Previous
abstract 
Next
abstract
Session 118 - Warner Prize Lecture.
Invited session, Thursday, January 18
1st Floor, La Villita Assembly Building
Positrons created by photon-photon collisions can be a source of both rest mass and opacity. They can become dominant in astrophysical systems where the energy densities are large and particles are accelerated to relativistic energies. Examples include accreting black holes and neutron stars, relativistic jets, pulsars, gamma-ray bursts and the cosmological universe.
The X-ray and gamma-ray spectra of active galactic nuclei and X-ray transients are probably determined by pair creation in accretion disk coronae. Positron creation determines the rest mass flux, speed, and photon emission from relativistic jets in radio galaxies, blazars and Galactic black hole candidates such as GRO J1655-40. It also determines the efficiency of energy extraction from rotating black holes, which may be the energy sources for at least some of these jets. Positron creation in neutron star magnetospheres is crucial to the operation of pulsars. The mechanism of positron creation varies depending on the strength and topology of the pulsar magnetic field, and the spin period of the neutron star. Pair-creation on cosmological background radiations prevents energetic gamma-rays produced by cosmological sources from reaching earth.
After reviewing these topics, we
summarize some recent results on positron creation in black hole
and pulsar magnetospheres, with applications to jets in radio
galaxies and to the observability (death-line)
of millisecond pulsars. We also describe a novel way
to use cosmological gamma-ray sources to measure the intergalactic
background radiation and (primordial?)
intergalactic magnetic fields at high redshifts,
in the range 10^-23\,\mboxG
