COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
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.
Publication Source (Journal or Book title)
Astronomy and Astrophysics
Collmar, W., Bennett, K., Bloemen, H., Blom, J., Hermsen, W., Lichti, G., Pohl, M., Ryan, J., Schönfelder, V., Stacy, J., Steinle, H., & Williams, O. (1997). COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission. Astronomy and Astrophysics, 328 (1), 33-42. Retrieved from https://repository.lsu.edu/physics_astronomy_pubs/5184