Relativistic Winds from Compact Gamma-Ray Sources: II. Pair Loading and Radiative Acceleration in Gamma-ray Bursts

We consider the effects of rapid pair creation by an intense pulse of gamma-rays propagating ahead of a relativistic shock. Side-scattered photons colliding with the main gamma-ray beam amplify the density of scattering charges. The acceleration rate of the pair-loaded medium is calculated, and its limiting bulk Lorentz factor related to the spectrum and compactness of the photon source. One obtains, as a result, a definite prediction for the relative inertia in baryons and pairs. The deceleration of a relativistic shock in the moving medium, and the resulting synchrotron emissivity, are compared with existing calculations for a static medium. The radiative efficiency is increased dramatically by pair loading. When the initial ambient density exceeds a critical value, the scattering depth traversed by the main gamma-ray pulse rises above unity, and the pulse is broadened. These considerations place significant constraints on burst progenitors: a pre-burst mass loss rate exceeding 10^{-5} M_\odot per year is difficult to reconcile with individual pulses narrower than 10 s, unless the radiative efficiency is low. An anisotropic gamma-ray flux (on an angular scale \Gamma^{-1} or larger) drives a large velocity shear that greatly increases the energy in the seed magnetic field forward of the propagating shock.Comment: 19 pp., LaTeX (aaspp4.sty), revised 12/23/99, Ap. J. in press; summary section added and several minor improvements in presentatio

Two neutron decay of 16Be

Recently, the first example of two-neutron decay from the ground state of an unbound nucleus, $^{16}$Be, was seen. Three-body methods are ideal for exactly treating the degrees of freedom important for these decays. Using a basis expansion over hyperspherical harmonics and the hyperspherical R-matrix method, we construct a realistic model of $^{16}$Be in order to investigate its decay mode and the role of the two-neutron interaction. The neutron-$^{14}$Be interaction is constrained using shell model predictions. We obtain a ground state for $^{16}$Be that is over-bound by approximately 1 MeV with a width of approximately 0.23 MeV. This suggests, that for such systems, the three-body force needs to be repulsive.Comment: 4 pages, 1 figure, contribution to the proceedings for the 21st International Conference on Few Body Problems in Physics, Chicago, Illinois, US

Turnip yellow mosaic virus: biochemical aspects of the infection process

Investigators have demonstrated that the RNA from a variety of plant viruses can function as mRNA in in vitro protein synthesising systems. An active homologous plant cell-free amino acid incorporating system has been isolated from Brassica chinensis L., leaves, and partially characterised employing polyuridylic acid(poly(U)) and turnip yellow mosaic virus(TYMV) RNA as messengers. Activity of this system has been compared with an active system isolated from developing seeds of Vicia faba L., and with mixed B. chinensis - V. faba systems. Bacterial contamination was found to be low in such incubations.[14(_c)] phenylalanine aminoacylation of B. chinensis tRNA was maximal at 17.5 mM Mg(^2+) under which conditions TYMV RNA was found to accept [14(_c)] valine. Maximum [14(_c)] phenylalanine poly(U) directed incorporation occurred at 8 mM Mg(^2+) in the V. faba and 6 mM Mg(^2+) in the B. chinensis system. TYMV RNA was relatively inactive when employing a single [14(_c)] amino acid, but was active in the B. chinensis system when employing a [14(_c)] amino acid mixture (maximum incorporation at 6mM Mg(^+2) and 70 mM K(^+)Partial characterisation of TYMV RNA (and other system components) was carried out using polyacrylamide gel and analytical ultracentrifugation techniques, indicating the infectious viral RNA to have a high sedimentation coefficient (c. 33 S).Various extraction procedures of plant and viral components are discussed with reference to ease of isolation and resultant biological activity. Microsomes isolated rapidly from young B. chinensis leaves were found to be most active in amino acid incorporation. The application of these experiments to studies on biochemical aspects of the TYMV infection process are considered in the light of other work in this field. It would appear that resistance of V. faba to infection by TYMV does not reside at the ribosomal level, and that TYMV protein synthesis occurs, at least in vitro, on the '80 S' rather than the '70 S' ribosome