TeV and keV-MeV Excesses as Probes for Hadronic Process in BL Lacertaes

TL;DR

This paper models the spectral energy distributions of certain BL Lac objects using a leptohadronic approach, explaining TeV and keV-MeV excesses as signatures of hadronic processes involving proton-photon interactions.

Contribution

It introduces a single-zone leptohadronic model that accounts for both TeV and keV-MeV excesses in BL Lacs, linking these features to hadronic interactions and cascade emissions.

Findings

TeV excess explained by pγ process via π0 decay.

Cascade electrons produce keV-MeV excess as a bump or plateau.

Proton power estimates are still above Eddington limit, challenging current black hole physics.

Abstract

A hard TeV $\gamma$-ray component excess over the single-zone leptonic model prediction (TeV excess) is observed in the spectral energy distributions (SEDs) of some BL Lacs. Its origin is uncertain. We revisit this issue with four BL Lacs (1ES 0229+200, 1ES 0347--121, 1ES 1101--232, and H2356--309), in which the TeV excess is detected in their intrinsic SEDs. We represent their SEDs with a single-zone leptohadronic model, where radiations of the electrons and protons as well as the cascade electrons produced by the $\gamma\gamma$ and p$\gamma$ interactions within their jets are considered. We show that the observed SEDs below the GeV gamma-ray band are attributed to the synchrotron radiations and self-Compton process of the primary electrons, and the TeV excess is explained with the $\gamma$-ray emission from the p$\gamma$ process via the $\pi^{0}$ decay. The cascade emission of the electrons produced via the $\gamma\gamma$ and p$\gamma$ interactions results in a keV-MeV excess in the SEDs, illustrated as a bump or plateau. This extra photon field enhances the production of TeV photons from the $p\gamma$ process, resulting in a reduction of the proton power by about one order of magnitude. However, the derived powers are still 3--4 orders of magnitude larger than the Eddington limit, being challenged by the current black hole accretion physics. Applying our model to Mrk 421, we propose that synergic observations with current and upcoming TeV and keV-MeV telescopes for its tentative TeV and MeV excesses can give insights to the hadronic process in its jet.