Introduction

The performance characteristics required of the VERITAS array of imaging Cherenkov telescopes are derived from its scientific goals. VERITAS was designed to optimize sensitivity and flexibility for observing a variety of sources, each of which may require a different subset of capabilities (e.g., continuous monitoring, large field of view, good flux sensitivity, large collection area, accurate angular resolution, low energy threshold, accurate energy resolution, prompt response, or broad energy coverage). With these features, VERITAS can observe known and anticipated sources in several observing modes. In addition, the array permits flexible reconfiguration to pursue unanticipated discoveries. Briefly summarized, the VERITAS scientific objectives are:

AGN studies:

The highly variable flaring activity observed in known TeV sources suggests that continuous monitoring of many AGN, even using instruments with somewhat reduced capabilities, may be an efficient means of detecting these objects. The determination of energy spectra and study of short time-scale variability require the best energy resolution and the largest collection area achievable. Extending the energy range to below 100GeV is crucial for detecting high redshift objects and additional classes of AGN with cutoffs below a few hundred GeV.

Galactic plane survey:

A large field of view, good flux sensitivity, large collection area are required to discover stable and variable sources through a large area survey. Also, broad energy coverage will increase sensitivity to objects with a variety of emission spectra. Because this requires substantial observing time, this survey can be accomplished most efficiently if the array can be split into sub-arrays.

Unidentified EGRET sources:

Accurate angular resolution (for source position reconstruction), wide field of view (for poorly constrained source positions), and low energy threshold (to detect previously unknown pulsars) are essential to understanding the nature of the unidentified EGRET sources.

Supernova remnants:

Excellent sensitivity in the 200GeV - 2TeV energy range is needed for this task. Also required is excellent angular resolution to study these extended sources and accurate spectra over a broad energy range to resolve contributions to the emission.

EBL studies:

Accurate energy spectra from sources at a range of redshifts are needed to resolve the effects of attenuation of gamma-rays by pair-production with extragalactic background light (EBL). Large photon statistics (i.e., large effective area) and good energy resolution are needed for accurate spectral measurements. Also, spectroscopy in the region around 100GeV is of particular importance for estimating the EBL spectral density in the whole wavelength range from 0.1-100$\mu$m because the amount of absorption from the EBL is expected to rapidly decrease there.

GRBs:

Prompt response to detect a rapidly decreasing flux, a large field of view for poorly located sources, and the ability to operate below 100GeV to extend the visibility range to high redshifts are desired.

Dark matter candidates:

Broad energy coverage, good energy resolution and good flux sensitivity are the most important requirements for detecting a neutralino annihilation line whose specific energy and flux is not well-constrained.

Pulsars:

Operation below 100GeV is required due to the expected spectral cutoffs in these objects.

The energy ranges of these areas of interest as well as the range of sensitivity of gamma-ray telescopes is shown in Figure 1.

  

Figure 1: Energy ranges covered with good sensitivity by the seven-telescope VERITAS (Array), a three-telescope array (Sub-array), and GLAST compared to the relevant energy ranges for studying some scientific topics of interest to VERITAS. The hashed area emphasizes the region not covered by the three-telescope sub-array.