[Paper Review] Exploring the FRI/FRII radio dichotomy with the Fermi satellite

P. Grandi, E. Torresi|arXiv (Cornell University)|May 8, 2012

Astrophysics and Cosmic Phenomena4 references7 citations

TL;DR

This study investigates the FRI/FRII radio dichotomy using Fermi-LAT data, finding that high-energy gamma-ray emission in both FR I and FRII radio galaxies originates in compact jet regions near the black hole, not in extended lobes. Despite similar emission sites, FRIIs are significantly underrepresented in the GeV sky, suggesting intrinsic jet differences—such as weaker Doppler boosting, less structural complexity, or reduced particle acceleration efficiency—rather than distance or faintness explain their elusiveness.

ABSTRACT

Misaligned Active Galactic Nuclei (MAGNs), i.e., radio galaxies and quasars with the jet not directly pointing at the observer, are a new class of GeV emitters. In low power radio galaxies (i.e., FRIs), gamma-rays are mainly produced in compact jet regions, although in at least one case, Centaurus A, high energy photons from the radio lobes have been also observed. The first localization of the gamma-ray dissipation zone in a high power radio galaxy (i.e., FRII) excludes major contributions from extended regions. The study of the FRII source 3C111 indicates that gamma-ray photons are produced in the jet. The site, coincident with the radio core, is estimated to be at a distance

Motivation & Objective

• To investigate why FRII radio galaxies are underrepresented as GeV gamma-ray emitters despite their high radio power.
• To localize the gamma-ray emission site in FRII sources using Fermi-LAT data and multiwavelength variability analysis.
• To test whether the paucity of FRII detections is due to intrinsic jet differences or observational biases such as distance or flux faintness.
• To compare FRI and FRII radio and gamma-ray properties across four complete radio source catalogs to assess selection effects.

Proposed method

• Analysis of Fermi-LAT data for 11 misaligned active galactic nuclei (MAGNs), including 3C 111 and Pictor A, to localize gamma-ray emission regions.
• Use of causality arguments and variability timescales to constrain the size and location of the emitting region: $ R \leq c\Delta t\delta/(1+z) $, where $ \delta $ is the Doppler factor.
• Application of spectral energy distribution (SED) modeling to test one-zone homogeneous vs. structured jet models, including SSC and EC processes.
• Comparison of FRI and FRII populations in four complete radio catalogs (3CR, 3CRR, MS4, 2Jy) to assess detection rates and selection effects.
• Use of multiwavelength variability data to assess whether flares occur in the core or along the jet, informing jet structure and emission mechanisms.
• Evaluation of jet models such as spine-layer, decelerating flow, and colliding shells to explain observed gamma-ray properties and emission site constraints.

Experimental results

Research questions

• RQ1 Where is the gamma-ray emission site localized in the FRII radio galaxy 3C 111, and what does this imply about the jet structure?
• RQ2 Why are FRII radio galaxies underrepresented as GeV gamma-ray sources despite their high radio power?
• RQ3 Is the low detection rate of FRIIs due to their greater distance and fainter fluxes, or due to intrinsic differences in jet physics?
• RQ4 Do FRI and FRII jets differ in their emission mechanisms, such as dominance of SSC versus EC processes?
• RQ5 How do jet structural models (e.g., spine-layer, decelerating flow) explain the observed gamma-ray properties and emission site constraints?

Key findings

• The gamma-ray emission in the FRII radio galaxy 3C 111 is localized within 0.3 pc of the black hole, consistent with the radio core, and not in the extended lobes or hotspots.
• The FRII source 3C 111 shows no significant contribution from radio lobes, indicating that the dissipation zone is compact and core-dominated.
• Only 1% of FRII sources in four complete radio catalogs have a Fermi-LAT counterpart, confirming their underrepresentation in the GeV sky.
• The paucity of FRII detections is unlikely due to distance or flux faintness, as FRIIs are more powerful and thus should be detectable if emission were similar to FRIs.
• The results suggest that intrinsic jet differences—such as weaker Doppler boosting, less structural complexity, or reduced particle acceleration efficiency—may explain the gamma-ray elusiveness of FRIIs.
• The study supports that FRII jets may be less structured (e.g., dominated by a fast spine) and less efficient at producing high-energy photons compared to FRIs, which may have more prominent, decelerated external layers.