthen, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 December 2025
Accepted: 20 February 2026

Abstract

We present the results of a multi-wavelength study of a population of X-ray bright (log(F_{0.2 − 12 keV}) > − 12.5), non-γ-ray detected high and extreme high synchrotron peak (HSP, EHSP; log(ν_{peak,  Hz}) > 16) BL Lacs to (i) put stronger constraints on the synchrotron peak location and shape and (ii) model their expected behaviour in the very high-energy band. First, we performed an X-ray spectral analysis, using XMM-Newton, Chandra, Swift–XRT, and eROSITA data, and fitting the spectra using both a power-law and a log parabola model. Of 78 sources in the initial sample, 17 were best described by a log parabola model, a result that supports a scenario where the synchrotron peak falls in the X-ray band. Among these 17 sources, we further selected the ten objects dominated by the jet emission, with no significant contamination of the host galaxy. We performed a γ-ray analysis of Fermi-LAT data for these objects, obtaining upper limits providing information on their flux in the 100 MeV–300 GeV energy range. We then modelled the broadband SED of these objects with JetSeT using two models: one assuming a log parabola for the electron distribution and the other one with a broken power-law electron distribution, using parameters consistent with those describing the emission of the prototypical EHSP 1ES 0229+200. We found the models to be generally consistent with the available multi-wavelength detections and upper limits. Furthermore, they confirmed that a subsample of sources could display relevant emission in the TeV energy range, even potentially reaching the threshold for detectability by the Cherenkov Telescope Array Observatory.