We consider the implications of the recent determination of the universal infrared background for the propagation of photons up to 20 TeV from the active galaxy Markarian 501 as observed by HEGRA. At 20 TeV the mean free path for photon-photon collisions on the infrared background would be much shorter than the distance to Markarian 501, implying absorption factors of the order of exp(−10), or greater, and consequently an excessive power output for this active galaxy. Possible solutions of this problem are discussed.

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries. The SKA will be 50 times more sensitive than any existing radio facility. A majority of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from 300 MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is aimed squarely in this frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phase-array feed systems on parabolic reflectors. This large field-of-view makes ASKAP an unprecedented synoptic telescope poised J. Wall is the overall editor.

We describe the results of a hybrid matrix{Monte Carlo calculation of cascading of UHE cosmic rays and -rays through the cosmic background radiation elds over cosmological distances. We calculate the -ray and neutrino emission that results from the cascade, as well as the e ect of cascading on the primary spectrum. We discuss the results for various cosmic ray injection spectra and primary species. Certain models for the production of the highest energy cosmic rays are ruled out.

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through largearea imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline an ambitious science program for ASKAP, examining key science such as understanding the evolution, formation and population of galaxies including our own, understanding the magnetic Universe, revealing the transient radio sky and searching for gravitational waves.

Observations of the FR I radio galaxy Centaurus A in radio, X-ray, and gamma-ray bands provide evidence for lepton acceleration up to several TeV and clues about hadron acceleration to tens of EeV. Synthesising the available observational constraints on the physical conditions and particle content in the jets, inner lobes and giant lobes of Centaurus A, we aim to evaluate its feasibility as an ultra-high-energy cosmic-ray source. We apply several methods of determining jet power and affirm the consistency of various power estimates of ∼1 × 10 43 erg s −1 . Employing scaling relations based on previous results for 3C 31, we estimate particle number densities in the jets, encompassing available radio through X-ray observations. Our model is compatible with the jets ingesting ∼3 × 10 21 g s −1 of matter via external entrainment from hot gas and ∼7 × 10 22 g s −1 via internal entrainment from jet-contained stars. This leads to an imbalance between the internal lobe pressure available from radiating particles and magnetic field, and our derived external pressure. Based on knowledge of the external environments of other FR I sources, we estimate the thermal pressure in the giant lobes as 1.5 × 10 −12 dyn cm −2 , from which we deduce a lower limit to the temperature of ∼1.6 × 10 8 K. Using dynamical and buoyancy arguments, we infer ∼440−645 Myr and ∼560 Myr as the sound-crossing and buoyancy ages of the giant lobes respectively, inconsistent with their spectral ages. We re-investigate the feasibility of particle acceleration via stochastic processes in the lobes, placing new constraints on the energetics and on turbulent input to the lobes. The same "very hot" temperatures that allow self-consistency between the entrainment calculations and the missing pressure also allow stochastic UHECR acceleration models to work.