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Astron. Astrophys. 349, 11-28 (1999)

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7. Conclusions

In this paper we have presented the 1997 Mkn 501 time averaged spectrum as measured with the HEGRA IACT system in the energy range from 500 GeV to 24 TeV. The absence of strong temporal evolution as well as a significant correlation of the emission strength and the spectral shape in the energy region from 500 GeV to 15 TeV made the determination of a time averaged spectrum astrophysically meaningful. Due to unprecedented [FORMULA]-ray statistics and the 20% energy resolution of the instrument, it was possible to detect for the first time [FORMULA]-rays from an extragalactic source with energies well beyond 10 TeV, and to measure a smooth, curved energy spectrum deeply into the exponential regime. We found, that the spectrum above 0.5 TeV is well described by a power law with an exponential cutoff [FORMULA] = [FORMULA], the highest recorded photon energies being 16 TeV or more.

The detection of TeV [FORMULA]-rays from Mkn 501 leads to the unavoidable conclusion that the observed [FORMULA]-radiation is produced in a relativistic jet with a Doppler factor [FORMULA]. Actually, assuming a pure power-law production spectrum of [FORMULA]-rays we may naturally explain the exponential cutoff in the observed spectrum by an internal [FORMULA]-[FORMULA] absorption in the jet. Because of the strong dependence of the optical depth on the jet's Doppler factor, this hypothesis gives an accurate determination of the latter, [FORMULA]. Remarkably, the uncertainty of this estimate, which is mainly due to the uncertainty in the value of the Hubble constant, [FORMULA] and in the measured energy of the exponential cutoff [FORMULA], does not exceed [FORMULA]. However, since there could be other reasons for the steepening of the TeV spectrum, this estimate can only be considered as a robust lower limit on [FORMULA]. In particular, the steepening of the [FORMULA]-ray spectrum at the highest energies could be attributed to an exponential cutoff in the spectrum of accelerated particles, as well as - in the case of the inverse Compton origin of [FORMULA]-rays - to the Klein-Nishina effect.

In addition, a modification of the intrinsic (source) spectrum of TeV [FORMULA]-rays takes place during their passage through the intergalactic medium. The recent claims about tentative detections of the diffuse extragalactic background radiation by the DIRBE instrument aboard the COBE at near infrared ([FORMULA]) and far infrared ([FORMULA]) wavelengths, both at the [FORMULA] flux level imply a strong effect of intergalactic [FORMULA]-[FORMULA] extinction on the observed Mkn 501 spectrum over the entire energy region measured by HEGRA. In particular, the shape of the highest energy part of the observed [FORMULA]-ray spectrum combined with the DIRBE flux at [FORMULA] requires a very steep ([FORMULA] with [FORMULA]) spectrum of the DEBRA with a characteristic flux at mid infrared wavelengths ([FORMULA]) around 1 to 2 [FORMULA]. Due to the expected flat spectrum of DEBRA at near infrared wavelengths (close to [FORMULA]) the modification of the [FORMULA]-ray spectrum is less prominent at energies of a few TeV, although the absolute extinction could be very large. This does not allow us to draw definite conclusions about the absorption effect based on the analysis of the shape of the [FORMULA]-ray spectrum. Nevertheless, in the case of Mkn 501 this ambiguity can be significantly reduced by rather general arguments regarding the [FORMULA]-ray energetics . Indeed, even relatively modest assumption about the optical depth of about [FORMULA] which corresponds to a flux of the DEBRA in the K-band ([FORMULA]) of about [FORMULA] creates uncomfortable conditions for any realistic models of the high energy radiation from Mrk 501. Thus, this value of the flux of the DEBRA may be considered as a rather strong upper limit comparable with the DIRBE upper limit at this wavelength.

To summarize, our excursion through the nonthermal physics of AGNs shows that [FORMULA]-ray observations alone do not allow a unique interpretation of a spectrum like the one we have presented for Mkn 501, even though one can make a number of highly interesting inferences. In particular, our current poor knowledge about the intrinsic spectrum of Mkn 501 does not allow us to make definite conclusions about the effect of the intergalactic absorption of TeV [FORMULA]-rays. And vice versa , the uncertainty in the density of DEBRA does not allow us to take into account the effect of the intergalactic absorption, and thus to "reconstruct" the intrinsic [FORMULA]-ray spectrum. The knowledge of the latter is an obvious condition for the quantitative study of the acceleration and radiation processes in the source. Therefore we believe that decisive progress in this field could be achieved by the analysis of both the spectral and temporal characteristics of X-ray and TeV [FORMULA]-ray emissions obtained during the multiwavelength campaigns of several X-ray selected BL Lac objects at different states of activity, and located at different distances within 1 Gpc. Due to the time variability of such sources this requires simultaneous observations together with extensive theoretical modeling. Given these we expect to obtain independent knowledge of the diffuse intergalactic radiation fields which can be compared with direct measurements and models of galaxy formation. We hope that the successful multiwavelength campaigns of Mkn 421 and Mkn 501 in 1998 with participation of several X-ray satellites and the HEGRA IACT system will provide highly interesting results in this area.

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999