Flux sensitivity

The performance of VERITAS is summarized by its flux sensitivity versus energy. The minimum detectable flux of gamma-rays is defined by the confidence level required for detection or the statistics of the detected photons. We require a 5$\sigma$ excess of gamma-rays above the background, or 10 photons (below this, Poisson statistics must be used). We estimate the flux sensitivity for 50 hours of observations on an object with a spectrum given by dN/dE$\propto$E^-2.5, as seen from the Crab Nebula in this energy range. We apply direction and image shape cuts to reject background events.

  

Figure 18: The sensitivity of VERITAS to point-like sources in 50 hours of observing. The dominant background as a function of energy threshold is indicated (see text for details). The two curves at low energies indicate the sensitivity of VERITAS in dark (lower curve) and bright (upper curve) NSB regions.

The gamma-ray flux sensitivity of VERITAS for point sources as a function of array energy threshold is shown in Figure 18. The complex shape of the sensitivity curve is caused by different energy regions being dominated by the different backgrounds shown in Figure 18. For energies above 2 - 3TeV, the sensitivity of VERITAS is limited by photon statistics. Larger telescope fields of view can improve this sensitivity in the future, as can large zenith angle observations. In the region near 1TeV, the sensitivity is limited by those rare cosmic-ray protons which mimic gamma-rays by converting most of their energy into an electromagnetic cascade in the first few interactions (prompt decay of $\pi^0$). In the energy region between 200GeV and $\sim$1TeV, the background rejection of VERITAS is so good that diffuse cosmic-ray electrons are the dominant background instead of hadronic cosmic rays. The region below 200GeV is limited by hadronic cosmic-ray events whose reconstruction is significantly affected by small fluctuations in the NSB. The more sensitive of the two curves indicates a relatively dark observation region (like an AGN with no bright stars in the FoV) while the less sensitive curve indicates a region where the NSB light is approximately 4 times brighter (as in some regions of the Galactic plane).