1 9 A ug 1 99 9 Compton Electrons and Electromagnetic Pulse in Supernovae

When gamma-rays emerge from a central source they may undergo Compton scattering in surrounding matter. The resulting Compton-scattered electrons radiate. Coherent radiation by such Compton electrons follows nuclear explosions above the Earth's atmosphere. Particle acceleration in instabilities produced by Compton electron currents may explain the radio emission of SN1998bw. Bounds on coherent radiation are suggested for supernovae and gamma-ray bursts; these bounds are very high, but it is unknown if coherent radiation occurs in these objects. High altitude (exoatmospheric) nuclear explosions are well known to produce striking electromagnetic phenomena on the surface of the Earth. These phenomena, termed HEMP (High altitude ElectroMagnetic Pulse) or EMP (Karzas and Latter 1962, 1965) occur when prompt gamma-rays following nuclear fission, radiative neutron capture or inelastic neutron scattering suffer Compton scattering in the upper atmosphere. The Compton electrons, with energies typically ∼ 1 MeV, are preferentially directed along the direction of the incident gamma-rays, radially away from the gamma-ray source, and move at a speed close to the speed of light. They are deflected by the geomagnetic field and radiate synchrotron radiation. Because the gamma-rays and Compton electrons are produced over an interval < 10 −7 sec, shorter than the characteristic gyroperiod of the radiation (∼ 10 −6 sec, allowing for the relativistic energy of the electrons) this radiation is coherent; it may be regarded as the effect of a continuously distributed time-dependent current density, rather than as the radiation of individual electrons. The number of radiating electrons is very large so that the currents and coherent radiation intensity are high, and are limited by the condition that the radiation field not exceed the geomagnetic field, for the radiation field of the Compton current acts to screen the geomagnetic field. In fact, the radiation may be crudely approximated as the diamagnetic field exclusion by the conducting swarm of Compton electrons. In atmospheric EMP the Compton electrons produce large numbers of low energy electrons by collisional ionization, and the effects of these electrons on the emergent radiation are the chief subject of the published calculations. Analogous phenomena may be produced by astronomical events. Karzas and Latter (1965) indicate a threshold gamma-ray fluence for the observation of EMP of ∼ 10 −6 erg/cm 2. This is less than the fluence of many observed gamma-ray bursts (GRB), in some cases by more than two orders of magnitude. However, GRB do not produce observable EMP in the Earth's …