Luminous and high-frequency peaked blazars: the origin of the $\gamma$-ray emission from PKS 1424+240

Abstract

The current generation of Cherenkov telescopes, together with Fermi-LAT, have greatly increased our knowledge of $\gamma$-ray blazars. Among them, the high-frequency-peaked BL Lacertae object (HBL) PKS1424+240 (z $\simeq$ 0.6) is the farthest persistent emitter of very-high-energy (VHE; E $\ge$ 100 GeV) $\gamma$-ray photons. Current emission models can satisfactorily reproduce typical blazar emission assuming that the dominant emission process is synchrotron-self-Compton (SSC) in HBLs; and external-inverse-Compton (EIC) in LBLs and FSRQs. Hadronic models are also able to correctly reproduce the $\gamma$-ray emission from blazars, although they are in general disfavoured for bright quasars and rapid flares.The blazar PKS1424+240 is a rare example of a luminous HBL, and we aim to determine which is the emission process most likely responsible for its $\gamma$-ray emission. This will impact more generally our comprehension of blazar emission models, and how they are related to the luminosity of the source and the peak frequency. We investigate different blazar emission models applied to the SED of PKS1424+240. Among leptonic models, we study a 1-zone SSC model, a 2-zone SSC model, and an EIC model. We then investigate a blazar hadronic model, and finally a scenario in which the $\gamma$-ray emission is associated with cascades in the line-of-sight produced by cosmic rays from the source. After a systematic study of the parameter space of the one-zone SSC model, we conclude that this scenario is not compatible with $\gamma$-ray observations of PKS1424+240. A two-zone SSC scenario can alleviate this issue, as well as an EIC solution. For the latter, the external photon field is assumed to be the infra-red radiation from the dusty torus. Alternatively, hadronic models can satisfactorily reproduce the $\gamma$-ray emission from PKS1424+240, both as in-source emission and as cascade emission.

11 pages, 8 figures; in press in A&A; abstract abridged

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• Testing CIBER cosmic infrared background measurements and axionlike particles with observations of TeV blazars
• A Two-Zone Model as origin of Hard TeV Spectrum in Extreme BL Lacs
• Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240
• Looking for the possible gluon condensation signature in sub-TeV gamma-ray spectra: from active galactic nuclei to gamma ray bursts
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• Very-high-energy gamma-ray detection and long-term multi-wavelength view of the flaring blazar B2 1811+31
• The ablation of gas clouds by blazar jets
• Multiwavelength temporal and spectral study of TeV blazar 1ES 1727+502 during 2014 to 2021
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