Fermi/LAT observations of Lobe-dominant Radio Galaxy 3C 207 and Possible Radiation Region of the Gamma-Rays

TL;DR

This study analyzes nine years of Fermi/LAT data of the radio galaxy 3C 207, suggesting that its gamma-ray emission likely originates from its knots, particularly knot-A, via the IC/CMB process, indicating relativistic motion.

Contribution

It provides evidence that gamma-ray emission in 3C 207 originates from its knots, especially knot-A, and demonstrates the importance of relativistic motion in modeling its spectral energy distribution.

Findings

Gamma-ray emission is steady over 9 years, indicating a non-core origin.

Single-zone models without relativistic motion underestimate gamma-ray flux.

Relativistic motion of knot-A explains the observed gamma-ray emission.

Abstract

3C 207 is a lobe-dominant radio galaxy with one sided jet and the bright knots in kpc-Mpc scale were resolved in the radio, optical and X-ray bands. It was confirmed as a gamma-ray emitter with Fermi/LAT, but it is uncertain whether the gamma-ray emission region is the core or knots due to the low spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data in the past 9 years. Different from the radio and optical emission from the core, it is found that the gamma-ray emission is steady without detection of flux variation over 2 sigma confidence level. This likely implies that the gamma-ray emission is from its knots. We collect the radio, optical, and X-ray data of knot-A, the closest knot from the core at 1 arcsec, and compile its spectral energy distribution (SED). Although the single-zone synchrotron+SSC+IC/CMB model by assuming knot-A at rest can reproduce the SED in the radio-optical-X-ray band, the predicted gamma-ray flux is lower than the LAT observations and the derived magnetic field strength deviates the equipartition condition with 3 orders of magnitude. Assuming that knot-A is relativistically moving, its SED from radio to gamma-ray bands would be well represented with the single-zone synchrotron+SSC+IC/CMB model under the equipartition condition. These results likely suggest that the gamma-ray emission may be from knot-A via the IC/CMB process and the knot should have relativistical motion. The jet power derived from our model parameters is also roughly consistent with the kinetic power estimated with the radio data.