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 -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 \gamma -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 \Delta \alpha range between 0.6 and 1.7 , and for the break energy E _ { b } 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 -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 \gamma -ray flaring phases shows that the major energy release across the entire electro-magnetic spectrum is measured at MeV-energies .