Relationship between gamma-ray loudness and X-ray spectra of radio galaxies

Abstract

The Fermi satellite has detected ~ 60 radio galaxies (RGs). In this study, we investigate the difference in the properties of X-ray spectra between GeV-loud RGs and GeV-quiet RGs. Our sample comprises 68 objects: 36 RGs detected with Fermi and 32 RGs not detected with gamma-rays. We analyzed the X-ray spectra of these 68 objects using data from the XMM-Newton, Chandra, NuSTAR, and Swift satellites. Our results show that most GeV-loud RGs do not exhibit significant absorption, while $\sim \!50\%$ of the GeV-quiet RGs exhibit significant absorption. This suggests that the jet of GeV-loud RGs is viewed from a small angle, and thus the emission is not easily blocked by the torus. Moreover, we reported that RGs with a heavy absorption are mostly in the X-ray luminosity range of 10⁴³-10⁴⁵ erg s^-1; however, few RGs with lower and higher luminosity suffer from heavy absorption. This is the same trend as that of Seyfert galaxies.