Astron. Astrophys. 324, 395-409 (1997)

7. General results

In this section we present some general results which we collected during a series of simulations with varying parameters.

An interesting feature is that the time-averaged SSC emission of a pair distribution with a lower cut-off follows a power-law in the X-ray range with energy spectral index which is consistent with the X-ray spectra of flat-spectrum radio quasars (FSRQs) and low-frequency peaked BL Lacs (LBLs) where the hard X-ray emission can be attributed to jet emission. The X-ray spectrum slightly hardens with increasing lower cut-off of the particle injection spectrum.

Our simulations showed that assuming a particle spectrum extending up to - where some EIC scattering occurs in the Klein-Nishina regime, the EIC spectrum is harder than in the case of pure Thomson scattering where we recover the classical result (Dermer & Schlickeiser 1993 a) above the break in the photon spectrum caused by incomplete Compton cooling.

A harder time-integrated EIC spectrum (assuming the same spectral index of the injected pair distributions) can also result if cooling due to EIC is very inefficient (i. e. for a blob at large distance from the accretion disk) implying that the effect of dilution of the soft photon field according to its dependence dominates the evolution of the EIC spectrum.

If a second mechanism contributes to the cooling of the radiating pair population, the resulting external inverse-Compton photon spectra do not significantly differ from the case of a pure EIC model. Equally the assumption of a lower cutoff does not have an important effect on the high-energy -ray spectrum. The hard X-ray to soft -ray spectrum from EIC becomes harder with increasing lower cutoff, implying that in case of a high lower cutoff this part of the photon spectrum is always dominated by SSC.

Time-averaged SSC-dominated -ray emission can not account for strong spectral breaks in the MeV range as observed in several FSRQs, even if a high lower cut-off in the particle in the initial particle distributions is assumed. This fact is well illustrated by Figs. 8 and 10. In contrast, such a cut-off can produce sharp spectral breaks () in the time-averaged EIC spectrum.

We find that even in the case of an accretion disk of very low luminosity and injection of the pairs occurring relatively high above the disk EIC scattering will give a significant contribution to the photon spectrum in the GeV - TeV range (see Fig. 10). If our assumption of a two-component -ray spectrum with neither SSC nor EIC being negligible is correct, we conclude that the intrinsic GeV - TeV spectrum of blazars will in general not be a simple power-law extrapolation of the EGRET spectrum.

The particle density plays a central role for the broadband emission from ultrarelativistic plasma blobs. The -ray spectra of blazars can well be reproduced by dense blobs (). But in this case, escape of high-energy -rays without significant - absorption implies the presence of a very low magnetic field and a very weak synchrotron component. This problem is very severe for high-frequency peaked BL Lacs (as Mrk 421) where the -ray component can extend up to TeV energies at which they can be absorbed very efficiently by the synchrotron component if one assumes that both components are produced in the same volume.

In the case of very high density ( cm^-3) the cut-off to lower frequencies in the synchrotron spectra is not caused by synchrotron-self absorption but by the Razin-Tsytovich effect. The same effect would also suppress synchrotron cooling below the Razin-Tsytovich frequency (Eq. [25]) which could consequently lead to a storage of the kinetic energy in pairs as long as the magnetic field does not change dramatically and the jet remains well collimated. As mentioned above, this effect can only be important if the synchrotron component (also above the Razin-Tsytovich frequency) is very low compared to the -ray component during a -ray outburst.

Then, a time lag between a -ray flare and the associated radio outburst can give information about the geometry of the jet structure. We suggest that this fact could be a major reason for the difference between FR I and FR II radio galaxies: If the jet is well-collimated over kpc scales, the Razin-Tsytovich effect prevents efficient cooling of the relativistic pairs to Lorentz factors lower than the Razin Lorentz factor, and the kinetic energy of the jet material can only be released in the radio lobes of FR II galaxies. If collimation is inefficient and the jet widens up rapidly after injection, its kinetic energy can be dissipated at short distances from the central engine, which would result in an FR I structure.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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