A Plasma Instability Theory of Gamma-Ray Burst Emission

A new theory for gamma-ray burst radiation is presented. In this theory, magnetic fields and relativistic electrons are created through plasma processes arising as a relativistic shell passes through the interstellar medium. The gamma-rays are produced through synchrotron self-Compton emission. It is found that shocks do not arise in this theory, and that efficient gamma-ray emission only occurs for a high Lorentz factor and a high-density interstellar medium. The former explains the absence of gamma-ray bursts with thermal spectra. The latter provides the Compton attenuation theory with an explanation of why the interstellar medium density is always high. The theory predicts the existence of a class of extragalactic optical transient that emit no gamma-rays.Comment: Presented at the 20 Texas Symposium on Relativistic Astrophysics, December 1998, Paris, France. To appear on the proceedings compact dis

Selachian faunas from the earliest Cretaceous Purbeck Group of Dorset, southern England.

Abundant selachian remains have been recovered from a number of horizons through the Purbeck Group at Durlston Bay, Lulworth Cove and Stair Hole in southern England. The remains, primarily teeth, but additionally fin spines and dermal denticles, belong to selachians from two major groups, the Hybodontoidea and the Rhinobatoidei. The assemblage of hybodont sharks is quite diverse, comprising six species from the four genera ‘Hybodus’, Egertonodus, Polyacrodus and Lonchidion. The rhinobatoid rays include two species, one belonging to the genus Belemnobatis and another, larger, indeterminate ray. Within the Purbeck fauna, two species are new: Lonchidion inflexum sp. nov. and Belemnobatis variabilis sp. nov. Within the entirely non-marine succession of the Purbeck Group, the beds containing ray teeth also contain molluscs indicative of more saline intervals. In all of the sampled beds, the hybodont faunas recovered were relatively homogenous