An X‐Ray Study of Magnetic Field Strengths and Particle Content in the Lobes of FR II Radio Sources

We present a Chandra and XMM-Newton study of X-ray emission from the lobes of33 classical double radio galaxies and quasars. We report new detections oflobe-related X-ray emission in 11 sources. Together with previous detections wefind that X-ray emission is detected from at least one radio lobe in ~75percent of the sample. For all of the lobe detections, we find that themeasured X-ray flux can be attributed to inverse-Compton scattering of thecosmic microwave background radiation, with magnetic field strengths in thelobes between (0.3 - 1.3) B_eq, where the value B_eq corresponds toequipartition between the electrons and magnetic field assuming a fillingfactor of unity. There is a strong peak in the magnetic field strengthdistribution at B ~ 0.7 B_eq. We find that > 70 percent of the radio lobes areeither at equipartition or electron dominated by a small factor. Thedistribution of measured magnetic field strengths differs for narrow-line andbroad-line objects, in the sense that broad-line radio galaxies and quasarsappear to be further from equipartition; however, this is likely to be due to acombination of projection effects and worse systematic uncertainty in the X-rayanalysis for those objects. Our results suggest that the lobes of classicaldouble radio sources do not contain an energetically dominant protonpopulation, because this would require the magnetic field energy density to besimilar to the electron energy density rather than the overall energy densityin relativistic particles.Comment: 44 pages, 4 figures. Accepted for publication in Ap