HEAD Division Meeting 1999, April 1999
Session 24. Blazars
Oral, Tuesday, April 13, 1999, 4:30pm-6:02pm, Colonial Room

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[24.06] New High-Energy Results for the Gamma-Ray Blazar PKS 0208-512

J.G. Stacy (LSU \& SUBR), W.T. Vestrand (UNH), P. Sreekumar (NASA/GSFC \& USRA), S.C. Kappadath (LSU), J.J. Blom (SRON-Utrecht)

The bright, variable gamma-ray blazar PKS 0208-512 is one of the most prominent gamma-ray sources in the extragalactic sky. It is one of a small number of active galaxies that has been detected by both the COMPTEL and EGRET instruments aboard the Compton\/ Gamma Ray Observatory (CGRO) from MeV to GeV energies. At EGRET GeV gamma-ray energies PKS 0208-512 exhibits significant flux and spectral variations, with strong evidence for spectral hardening of its emission during active periods. At the MeV gamma-ray energies observable by COMPTEL, PKS 0208-512 has been reported as an active galaxy with excess emission in the 1 to 3 MeV band (an ``MeV blazar''). Here, we present the results of a major re-analysis of the high-energy data acquired simultaneously with both the COMPTEL and EGRET instruments for this source. In particular, over the COMPTEL energy range our analysis incorporates refined event-selection criteria, based on an improved understanding of the instrumental response, and on extensive experience acquired during the analysis of the cosmic diffuse background radiation in the MeV band. We re-evaluate the significance of the excess MeV emission reported from PKS 0208-512, and assess the implications regarding the duty cycle of such MeV flaring activity in gamma-ray blazars. Finally, we briefly describe how these gamma-ray measurements will be combined with both previous and ongoing multiwavelength observations of PKS 0208-512, and how these results will aid in addressing questions related to the origin of the high-energy emission observed from gamma-ray blazars.

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