Introduction

VERITAS is a major new ground-based observatory for studying nonthermal astrophysics in the gamma-ray band above 100 GeV. VERITAS is currently located at the Fred Lawrence Whipple Observatory Base Camp (111º W, 32º N) in southern Arizona, USA. It will have more than an order of magnitude greater sensitivity than the Whipple gamma-ray telescope, its predecessor and the pioneering instrument in the field.

Stereo observations with the first two of four telescopes began in January, 2006. For this commissioning period, known TeV sources were observed. Results from one of these sources, the blazar Mrk421, are presented here to demonstrate the performance of the first two telescopes.

VERITAS Telescope 1

12-m diameter reflector 499-PMT camera Electronics Shed

• Davies-Cotton f/1.0 Optics. Total mirror area =

106 m

2 • Installed at Whipple Basecamp on Mt. Hopkins (1275m elevation) in January 2005

Camera Design

499-Pixel Camera Photonis XP2970/02 PMTs 3.5

º

FoV 0.15

º

pixel spacing

Lightcones installed 01/06



~30% increase in photon collection.

Mechanical Performance

• Alt-Az mount • Slew speed

1.0º/sec

• Tracking accuracy – Raw pointing error RMS –

~20-30”

g -ray source location good

• Precision continuous pointing monitor under development

Very Energetic Radiation Imaging Telescope Array System

T4 T3 T1 T2 T2 T3 109m 82m 85m T1 T4 35m Composite Camera View

Stereoscopy

• Night-sky and muon events strongly suppressed at trigger level by two-telescope trigger requirement.

• Source position in field of view reconstructed using intersection of image axes.

•

Improved angular and energy resolution

T1 T2

Stereo view of a large cosmic-ray event.

Individual Camera Views

T1 T2

Multiple views of the shower allow a more accurate determination of its core location and arrival direction, yielding an enhanced angular resolution and improved energy resolution.

•

Lower energy threshold

The background due to local muons is an important factor with regard to the lower limit on the energy threshold of a single telescope; thus eliminating that background, together with increasing the total mirror area and improving the discrimination between gamma rays and cosmic rays all act to lower the energy threshold.

Stereo view of a smaller event, along with summed FADC traces (2-ns samples).

Stereo On/Off observations of Mrk421

• 7 pairs (3.3 hrs On time) from April, 2006.

• Additional stereo observations in wobble mode also yielded detections of Mrk421 and Mrk501.

• Left: 2-D significance map using a ring background model.

• Center: q 2 plot showing good agreement between On and Off observations away from the source and a strong gamma-ray signal at q 2 < 0.03 deg 2 .

• Right: Alpha plots for the individual telescopes, showing comparable performance by the individual telescopes.

Preliminary

T1

Preliminary Preliminary

On Off On

T2

Preliminary

Off

Trigger System

Single Telescope

Level 1: Programmable CFD for each pixel Level 2: Pattern selection trigger requires any 3 adjacent pixels to fire within 10 ns Trigger rate

~ 150 Hz

With light cones

~ 225 Hz

NSB

Hardware Stereo: Array Trigger

Level 3: Array trigger requires multiple telescope triggers at hardware level.

Installed March 2006 Delay corrections dependent on pointing direction Coincidence window:

~ 100 ns

Trigger rate

~ 100 Hz

Deadtime

~ 5 %

Air Showers

Status and Plans

Site prepared for four telescopes at Horseshoe Canyon, Kitt Peak National Observatory. Access currently restricted; construction of array nearly complete at temporary location: Fred Lawrence Whipple Observatory (FLWO) Base Camp.

We will operate a three-telescope array during Fall 2006 and the full four-telescope array beginning January 2007.

Two-year observing program (2007/2008) planned for FLWO site: • Sky Survey of Cygnus region • AGN monitoring and searches for new AGN; EBL studies • SNRs and PWNe studies: spectra and morphology • Dark Matter search • Other topics Compared to Kitt Peak, FLWO site has comparable collection area and < 20% higher energy threshold (see figures below).

Optical Performance

• 350 mirror facets – glass, aluminum coated on-site • reflectivity

>90%

nm at 320

PMT size

Gamma/Hadron Separation

Background rejection is based on differences in the air-shower development between gamma-ray and cosmic-ray primaries (and, for targets with a known position in the FoV, on the shower direction).

MSW On (blue) and Off (black pts) -ray excess

• Leads to morphological differences in camera images.

• Gamma-ray showers are primarily electromagnetic.

 compact, regular images.

• Cosmic-ray showers are primarily hadronic.

 images larger, often uneven light distribution (subshowers).

• Parameterize images as ellipses and use ellipse “length” and “width” (semi-major and semi-minor axes) to reject cosmic-ray primaries.

• Combine images from multiple telescopes with weightings determined via simulations: mean scaled length (MSL) and width (MSW).

MSL On (blue) and Off (black pts)

g

-ray excess On - Off On - Off Acknowledgements

This research is supported by grants from the U.S. Department of Energy, the National Science Foundation, the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland, and by PPARC in the UK.