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

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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 α of 1.9±0.4. However, spectra collected from different observational periods reveal different spectral shapes at MeV-energies: a hard state with α=1.4±0.4 during flaring observations reported by EGRET, and a soft state of α=2.6±0.7 during other times. The combined simultaneous EGRET and COMPTEL spectra indicate these two spectral states as well. During the low luminosity γ-ray state 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 γ-ray state 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 Δα range between 0.6 and 1.7, and for the break energy Eb between ∼5 MeV and ∼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 γ-ray flaring 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 γ-ray flaring phases shows that the major energy release across the entire electromagnetic spectrum is measured at MeV-energies.

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Astronomy and Astrophysics

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