Fermi-LAT Discovery of a Gamma-Ray Outburst from the Peculiar Compact Steep Spectrum Radiogalaxy 3C 216

Abstract

3C 216 is an extragalactic radio source classified as a compact steep spectrum (CSS) object, associated with the source 4FGL J0910.0+4257 detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The source exhibits extended radio structures as well as an inner relativistic jet. In general, jets accelerated by active galactic nuclei (AGNs) are efficient sources of nonthermal radiation, spanning from the radio band to X-ray and gamma-ray energies. Due to relativistic beaming, much of this radiation, particularly in the high-energy domain, is concentrated within a narrow cone aligned with the jet’s direction. Consequently, high-energy emission is more easily detected in blazars , where the jet is closely aligned with the line of sight of the observer. Beginning in 2022 November, Fermi-LAT observed increased gamma-ray activity from 3C 216, culminating in a strong outburst in 2023 May. This event was followed up by observations from the Neil Gehrels Swift Observatory telescope. In this work, we perform a careful analysis of the multifrequency data (gamma ray, X-ray, UV, optical) collected during this observational campaign. We find that the spectral energy distribution (SED) of the flaring source evolves in a coherent way, supporting a common origin for the multifrequency emission. These results suggest that the SED observed during the outburst was dominated by a single emission zone, where synchrotron self-Compton (SSC) processes played a primary role. Since single-zone SSC models have typically fewer free parameters than multizone alternatives, they are a powerful probe of the physical conditions of the high-energy emitting regions. Therefore, observing SSC radiation even in CSS sources improves our understanding of the production of high-energy radiation in AGN jets.

Keywords

• Gamma-ray sources
• High energy astrophysics
• Radio sources