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Astron. Astrophys. 353, 847-852 (2000)

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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 [FORMULA], 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.

Upper limits of a few times [FORMULA] cm-2 s-1 are obtained after a few hours of observation. The correction of the absorption by the CIB of Fig. 1 (computed up to 3 TeV) increases the upper limits by a factor of 2 to 60 (for a slope of -2.6) depending on the effective threshold and the source redshift.


Table 3. 99% confidence level upper limits derived in this work: (a) right ascension of the source, 2200a designates BL Lacertae in 1997, and 2200b BL Lacertae in 1998, (b) mean energy threshold of the observations in GeV for the -2.6 slope (the effective threshold is approximately 200 GeV lower for the -3.6 slope), (c) upper limits in Crab units, (d) upper limits on the integral flux above the threshold energy in units of [FORMULA] cm-2 s-1, and (e) upper limits on the integral flux between the threshold energy and 3 TeV after correction for the CIB absorption (using LCDM model by Primack et al. 1999) in units of [FORMULA] cm-2 s-1. The slope of the assumed differential spectrum is -2.6 or -3.6 (see text for details).

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.

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000