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Astron. Astrophys. 328, 33-42

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COMPTEL observations of the quasar PKS 0528+134
during the first 3.5 years of the CGRO mission

W. Collmar1, K. Bennett4, H. Bloemen2, J.J. Blom2, W. Hermsen2, G.G. Lichti1, M. Pohl1, J. Ryan3, V. Schönfelder1, J.G. Stacy3, H. Steinle1, and O.R. Williams4

1Max-Planck-Institut für extraterrestrische Physik, P.O. Box 1603, D-85740 Garching, Germany
2SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3University of New Hampshire, Institute for the Study of Earth, Oceans and Space, Durham NH 03824, USA
4Astrophysics Division, Space Science Department of ESA/ESTEC, 2200 AG Noordwijk, The Netherlands

Received 14 February 1997 / Accepted 4 July 1997


The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have been analyzed. During the first two years (CGRO Phases I and II), PKS 0528+134 was most significantly detected at energies above 3 MeV. During the last year (CGRO Phase III), there is only evidence for the quasar at energies below 3 MeV indicating a spectral change. Detections and non-detections during individual observations indicate a time variable MeV-flux. The time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well represented by a power-law shape with a photon index $\alpha$ of 1.9$\pm$0.4. However, spectra collected from different observational periods reveal different spectral shapes at MeV-energies: a hard state with $\alpha$=1.4$\pm$0.4 during flaring observations reported by EGRET, and a soft state of $\alpha$=2.6$\pm$0.7 during other times. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During the low luminosity $\gamma$-raystate no spectral break is obvious from the combined COMPTEL and EGRET measurements. Only by inclusion of OSSE data is a spectral bending indicated. For the high luminosity $\gamma$-raystate however, the combined COMPTEL and EGRET data themselves require a spectral bending at MeV-energies. By fitting a broken power-law shape, the best-fit values for the break in photon index $\Delta\alpha$ range between 0.6 and 1.7, and for the break energy Eb between $\sim$5 MeV and $\sim$20 MeV. Because the flux values measured by COMPTEL below 3 MeV in both states are roughly equal, the observations would be consistent with an additional spectral component showing up during $\gamma$-rayflaring phases of PKS 0528+134. Such a component could be introduced by e.g. a high-energy electron-positron population with a low-energy cutoff in their bulk Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for the $\gamma$-rayflaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies.

Key words: gamma rays: observations - galaxies: active - galaxies: quasars: individual: PKS 0528+134

Send offprint requests to: W. Collmar; e-mail: wec@mpe-garching.mpg.de

© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998