Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003apj...598..216s&link_type=abstract
The Astrophysical Journal, Volume 598, Issue 1, pp. 216-231.
Astronomy and Astrophysics
Astronomy
4
Galaxies: Active, Gamma Rays: Observations, Galaxies: Quasars: Individual: Alphanumeric: Pks 0208-512
Scientific paper
We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6×107 Msolar at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ>~3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.
Sreekumar Parameswaran
Stacy John Gregory
Vestrand Thomas W.
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