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up to the end of 1994 more than 50 extragalactic radio sources have been detected with egret as emitters of high - energy @xmath0-rays .
the majority of the sources are quasars and optically violent variables ( ovv ) and a number are classified as bl lacertae ( bl lac ) objects .
individual @xmath0-ray spectra in the egret range can generally be well described by power - laws ( von montigny , this volume ) . in case of the bl lac
mrk421 the spectrum extends up to tev energies ( punch et al .
1992 ) . in this paper
we analyse the class - averaged spectra of agn by summing the observed intensity and the statistic of power - law fits to the observed emission .
we also derive the spectrum of the average @xmath0-ray agn which is to be compared to the spectrum of the diffuse extragalactic background .
here we have summed all egret data of phases 1,2 , and 3 . with the standard likelihood techniques we have searched for point sources ,
of which we can identify 44 as agn , 11 bl lac s and 33 quasars and ovv s .
for all 44 agn we have performed a spectral analysis .
there is no cut - off visible in the @xmath0-ray spectra with the possible exception of a weak deficiency below 100 mev for the quasars which may be the outer extension of the usual roll - over at a few mev .
we have also searched for systematical deviations from power - law behaviour in the @xmath0-ray spectra of the 44 agn . for each individual agn
we have fitted a power - law spectrum to the data .
the weighted difference between this fit and the measured intensity in the ten energy bands , i.e. @xmath1 , has been summed for all quasars and bl lac s , respectively , to obtain the average deviation .
no significant deviations from power - law behaviour in the average agn spectrum is observed , neither for quasars and ovv s nor for bl lac s .
we have summed the observed intensity in ten energy bands to derive the spectrum of the average agn .
this spectrum is what we would get as contribution to the diffuse extragalactic background if the agn were unresolved .
the @xmath0-ray intensity of all 44 agn sums up to around 7 % of the diffuse background .
it is interesting to see that the @xmath0-ray spectrum of the average bl lac is harder than that of the average quasar and ovv ( with formal significance 1.7@xmath2 ) .
this does not imply that in single viewing periods bl lac s have always harder spectra than quasars .
in fact we see a remarkable spread of spectral indices for both classes of objects when individual viewing periods are considered . but
this concerns individual sources .
the spectrum of the average in both object classes is different , and therefore they contribute with different spectral characteristic to the diffuse @xmath0-ray background .
the average spectrum of all agn is dominated by that of the quasars and it differs with 2.7@xmath2 significance from that of the observed diffuse extragalactic background ( kniffen et al .
1997 ) which is similar to the average bl lac intensity spectrum .
interestingly , the bl lac s have on average a much smaller redshift with values between 0.031 and 0.94 , while more than 50% of the objects in the quasar and ovv class have redshifts in excess of 1.0 .
this indicates that in case of quasars we observe a fair range of the luminosity function directly , in contrast to the bl lac case where we see only the tip of the iceberg . in other words , we expect the @xmath0-ray @xmath3 distribution of bl lac s to peak at lower @xmath0-ray fluxes than that of quasars and ovv s . as a result
the contribution of bl lac s to the diffuse extragalactic @xmath0-ray background may be strong despite the small number of directly observed objects .
hence it may be that bl lac s provide the bulk of the @xmath0-ray background .
a large fraction of agn is variable at @xmath0-ray energies .
any cut - off arising from opacity effects will be more prominent at high flux levels since then the intrinsic photon density of the source is high .
we have therefore chosen a subsample of agn for which at least a moderate level of variability can be found . in total
we are left with 26 quasars and ovv s and only 6 bl lac s .
the analysis is now similar to that described in the previous section except that the spectra are not derived on the basis of the summed data of phases 1 - 3 but only on data of the viewing periods in which the sources showed the highest flux levels .
there is not very much change compared to the average behaviour in case of bl lac s .
one has to keep in mind that we are now left with 6 objects and the statistic is not sufficient to distinguish general trends from pathological individuals .
the behaviour of quasars and ovv s during their peak phase is more interesting . at first
we see that the flare spectra are harder than the time - average , at least up to a few gev .
this is a confirmation of a claim by mcke et al .
( 1996 ) who found a hardening of the @xmath0-ray spectra with increasing flux level for 8 highly variable @xmath0-ray agn .
we also see that at energies below 70 mev and at energies above 4 gev the peak spectra show some evidence of a cut - off . to get a better idea of the significance level of the cut - offs
we have repeated the power - law fits for the peak phases of quasars and ovv s under the constraint that now the fit is based on the energy band of 70 mev to 4 gev and then extrapolated to calculate the true deviations in the outer energy bands .
the result is shown in fig.1 . at energies below 70 mev
there is a deficiency of intensity compared to power - law behaviour with total statistical significance of 3.6@xmath2 while at high energies above 4 gev we observe an intensity deficit with 2.5@xmath2 significance .
this result is stable with respect to the choice of sources .
we have omitted the sources which have less than 6@xmath2 significance at the time of flare and the outcome remains unchanged .
we have also included secondary flares , i.e. viewing periods in which the sources have been either within 2@xmath2 of the peak or have been observed with @xmath4 above 100 mev , and again the result is unchanged .
we have further tested the reliability of our method by monte - carlo simulations .
these simulations would detect systematic problems in the analysis tools , which may arise from the small photon numbers both at low and at high @xmath0-ray energies .
we did not detect significant systematic deviations from a gaussian distribution of the variable @xmath5 . even accounting for calibration uncertainties at low @xmath0-ray energies
the statistical uncertainties are much larger than the systematical uncertainties so that the former are a fair measure of the total uncertainty .
most quasars show a spectral break at mev energies .
it is , however , questionable whether such an extended spectra turnover is sufficient to account for the observed deficit below 70 mev , which is a factor 10 higher in energy than the typical break energy .
we prefer to interprete the result in the sense that the @xmath0-ray spectrum of quasars and ovv s is not a one component spectrum , but rather the superposition of different emission processes .
simulations show that a low energy cut - off in the injection spectrum of radiating electrons can account for the observed behaviour ( bttcher and schlickeiser 1996 ) .
the fact that we see this cut - off only at flare states , when photon densities are high , points at opacity effects as cause . in case of backscattered accretion disk photons
the opacity will sharply increase at a few gev .
if the efficiency of backscattering is high , which is probably the case for quasars and ovv s , the optical depth will exceed unity and a cut - off will result .
however , correlations between optical depth and the flux at a few 100 mev will occur only when the @xmath0-ray outburst is caused by an increased flux of target photons .
one may also think of photon - photon pair production on the high energy end of the self produced synchrotron spectrum . here
a correlation with the flux level can be naturally explained .
simulations show that at least in simple geometries the synchrotron - self - compton component tends to swamp the high energy end of the synchrotron spectrum ( bttcher , pohl and schlickeiser , in prep . ) , so that there is no natural reason to let this effect become important at a few gev @xmath0-ray energy .
bttcher m. , schlickeiser r. : 1996 , , 306 , 86 kniffen d.a .
et al . : 1997 , , in press mcke a. et al . : 1996 , proceedings of iau symp .
175 , kluwer , dordrecht , p.285 punch m. et al . : 1992 , nature , 358 , 477
in this paper i investigate the @xmath0-ray spectra of agn by summing up the intensity and the power - law fit statistic of quasars and ovv s and bl lac s separately . 
 the spectrum of the average agn is softer than that of the extragalactic @xmath0-ray background . 
 it may be that bl lac s , of which the average has a harder spectrum than quasars , make up the bulk of the extragalactic background .    
 we also find cut - offs both at low and at high energies in the spectra of quasars and ovv s , however only at the time of @xmath0-ray outbursts . 
 while the cut - off at high energies may have something to do with opacity , the cut - off at low energies may be taken as indication that the @xmath0-ray emission of quasars is not a one component spectrum .