## 5. Experimental results
In this section we first present the 1997 Mkn 501 time averaged energy
spectrum. As discussed already in the introduction the derivation of a
time averaged spectrum is meaningful since the changes in the spectral
shape during the HEGRA observations were rather small, i.e. they were
too small to be assessed with an accuracy of typically between 0.1 and
0.3 in the diurnal spectral indices. Moreover, as described in Paper 1, dividing the data into groups according to the absolute flux level
or according to the rising or falling behavior of the source activity
yielded mean spectra which did not differ significantly from each
other in the one to ten TeV energy range. The weakness of the
correlation between the absolute flux and the spectral shape will
further be substantiated below over the energy region from
500 GeV to 15 TeV. Nevertheless, the importance of the
spectral constancy should not be overestimated. If the spectral
variability is not tightly correlated with the absolute flux, diurnal
spectral variability characterized by a change of the spectral index
at several TeV by approximately 0.1
is surely consistent with the HEGRA data. The time-averaged energy
spectrum is shown in Fig. 9. For the determination of the spectrum
also at energies below 800 GeV, only the data from zenith angles
smaller 30 , , and TeV. The systematic errors on the fit parameters result from worst case assumptions concerning the systematic errors of the data points, and their correlations and include the error caused by the 15% uncertainty in the energy scale. The errors on the fit parameters, especially on and , are strongly correlated. The variation of only one of the parameters within the quoted error range yields spectra which are inconsistent with the measured spectrum. The data points and their errors are summarized in Table 1.
In the highest energy bin (19 TeV to 24 TeV) 40 excess events are found above a background of 13 events, corresponding to a nominal significance of S = ( - ) / of 3.7 . However, due to the steep spectrum in this energy range, a part of these events may represent a spill-over from lower energies. To provide an absolutely reliable lower limit on the highest energies in the sample, the spectrum was fit to the form of Eq. 6, but with a sharp cutoff at : . The best fit is achieved with TeV; the lower limit is TeV. Fig. 10 illustrates the spectral energy distribution,
as determined from the small zenith
angle data (30
Fig. 11 (upper panel) shows the spectral energy distribution for the overall data sample and for periods of high and low flux separately: dN/dE(2 TeV) determined on diurnal basis above 3 and below 1.6 , with a ratio of the mean fluxes close to 5. The high and low flux spectra agree within statistical errors, as shown by the ratio of both spectra, presented in Fig. 11 (lower panel). The systematic error is to good approximation the same for both data samples and cancels out in the ratio. The result thus confirms our previous conclusion about the flux-independence of the spectrum of Mkn 501 in 1997 between 1 and 10 TeV (Paper 1). Now the statement is extended to the broader energy region, from 500 GeV to 15 TeV. From 1 TeV to several TeV the slope of the spectrum is determined with high statistical accuracy, e.g. a power law fit in the energy region from 1 TeV to 5 TeV gives a differential index of -2.23 and for the high and the low flux spectrum respectively. In the narrow energy range from 500 GeV to 1 TeV the statistical uncertainty on the spectral index is considerably larger, we compute 0.2 for the high flux sample and 0.4 for the low flux sample. Therefore, our 1997 Mkn 501 data would not contradict a correlation of emission strength and spectral shape below 1 TeV as tentatively reported by the CAT-group (Djannati-Atai et al. 1999).
For completeness, the HEGRA IACT system data are plotted in Fig. 12 jointly with the HEGRA CT1 (Aharonian et al. 1999c), the CAT (Barrau 1998), the Telescope Array (Hayashida et al. 1998), and the Whipple (Samuelson et al. 1998) results concerning the Mrk 501 energy spectrum during the 1997 outburst. Generally a good agreement can be recognized in the overlapping energy regions, except for a steeper Telescope Array spectrum.
© European Southern Observatory (ESO) 1999 Online publication: August 25, 1999 |