Context. The phase of radio galaxy evolution after the jets have switched off, often referred to as the remnant phase, is poorly understood and very few sources in this phase are known. Aims. In this work we present an extensive search for remnant radio galaxies in the Lockman Hole, a well-studied extragalactic field. We create mock catalogues of low-power radio galaxies based on Monte Carlo simulations to derive first-order predictions of the fraction of remnants in radio flux limited samples for comparison with our Lockman-Hole sample. Methods. Our search for remnant radio galaxies is based on LOFAR observations at 150 MHz combined with public survey data at higher frequencies. To enhance the selection process, and obtain a more complete picture of the remnant population, we use spectral criteria such as ultra-steep spectral index and high spectral curvature, and morphological criteria such as low radio core prominence and relaxed shapes to identify candidate remnant radio galaxies. Mock catalogues of radio galaxies are created based on existing spectral and dynamical evolution models combined with observed source properties. Results. We have identified 23 candidate remnant radio galaxies which cover a variety of morphologies and spectral characteristics. We suggest that these different properties are related to different stages of the remnant evolution. We find that ultra-steep spectrum remnants represent only a fraction of our remnant sample suggesting a very rapid luminosity evolution of the radio plasma. Results from mock catalogues demonstrate the importance of dynamical evolution in the remnant phase of low-power radio galaxies to obtain fractions of remnant sources consistent with our observations. Moreover, these results confirm that ultra-steep spectrum remnants represent only a small subset of the entire population (∼50%) when frequencies higher than 1400 MHz are not included in the selection process, and that they are biased towards old ages.

The Chandra X-ray observatory has discovered dozens of resolved, kiloparsec-scale jets associated with powerful quasars in which the X-ray fluxes are observed to be much higher than the expected level based on the radio-optical synchrotron spectrum. The most popular explanation for the anomalously high and hard X-ray fluxes is that these jets do not decelerate significantly by the kiloparsec scale, but rather remain highly relativistic (Lorentz factors Γ ∼ 10). By adopting a small angle to the line of sight, the X-rays can thus be explained by inverse Compton upscattering of cosmic microwave background (CMB) photons (IC/CMB), where the observed emission is strongly Doppler boosted. Using over six years of Fermi monitoring data, we show that the expected hard, steady gamma-ray emission implied by the IC/CMB model is not seen in PKS 0637-752, the prototype jet for which this model was first proposed. IC/CMB emission is thus ruled out as the source of the X-rays, joining recent results for the jets in 3C 273 (using the same method) and PKS 1136-135 (using UV polarization). We further show that the Fermi observations give an upper limit of δ < 6.5 for the four brightest X-ray knots of PKS 0637-752, and derive an updated limit of δ < 7.8 for knots A and B1 of 3C 273 (assuming equipartition). Finally, we discuss the fact that high levels of synchrotron X-ray emission in a slow jet will unavoidably lead to a level of angle-integrated TeV emission which exceeds that of the TeV BL Lac class.

We have obtained a series of deep X-ray images of the nearby galaxy M83 using Chandra, with a total exposure of 729 ks. Combining the new data with earlier archival observations totaling 61 ks, we find 378 point sources within the D 25 contour of the galaxy. We find 80 more sources, mostly background active galactic nuclei (AGNs), outside of the D 25 contour. Of the X-ray sources, 47 have been detected in a new radio survey of M83 obtained using the Australia Telescope Compact Array. Of the X-ray sources, at least 87 seem likely to be supernova remnants (SNRs), based on a combination of their properties in X-rays and at other wavelengths. We attempt to classify the point source population of M83 through a combination of spectral and temporal analysis. As part of this effort, we carry out an initial spectral analysis of the 29 brightest X-ray sources. The soft X-ray sources in the disk, many of which are SNRs, are associated with the spiral arms, while the harder X-ray sources, mostly X-ray binaries (XRBs), do not appear to be. After eliminating AGNs, foreground stars, and identified SNRs from the sample, we construct the cumulative luminosity function (CLF) of XRBs brighter than 8 × 10 35 erg s −1. Despite M83's relatively high star formation rate, the CLF indicates that most of the XRBs in the disk are low mass XRBs.

Recent results based on the analysis of radio galaxies and their hot X-ray emitting atmospheres suggest that non-radiating particles dominate the energy budget in the lobes of FRI radio galaxies, in some cases by a factor of more than 1000, while radiating particles dominate the energy budget in FRII radio galaxy lobes. This implies a significant difference in the radiative efficiency of the two morphological classes. To test this hypothesis, we have measured the kinetic energy flux for a sample of 3C FRII radio sources using a new method based on the observed parameters of the jet terminal hotspots, and compared the resulting Q jet − L radio relation to that obtained for FRI radio galaxies based on X-ray cavity measurements. Contrary to expectations, we find approximate agreement between the Q jet − L radio relations determined separately for FRI and FRII radio galaxies. This result is ostensibly difficult to reconcile with the emerging scenario in which the lobes of FRI and FRII radio galaxies have vastly different energy budgets. However, a combination of lower density environment, spectral ageing and strong shocks driven by powerful FRII radio galaxies may reduce the radiative efficiency of these objects relative to FRIs and couteract, to some extent, the higher radiative efficiency expected to arise due to the lower fraction of energy in non-radiating particles. An unexpected corollary is that extrapolating the Q jet − L radio relation determined for low power FRI radio galaxies provides a reasonable approximation for high power sources, despite their apparently different lobe compositions.