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R. Gelderman (Western Kentucky Univ.), B.E. Woodgate (NASA/GSFC, LASP)
Almost four decades after the true nature of active galaxies and quasars was recognized, the composition of their remarkable jets is still unknown. It is well established that these outflows contain electrons moving along magnetic fields at relativistic speeds; however the identity of the positively charged component of the plasma is undetermined. Through modeling and observation of one of the archetypical radio galaxies, M87, our goal is to identify conclusive evidence for either electron-positron or electron-proton beams as the constituents of jets in AGN.
While theoretical arguments have been made in favor of both scenarios, no direct evidence has been found for either protons or positrons. The 511 keV annihilation line remains undetected from non-thermal sources. Recently however, Wardle and collaborators approached the problem indirectly and reported (1998 Nature) that the pattern of polarization emitted by the jet in 3C279 indicates the presence of positrons. We have approached the situation from another angle and are searching for Positronium recombination emission lines. Positronium is a bound combination of an electron and a positron analogous to, though with electron transition energy levels at twice the wavelength of, atomic hydrogen. Thus the Positronium equivalent of the Lyman alpha line will occur at 2432Å.
We will obtain longslit UV spectra of the M87 jet using the Space Telescope Imaging Spectrometer. The results of these observations will be compared with a theoretical prediction which includes Wallyn, et al.'s (1996) transition ratios expected from a nonthermal source. With theoretically and observationally determined values for the plasma flow and particle re-acceleration we shall be able to model the particle flux of the electrons (and positrons?) which are responsible for the synchrotron emission.
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