  The Cherenkov Telescope Array (CTA) is the future large observatory in the
very high energy (VHE) domain. Operating from 20 GeV to 300 TeV, it will be
composed of tens of Imaging Air Cherenkov Telescopes (IACTs) displaced in a
large area of a few square kilometers in both the southern and northern
hemispheres. The CTA/DATA On-Site Analysis (OSA) is the system devoted to the
development of dedicated pipelines and algorithms to be used at the CTA site
for the reconstruction, data quality monitoring, science monitoring and
realtime science alerting during observations. The OSA integral sensitivity is
computed here for the most studied source at Gamma-rays, the Crab Nebula, for a
set of exposures ranging from 1000 seconds to 50 hours, using the full CTA
Southern array. The reason for the Crab Nebula selection as the first example
of OSA integral sensitivity is twofold: (i) this source is characterized by a
broad spectrum covering the entire CTA energy range; (ii) it represents, at the
time of writing, the standard candle in VHE and it is often used as unit for
the IACTs sensitivity. The effect of different Crab Nebula emission models on
the CTA integral sensitivity is evaluated, to emphasize the need for
representative spectra of the CTA science targets in the evaluation of the OSA
use cases. Using the most complete model as input to the OSA integral
sensitivity, we obtain a significant detection of the Crab nebula (about 10% of
flux) even for a 1000 second exposure, for an energy threshold less than 10
TeV.
