Energy resolution

The energy resolution of VERITAS will be considerably better than that of the Whipple telescope for three reasons: (1) the shower core location will be known, (2) several telescopes will view each event at different distances from the shower core, and (3) each camera will have finer pixellation (0.15$^\circ$ vs. 0.25$^\circ$). The location of the core of the shower will be estimated with an accuracy of about 10m. With a single telescope, the core distance can be estimated using the location of the image centroid relative to the source position, but the correlation is not very tight and the corresponding energy estimate is not very accurate. For the Whipple telescope, the RMS energy resolution using this technique is $\Delta E/E \approx 0.35$ ([Mohanty et al. 1998]).

For our estimate of energy resolution for VERITAS, we used simulated showers from gamma-rays with energies >200GeV whose arrival direction was reconstructed to within 0.1$^\circ$ of the source position. We also required that each telescope record >20p.e., after image cleaning, to avoid images significantly contaminated by NSB fluctuations. The reconstructed energy for an individual telescope is a function of the amount of light in the shower, the distance of the telescope from the shower core, and the location of the image centroid relative to the source position. With the core location known, the centroid location gives information about the height of emission detected by the telescope, further improving the resolution compared to a single telescope. The final energy estimate for the array is the average from individual telescopes which detected at least 20p.e.

The resulting overall energy RMS resolution is $\mathbf \Delta {\bf E}/{\bf E} \approx$ 0.10 - 0.15 from 0.2 to 10TeV. The resolution improves slowly as the energy of the shower increases. Below 200GeV, the energy resolution degrades somewhat, but not significantly. This resolution will likely be improved through more sophisticated energy estimates and through the use of more restrictive cuts on events used in the energy analysis.