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

TL;DR

This paper models gamma-ray emissions in the radio galaxy NGC 1275, showing that cascade processes involving external radiation fields can explain observed high-energy gamma-ray data during flaring events.

Contribution

It introduces a refined Monte-Carlo simulation considering external radiation fields and magnetic effects to explain gamma-ray emissions in NGC 1275, a radio galaxy.

Findings

Cascade gamma-rays can explain Fermi LAT and MAGIC observations.

Model fitting during flaring periods constrains AGN environment parameters.

External radiation fields significantly influence gamma-ray cascades.

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) $\gamma$-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 $\gamma$-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 $\gamma$-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.