Suzaku Observations of gamma-Ray Bright Radio Galaxies: Origin of the X-ray Emission and Broad-Band Modelin

TL;DR

This study uses Suzaku and Fermi LAT data to analyze the X-ray and gamma-ray emissions of nearby radio galaxies, revealing jet origins and challenging existing unification models with high synchrotron peak frequencies.

Contribution

First comprehensive X-ray and gamma-ray analysis of three radio galaxies, demonstrating jet-dominated X-ray emission and high synchrotron peak frequencies, challenging previous unification scenarios.

Findings

Jet origin of X-ray emission supported by spectral features.

PKS0625-354 and 3C78 exhibit high synchrotron peak frequencies.

Broad-band modeling aligns with low bulk Lorentz factors in these sources.

Abstract

We performed a systematic X-ray study of eight nearby gamma-ray bright radio galaxies with Suzaku for understanding the origin of their X-ray emissions. The Suzaku spectra for five of those have been presented previously, while the remaining three (M87, PKS0625-354, and 3C78) are presented here for the first time. Based on the Fe-K line strength, X-ray variability, and X-ray power-law photon indices, and using additional information on the [O III] line emission, we argue for a jet origin of the observed X-ray emission in these three sources. We also analyzed five years of Fermi Large Area Telescope (LAT) GeV gamma-ray data on PKS0625-354 and 3C78 to understand these sources within the blazar picture. We found significant gamma-ray variability in the former object. Overall, we note that the Suzaku spectra for both PKS0625-354 and 3C78 are rather soft, while the LAT spectra are unusually hard when compared with other gamma-ray detected low-power (FR I) radio galaxies. We demonstrate that the constructed broad-band spectral energy distributions of PKS0625-354 and 3C78 are well described by a one-zone synchrotron/synchrotron self-Compton model. The results of the modeling indicate lower bulk Lorentz factors compared to those typically found in other BL Lac objects, but consistent with the values inferred from modeling other LAT-detected FR\,I radio galaxies. Interestingly, the modeling also implies very high peak ($\sim 10^{16}$ Hz) synchrotron frequencies in the two analyzed sources, contrary to previously-suggested scenarios for FR I/BL Lac unification. We discuss the implications of our findings in the context of the FR I/BL Lac unification schemes.