## 3. Upper limits in Crab units and in flux units
In the following analysis, measured fluxes or flux upper limits are
first determined in Crab units and are only subsequently converted
into absolute flux values or flux upper limits. Results in Crab units
have the advantage of relying exclusively on measured data and of
being free from systematic errors due to the Monte Carlo simulations
of the air showers and the detector response. Moreover, as several
Bl Lac objects have been observed with only three telescopes in
the CT-system, we can directly compare their observations to Crab data
taken under the same conditions, at the same epoch, avoiding thus
specific simulations. The trigger rate is constant within 5% for a
given zenith angle (Aharonian et al. 1999a), thus no strictly
simultaneous data can be compared safely. For each source and each
zenith angle interval the number of
events in the ON-source region (),
the number of events in the OFF-source region
(), and the observation time
is determined. To maximize the
statistics the analysis is based on "loose" cuts: the mean scaled
width of the showers (Konopelko 1995; Daum et al. 1997) has to be
smaller than 1.2 (to retain 80%
of photon induced showers) and the squared angular distance of the
reconstructed shower direction from the source direction has to be
smaller than 0.05 deg
Using the probability density function of the number of source events, we compute the upper limit of the number of counts from hours of source observations at 99% confidence level (Helene 1983). Similarly, we calculate the lower limit of the number of counts for hours of Crab observations. We compute the upper limit in Crab units from: . The energy threshold is computed by Monte-Carlo for each source as a function of the mean zenith angle of the object during the observations. For the CT-system, the energy threshold scales with zenith angle roughly as (Konopelko et al. 1998). Only data with good weather conditions are used and phototube voltage fluctuations are corrected. Assuming a source energy spectrum, the conversion of upper limits
in Crab units into upper limits in absolute flux units is
straightforward. In the following we use two slopes for observed
source spectrum: dN/dE , as measured
for the Crab nebulae around 1 TeV (Konopelko et al. 1998) and a
steeper one dN/dE . Above 0.5 TeV,
the integral Crab flux is =
cm © European Southern Observatory (ESO) 2000 Online publication: January 18, 2000 |