VERITAS

This page summarizes the sensitivity and performance of VERITAS.  In summer 2009 the array layout was changed; this provided a ~15% improvement in sensitivity, corresponding to a ~25% reduction in the time required to detect a source. Figures below show both the post- and pre-2009 sensitivities, as well as resolutions and thresholds from prior to the array move.

An overview of VERITAS performance is available here (pdf) . Questions can be addressed to Jamie Holder (jholder at physics dot udel dot edu) or Rene Ong (rene at astro dot ucla dot edu).

Key characteristics:

• energy range: 100 GeV to >30 TeV (spectral reconstruction starts at 150 GeV)
• energy resolution: 15% at 1 TeV
• peak effective area: 100,000 m²
• angular resolution: 0.1 deg at 1 TeV, 0.14 deg at 200 GeV (68% containment radius)
• source location accuracy: 50 arcseconds
• point source sensitivity (with new array configuration as of summer 2009): 1% Crab in < 30h, 10% in 30 min
• observation time per year: ~750 hours non-moonlight, ~200 hours moonlight. Typically 70-100 hours total per month over 10 months.

Critical Information:

• VERITAS observes only under clear, dark skies: Observations are not possible under very cloudy conditions, or when it is raining (see here for local weather conditions). Observations can be made under moonlight, when the moon is less than half full. The observatory shuts down for ~6 days every month around full moon.
• VERITAS does not observe during the summer: Observations are not possible during the summer months of July and August, due to local monsoon conditions.
• VERITAS is located near Tucson, Arizona: at +31° 40' 30.21", -110° 57' 7.77" ; Altitude 1268 m (4159 ft).
• VERITAS works best for sources at high elevation angles: For observations made below 60 deg elevation, Cherenkov telescopes have significantly reduced sensitivity and higher energy threshold. This limits most VERITAS targets to declinations 0 deg to +60 deg; exceptions can be made for particularly interesting targets slightly outside of this region (-10 deg to +70 deg). A useful source visibility tool is available here

Plots (new array configuration after Summer 2009):

(click to zoom)

• Observation time vs source strength for a 5 sigma signal or at least 10 events (elevation: 70 deg):

Plots (array configuration until Summer 2009):

(click to zoom)

• Observation time vs source strength for a 5 sigma signal or at least 10 events (elevation: 70 deg):

• Angular resolution (68% containment) vs primary energy (elevation: 70 deg)
  

• VERITAS sensitivity decreases with source offset from the centre of the field of view. Below are measured gamma-ray and background rates from observations on the Crab Nebula compared with Monte Carlo simulations. Background rates are multiplied by an offset for better visibility. The range of Monte Carlo prediction is given by the uncertainty of flux measurement on the Crab Nebula (elevation: 70 deg)

• Energy resolution vs primary energy (elevation: 70 deg)

• Energy threshold is defined as peak in the differential counting rate for a Crab-like source. Here we show the increase in energy threshold for sources at larger zenith angles (smaller elevation angles) (black: Crab-like spectrum event selection cuts, red: soft spectrum event selection cuts)