Relativistic Accretion

A brief summary of the properties of astrophysical black holes is presented. Various modes of accretion are distinguished, corresponding to accretion at rates from well below to well above the Eddington rate. The importance of mass loss is emphasized when the accreting gas cannot radiate and it is asserted that a strong wind is likely to be necessary to carry off mass, angular momentum and energy from the accreting gas. The possible importance of the black hole spin in the formation of jets and in dictating the relative importance of non-thermal emission over thermal radiation is discussed.Comment: To appear in "Astrophysical Discs", ASP Conference Series, 13 pages, latex, 0 figure

The surprising Crab pulsar and its nebula: A review

The Crab nebula and its pulsar (referred to together as "Crab") have historically played a central role in astrophysics. True to their legacy, several unique discoveries have been made recently. The Crab was found to emit gamma-ray pulsations up to energies of 400 GeV, beyond what was previously expected from pulsars. Strong gamma-ray flares, of durations of a few days were discovered from within the nebula, while the source was previously expected to be stable in flux on these time scales. Here we review these intriguing and suggestive developments. In this context we give an overview of the observational properties of the Crab and our current understanding of pulsars and their nebulae.Comment: Review for Reports on Progress in Physics, 31 pages, 11 figures. For a video abstract see the journal web page of the article or http://bcove.me/391e37d

Point source models for the gravitational lens B1608+656: Indeterminacy in the prediction of the Hubble constant

We apply elliptical isothermal mass models to reproduce the point source properties, i.e. image positions, flux density ratios and time delay ratios, of the quadruple lens B1608+656. A wide set of suitable solutions is found, showing that models that only fit the properties of point sources are under-constrained and can lead to a large uncertainty in the prediction of Ho. We present two examples of models predicting Ho=100 km/(s Mpc) (chi^2=4) and Ho=69 km/(s Mpc) (chi^2=24).Comment: 2 pages, 1 figure. To appear in "Gravitational Lensing: Recent Progress and Future Goals", Boston University, MA, July 1999, eds. T.G. Brainerd and C.S. Kochane

The SNAP Strong Lens Survey

Basic considerations of lens detection and identification indicate that a wide field survey of the types planned for weak lensing and Type Ia SNe with SNAP are close to optimal for the optical detection of strong lenses. Such a ``piggy-back'' survey might be expected even pessimistically to provide a catalogue of a few thousand new strong lenses, with the numbers dominated by systems of faint blue galaxies lensed by foreground ellipticals. After sketching out our strategy for detecting and measuring these galaxy lenses using the SNAP images, we discuss some of the scientific applications of such a large sample of gravitational lenses: in particular we comment on the partition of information between lens structure, the source population properties and cosmology. Understanding this partitioning is key to assessing strong lens cosmography's value as a cosmological probe.Comment: 6 pages, 4 figures. To appear in the conference proceedings of "Wide Field Imaging from Space" (published in New Astronomy Reviews), eds. T. McKay, A. Fruchter, and E. Linde