Discovery non-thermal X-ray from radio lobe of Cygnus A

Yuichi Yaji (Department Physics, Saitama University) , H. Seta (Saitama University), M. Tashiro (Saitama University), N. Isobe (RIKEN), M. Kino (JAXA), K. Asada (JAXA), H. Nagai (NAOJ), M. Kusunose (Kwansei Gakuin University)

Lobes of radio galaxies are enormous storages of large amount of non-thermal electrons and magnetic fields, both of which are thought to be transferred by jets from active galactic nuclei. Their energies can be comparable to those of thermal energies of intra-cluster medium (ICM), and resultant X-ray "cavities" in ICM are observed from several clusters. Observation of the inverse-Compton (IC) X-ray emission from non-thermal electrons in lobes is crucial to measure the electron and magnetic energy densities. However, it is difficult to precisely measure the IC X-ray from the lobes of radio galaxies located in clusters, because they are often contaminated by the thermal X-rays from ICM. In order to avoid the contamination, spatial resolution is the key. We analysed all the available archival data of the bright FR II radio galaxy Cygnus A (9 pointing in total). After the standard data screening, we obtained 230 ks of good exposure has been obtained. We carefully subtracted the ambient ICM emission, and have succeeded to detect a hard X-ray emission associated with the radio lobes. The spectra of the east and the west lobes are well described by power-law models with energy indices of ), we regard that the hard X-ray emission from the lobes is produced via IC process by the synchrotron electron in the lobes. Comparison of the determined X-ray flux and radio flux indicates that the electron energy highly dominates that of the magnetic field in the lobes. Comparison of the derived non-thermal pressure with that of ICM will also discussed in the paper.