## 5. Experimental results
The observations described in this paper do not reveal positive
evidence for VHE emission from any of the nine studied sources.
Table 3 summarizes the upper limits in Crab units
, the mean energy thresholds of the
observations, and the upper limits on the integral fluxes, assuming
the two different slopes for the spectrum, above the energy threshold
with and without correction of the absorption by the CIB (Primack LCDM
model). The threshold being defined as the product of the acceptance
of the detector and the source gamma-ray spectrum, it effectively
depends on the slope of the gamma-ray flux. So the table give
thresholds for the Crab slope (-2.6), and an effective threshold 30%
lower is used for computation of the steeper model. All upper limits
have a confidence level of 99%. As mentioned above, the flux upper
limits in Crab units are free from systematic errors. The major
systematic uncertainty of the upper limits in absolute flux units
derives from a 15% uncertainty in the energy scale of the CT-System.
This 15% uncertainty translates into a flux uncertainty of
approximately 20% and 40% for a integral source spectral index of -1.6
and -2.6 respectively.
Of course the effect of the CIB absorption makes the observed spectrum steeper and then the impact of a very hard intrinsic spectrum is attenuated (upper limits for the two slopes are less different after absortion than before). We note that the upper limits are derived from assumed slopes for spectra and a model for the CIB intensity. These uncertainties make upper limits rather qualitative. © European Southern Observatory (ESO) 2000 Online publication: January 18, 2000 |