We present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 classical double radio galaxies and quasars. We report new detections of lobe-related X-ray emission in 11 sources. Together with previous detections, we find that X-ray emission is detected from at least one radio lobe in $75% of the sample. For all of the lobe detections, we find that the measured X-ray flux can be attributed to inverse Compton scattering of the cosmic microwave background radiation, with magnetic field strengths in the lobes between 0.3B eq and 1.3B eq , where the value B eq corresponds to equipartition between the electrons and magnetic field, assuming a filling factor of unity. There is a strong peak in the magnetic field strength distribution at B $ 0:7B eq . We find that more than 70% of the radio lobes are either at equipartition or electron dominated by a small factor. The distribution of measured magnetic field strengths differs for narrow-and broad-line objects, in the sense that broad-line radio galaxies and quasars appear to be further from equipartition; however, this is likely to be due to a combination of projection effects and worse systematic uncertainty in the X-ray analysis for those objects. Our results suggest that the lobes of classical double radio sources do not contain an energetically dominant proton population, because this would require the magnetic field energy density to be similar to the electron energy density rather than the overall energy density in relativistic particles.

Superluminal motion is a common feature of radio jets in powerful g-ray-emitting active galactic nuclei. Conventionally, the variable emission is assumed to originate near the central supermassive black hole where the jet is launched on parsec scales or smaller. Here we report the discovery of superluminal radio features within a distinct flaring X-ray-emitting region in the jet of the nearby radio galaxy M87 with the Very Long Baseline Array. This shows that these two phenomenological hallmarks-superluminal motion and high-energy variability-are associated, and we place this activity much farther (≥120 pc) from the "central engine" in M87 than previously thought in relativistic jet sources. We argue that the recent excess very high energy TeV emission from M87 reported by the H.E.S.S. experiment originates from this variable superluminal structure, thus providing crucial insight into the production region of g-ray emission in more distant blazars.

We use new and archival Chandra data to investigate the X-ray emission from a large sample of compact hot spots of FR II radio galaxies and quasars from the 3C catalog. We find that only the most luminous hot spots tend to be in good agreement with the predictions of a synchrotron self-Compton model with equipartition magnetic fields. At low hot spot luminosities inverse Compton predictions are routinely exceeded by several orders of magnitude, but this is never seen in more luminous hot spots. We argue that an additional synchrotron component of the X-ray emission is present in low-luminosity hot spots and that the hot spot luminosity controls the ability of a given hot spot to produce synchrotron X-rays, probably by determining the high-energy cutoff of the electron energy spectrum. It remains plausible that all hot spots are close to the equipartition condition.

We have made deep Chandra observations of three powerful FRII radio sources: two quasars (3C 263 and 3C 351) and one radio galaxy (3C 330). X-ray emission from hotspots and lobes, as well as from the active nucleus, is detected in each source.We model the hotspots' synchrotron spectra using VLA, BIMA and Hubble Space Telescope data. In 3C 263 and 3C 330, the hotspots' X-ray emission is at a level consistent with being synchrotron self-Compton (SSC) emission, with a hotspot magnetic field close to the equipartition value. In the two hotspots of 3C 351, however, an SSC origin for the X-rays would require the magnetic field strength to be an order of magnitude below the equipartition value in our models: in addition, there are offsets between the radio, optical and X-ray emission from the secondary hotspot which are hard to explain in a simple SSC model. We discuss the emission mechanisms that may be responsible for these observations. On our preferred model, the X-ray emission from the radio lobes of the three sources is due to inverse-Compton scattering of the microwave background radiation. If this is the case, the magnetic field strengths in the lobes are typically about a factor -2 -2 below the equipartition values, assuming uniform lobe electron and magnetic field distributions.We detect extended X-ray emission, which we attribute to a cluster/group environment, around 3C 263 and 3C 330. This detection allows us to show that the lobes are close to pressure balance with their surroundings, as long as no non-radiating particles contribute to the internal pressure of the lobes.Subject headings: galaxies: active -X-rays: galaxies -X-rays: quasars -radiation mechanisms: non-thermal ROSAT observations of the archetypal FRII object Cygnus A (Harris, Carilli & Perley 1994) showed that the X-ray emission from its hotspots was consistent with the SSC process if the magnetic field strength was close to the equipartition or minimum-energy values (Burbidge 1956). This result is often taken as evidence for equipartition between magnetic fields and synchrotronemitting electrons in radio sources in general. Other ROSAT observations, however (e.g., Harris, Leighly & Leahy 1998), showed that some hotspots had X-ray emission too bright to be produced by the SSC mechanism with equipartition field strengths, suggesting that a different emission process was responsible. At the time of writing, after a number of new Chandra hotspot detections, this dichotomy remains, as shown in Table 1. Three interesting facts about this division are immediately apparent. Firstly, the number of objects whose emission processes are not from SSC near equipartition (hereafter 'non-SSCE' objects) is still a significant fraction of the total. Secondly, the non-SSCE objects all display broad emission lines, which may, after all, indicate some role for beaming in the hotspots. And, thirdly, the non-SSCE objects all have optical synchrotron hotspots.The major remaining questions in this area are therefore:1. Are magnetic field strengths close to equipartit...

The emission processes responsible for the observed X-rays from radio jets are commonly believed to be non-thermal, but in any particular case, it is unclear if synchrotron emission or one or more varieties of inverse Compton emission predominates. We present a formulation of inverse Compton emission from a relativistically moving jet ("IC/beaming") which relies on radio emitting synchrotron sources for which the energy densities in particles and fields are comparable. We include the non-isotropic nature of inverse Compton scattering of the relativistic electrons on photons of the cosmic microwave background (CMB) and provide beaming parameters for a number of jets. A list of X-ray emitting jets is given and the jets are classified on the basis of their morphology and spectral energy distribution to determine their likely emission process. We conclude that these jets have significant bulk relativistic velocities on kpc scales; that higher redshift sources require less beaming because the energy density of the CMB is significantly greater than locally; and that for some nearby sources, synchrotron X-ray emission predominates because the jet makes a large angle to the line of sight.