Hadronic Processes in Advection-Dominated Accretion Flow as the Origin of TeV Excesses in BL Lac Objects

TL;DR

This work addresses TeV excesses observed in several BL Lac objects that challenge one-zone leptonic models. It proposes a two-zone lepto-hadronic framework in which the jet's synchrotron and SSC emission accounts for radio-to-GeV–sub-TeV bands, while TeV photons originate from $p\gamma$ interactions of protons accelerated in an advection-dominated accretion flow and decaying $\pi^0$ mesons. The required proton population must be very hard, with $p\sim1.6-1.7$ and a cutoff $\varepsilon_{p,\rm cut}\sim30-90$ TeV, and the ADAF must have a large radius $R_{ m o}\sim10^3-10^4R_S$ to allow TeV escape; MRI-driven turbulence and magnetic reconnection are identified as plausible acceleration mechanisms. Cascaded electrons and $pp$ contributions are found to be subdominant or disfavored, respectively, and neutrino fluxes lie mostly below current IceCube sensitivities, offering specific observational tests for ADAF-driven hadronic scenarios in BL Lacs.

Abstract

The spectral energy distributions (SEDs) of certain BL Lac objects (BL Lacs) exhibit an additional hard $γ$-ray component in the TeV energy range that surpasses the predictions of the one-zone leptonic jet model. The origin of this excess emission remains unclear. In this study, we selected five BL Lacs whose SEDs display a very hard intrinsic spectrum in the TeV band and successfully reproduced their broadband SEDs using a two-zone lepto-hadronic model. Within this framework, the emission observed in the optical, X-ray, GeV $γ$-ray, and sub-TeV $γ$-ray bands is modeled using the synchrotron and synchrotron self-Compton radiation processes of the relativistic electrons in the jets. Meanwhile, the TeV excess is attributed to $γ$-ray emission resulting from the photomeson ($pγ$) process via $π^0$ decay occurring within advection-dominated accretion flows (ADAFs). This scenario requires a hard proton spectrum with a spectral index of $p \sim 1.6-1.7$ and a cutoff energy ranging from 30 to 90 TeV, as well as a relatively large ADAF radius. Such hard proton spectra suggest that the dominant acceleration mechanisms are likely magnetic reconnection and/or stochastic acceleration processes within ADAFs. Additionally, the emission from the cascaded electrons results in a bump in the keV--MeV band; however, it is overwhelmed by the jet emission. Although the hadronuclear ($pp$) process cannot be entirely ruled out, it would necessitate an even harder proton spectrum and a higher cutoff energy compared to the $pγ$ process, making it a less favorable explanation for the observed TeV excess.

Figures (2)

• Figure 1: Broadband SEDs of the five BL Lacs with fitting results. The observational data, represented by scattered points, are referenced to Section \ref{['sec_sample']}. Gray circles and triangles denote data in the observer frame, whereas the corresponding red symbols indicate data corrected using the EBL model of 2022ApJ...941...33F. The black solid lines represent the sum of each component emission for the sources, including the jet synchrotron radiation (red solid lines), the jet SSC radiation without EBL absorption (green solid lines), the ADAF spectrum (magenta solid lines), the $\gamma$-ray emission from the $\pi^0$ decay in the $p\gamma$ process (blue solid lines) and in the $pp$ process (gray dashed lines), and the radiation from cascaded electrons (red and green dashed lines) produced by photomeson and Bethe--Heitler processes in the ADAF. The differential sensitivity curves (violet short horizontal lines) of IceCube are taken from 2019EPJC...79..234A.
• Figure 2: The $\gamma\gamma$ optical depth curves for the ADAF model with varying radii in the source 1ES 0347--121.