Compton-induced $γ$-ray Cascade Emissions in Radio Galaxy NGC 1275

TL;DR

This study tackles the origin of high-energy γ-ray emission from misaligned AGN, focusing on NGC 1275, by employing a refined 3D Monte Carlo cascade code that incorporates both an isotropic BLR and an anisotropic Shakura-Sunyaev disk radiation field with a mild magnetic field. The authors demonstrate that Compton-induced γ-ray cascades, resulting from IC scattering of external photons by deflected e$^$ pairs, can reproduce the Fermi-LAT and MACE broadband SEDs during the 2022–2023 flare without relying on strong Doppler boosting, with the cascade behavior strongly dependent on the magnetic field, BLR energy density, and the primary γ-ray injection height. A key finding is a degeneracy between the BLR energy density $u_{BLR}$ and the injection height $z^{\gamma}_0$, and the data suggest that primary γ-rays likely originate in at least two spatial zones to match LAT and MACE observations. Overall, the work provides a Doppler-free mechanism to explain HE–VHE emission in radio galaxies and offers constraints on the location and environment of γ-ray production in such systems.

Abstract

Among active galactic nuclei (AGNi), blazars are the brightest emitters of high-energy (HE, $E \geq 100$ MeV) to very-high-energy (VHE, $E \geq 100$ GeV) $γ$-rays from their jets. Radio galaxies, being the misaligned parent population of the blazar class, were historically not detected at these frequencies. However, advances in experiments and observatories have led to their detection in the HE--VHE $γ$-ray band. In this work, we leverage and refine a Monte-Carlo photon and electron-positron (e$^\pm$) pair tracking code in the AGN environment of the radio galaxy NGC 1275. In the code, we consider the isotropic broad-line region (BLR) and anisotropic Shakura-Sunyaev (SS) accretion disk radiation fields, with mild magnetic fields in the AGN environment. We find that cascade $γ$-rays from inverse-Compton scattering by relativistic e$^\pm$ pairs of these external radiation fields can explain the Fermi Large Area Telescope's (LAT) and Major Atmospheric Cherenkov Experiment's observations from the radio galaxy NGC 1275. We present a set of plausible parameters obtained from the code by fitting the source's spectral energy distribution (SED) during flaring events reported during the period December 2022 to January 2023.

Figures (3)

• Figure 1: Cascade SED for different magnetic fields at BLR photon energy density of $50\times 10^{-3}$$\mathrm{erg}\cdot\mathrm{cm}^{-3}$ and injection height of 0.6 $\mathrm{R}_{BLR}$.
• Figure 2: Cascade SED for different BLR energy densities. In the left panel the magnetic field is 100 mG and the injection height is 0.8 $\mathrm{R}_{BLR}$, and 50 mG and 0.6 $\mathrm{R}_{BLR}$ in the right panel.
• Figure 3: Cascade SED for different primary photon injection heights, where the magnetic field is 100 mG and the BLR energy density is $u_{BLR} = 50\times 10^{-3}$$\mathrm{erg}\cdot\mathrm{cm}^{-3}$ in the left panel and $u_{BLR} = 10\times 10^{-3}$$\mathrm{erg}\cdot\mathrm{cm}^{-3}$ in the right panel.