VERITAS, an Imaging Atmospheric Cherenkov Telescope (IACT) system for
gammma-ray astronomy in the GeV-TeV range, has recently completed its first
season of observations with a full array of four telescopes. A number of
astrophysical gamma-ray sources have been detected, both galactic and
extragalactic, including sources previously unknown at TeV energies. We
describe the status of the array and some highlight results, and assess the
technical performance, sensitivity and shower reconstruction capabilities.Comment: Submitted to Proceedings of "4th Heidelberg International Symposium
  on High Energy Gamma-Ray Astronomy 2008

A. Cannon

A. Cesarini

A. D. Falcone

A. Furniss

A. Imran

A. Konopelko

A. McCann

A. W. Smith

A. Weinstein

B. Zitzer

C. M. Hui

D. A. Williams

D. B. Kieda

D. Hanna

D. Horan

D. Pandel

D. Steele

E. Aliu

E. Hays

F. Krennrich

Felix A. Aharonian

Frank Rieger

G. Finnegan

G. Gyuk

G. H. Gillanders

G. H. Sembroski

G. Maier

H. J. Rose

H. Krawczynski

J. A. Toner

J. E. Ward

J. Grube

J. H. Buckley

J. Holder

J. Kildea

J. Niemiec

J. P. Finley

J. Quinn

J. S. Perkins

K. L. Byrum

K. Ragan

L. C. Reyes

L. Ciupik

L. F. Fortson

L. Valcarcel

M. Beilicke

M. J. Lang

M. K. Daniel

M. Kertzman

M. McCutcheon

M. Pohl

M. Schroedter

M. Wood

N. Karlsson

O. Celik

P. Cogan

P. Colin

P. Fortin

P. Kaaret

P. Moriarty

P. T. Reynolds

R. A. Ong

R. Guenette

R. J. White

R. Mukherjee

R. Wagner

S. A. Wissel

S. J. Fegan

S. LeBohec

S. M. Bradbury

S. P. Swordy

S. P. Wakely

T. Arlen

T. B. Humensky

T. C. Weekes

T. Ergin

T. Nagai

V. A. Acciari

V. Bugaev

V. V. Vassiliev

W. Benbow

W. Cui

Werner Hofmann

Y. Butt

Y. C. K. Chow

English

arXiv

Crossref

Status of the VERITAS Observatory

VERITAS, an Imaging Atmospheric Cherenkov Telescope (IACT) system for gammma-ray astronomy in the GeV-TeV range, has recently completed its first season of observations with a full array of four telescopes. A number of astrophysical gamma-ray sources have been detected, both galactic and extragalactic, including sources previously unknown at TeV energies. We describe the status of the array and some highlight results, and assess the technical performance, sensitivity and shower reconstruction capabilities

Holder, J.

Acciari, V. A.

Aliu, E.

Arlen, T.

Beilicke, M.

Benbow, W.

Bradbury, S. M.

Buckley, J. H.

Bugaev, V.

Butt, Y.

Byrum, K. L.

Cannon, A.

Celik, O.

Cesarini, A.

Ciupik, L.

Chow, Y. C. K.

Cogan, P.

Colin, P.

Cui, W.

Daniel, M. K.

Ergin, Tülün

Falcone, A. D.

Fegan, S. J.

Finley, J. P.

Finnegan, G.

Fortin, P.

Fortson, L. F.

Furniss, A.

Gillanders, G. H.

Grube, J.

Guenette, R.

Gyuk, G.

Hanna, D.

Hays, E.

Horan, D.

Hui, C. M.

Humensky, T. B.

Imran, A.

Kaaret, P.

Karlsson, N.

Kertzman, M.

Kieda, D. B.

Kildea, J.

Konopelko, A.

Krawczynski, H.

Krennrich, F.

Lang, M. J.

LeBohec, S.

Maier, G.

McCann, A.

McCutcheon, M.

Moriarty, P.

Mukherjee, R.

Nagai, T.

Niemiec, J.

Ong, R. A.

Pandel, D.

Perkins, J. S.

Pohl, M.

Quinn, J.

Ragan, K.

Reyes, L. C.

Reynolds, P. T.

Rose, H. J.

Schroedter, M.

Sembroski, G. H.

Smith, A. W.

Steele, D.

Swordy, S. P.

Toner, J. A.

Valcarcel, L.

Vassiliev, V. V.

Wagner, R.

Wakely, S. P.

Ward, J. E.

Weekes, T. C.

Weinstein, A.

White, R. J.

Williams, D. A.

Wissel, S. A.

Wood, M.

Zitzer, B.

OpenMETU (Middle East Technical University)

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Status of the VERITAS Observatory
J. Holder∗ and V.A. Acciari, E. Aliu, T. Arlen, M. Beilicke, W. Benbow,
S.M. Bradbury, J.H. Buckley, V. Bugaev, Y. Butt, K.L. Byrum,A. Cannon,
O. Celik, A. Cesarini, L. Ciupik, Y.C.K. Chow, P. Cogan, P. Colin, W. Cui,
M.K. Daniel, T. Ergin, A.D. Falcone, S.J. Fegan, J.P. Finley, G. Finnegan, P. Fortin,
L.F. Fortson, A. Furniss, G.H. Gillanders, J. Grube, R. Guenette, G. Gyuk,
D. Hanna, E. Hays, D. Horan, C.M. Hui, T.B. Humensky, A. Imran, P. Kaaret,
N. Karlsson, M. Kertzman, D.B. Kieda, J. Kildea, A. Konopelko, H. Krawczynski,
F. Krennrich, M.J. Lang, S. LeBohec, G. Maier, A. McCann, M. McCutcheon,
P. Moriarty, R. Mukherjee, T. Nagai, J. Niemiec, R.A. Ong, D.Pandel, J.S. Perkins,
M. Pohl, J. Quinn, K. Ragan, L.C. Reyes, P.T. Reynolds, H.J. Rose, M. Schroedter,
G.H. Sembroski, A.W. Smith, D. Steele, S.P. Swordy, J.A. Toner, L. Valcarcel,
V.V. Vassiliev, R. Wagner, S.P. Wakely, J.E. Ward, T.C. Weekes, A. Weinstein,
R.J. White, D.A. Williams, S.A. Wissel, M. Wood, B. Zitzer†
∗Dept. of Physics and Astronomy and the Bartol Research Institute, University of Delaware, DE 19716, USA
†seehttp://veritas.sao.arizona.edu for a full list of affiliations
Abstract. VERITAS, an Imaging Atmospheric Cherenkov Telescope (IACT) system for gammma-ray astronomy in the
GeV-TeV range, has recently completed its first season of observations with a full array of four telescopes. A number of
astrophysical gamma-ray sources have been detected, both galactic and extragalactic, including sources previously unknown at
TeV energies. We describe the status of the array and some highlig t results, and assess the technical performance, sensitivity
and shower reconstruction capabilities.
Keywords: Gamma-ray telescopes
PACS: 95.45.+i, 95.55.Ka
INTRODUCTION
The current generation of TeV gamma-ray telescopes,
H.E.S.S., MAGIC and VERITAS, provide an order of
magnitude sensitivity improvement over previous instru-
ments, and have dramatically increased the number of
astrophysical gamma-ray sources, and source classes,
available for study. VERITAS is the most recent of these
observatories to come online, and has now completed its
first full season of observations.
The VERITAS array [1] comprises four, 12 m diame-
ter imaging atmospheric Cherenkov telescopes located in
Tucson, Arizona (31◦40′30′′ N,110◦57′07′′ W, 1268 m
above sea level) (Figure 1). Each telescope is equipped
with a pixellated photomultiplier tube (PMT) camera,
providing a∼ 3.5◦ field of view. The array layout gives
telescope baselines varying from 35 m to 108 m. Con-
struction of the array began with a prototype instrument
in Fall 2003; the first full telescope was commissioned in
winter 2005, and the full array completed in spring 2007.
VERITAS observations are made under clear sky con-
ditions from September to June, with a summer shut-
down due to local monsoon conditions. The 2007-2008
observing season provided∼ 700 hours of good, dark
sky observations as well as∼ 100 hours taken under
partial moonlight. Moonlight observations are generally
made with the standard high voltage and trigger condi-
tions, and provide a valuable increase in duty cycle: ob-
ervations of the newly discovered TeV AGN WComae
[2], made under moonlight, led to the detection of a flare
with a flux of∼ 25% of the Crab Nebula flux [3].
The Whipple 10 m telescope operates on a similar
schedule at a distance of 7 km from the VERITAS site,
and provides a trigger for high flux (> 10% Crab flux)
transient events in known TeV sources. A recent Whipple
10 m trigger led to the prompt measurement by VERI-
TAS of an extreme flaring state in the TeV blazar Markar-
ian 421, peaking at∼ 10 Crab [4].
FIGURE 1. The VERITAS Array
PERFORMANCE
Each PMT pixel in the telescope cameras is instrumented
with a custom-built 500 Mega-samples per second Flash
ADC [5]. The FADC traces are read-out when the ar-
ray trigger conditions are met (two out of four telescopes
triggered, each with the signals from at least three ad-
jacent PMTs crossing their individual constant fraction
discriminator thresholds). In the first stage of the analy-
sis, the signal traces are integrated. The resultant shower
images are then calibrated, cleaned and parameterized.
Gamma-ray candidate events are selected based primar-
ily on the image shape (using “reduced mean-scaled
width andlength” parameters) and the reconstructed an-
gular distance from the candidate source location,θ ([6],
and references therein). The following sections present
some of the critical performance metrics of the system.
Angular Reconstruction
Angular resolution is a function of both the analysis
procedure and the absolute pointing accuracy of the tele-
scopes in the array. Calibration of the telescope pointing
is performed on a monthly basis, and the results input to a
pointing model in the telescope tracking software which
corrects for the measured misalignments and flexures
during observations. This provides a systematic pointing
accuracy of∼ 0.02◦. Recently installed optical pointing
monitor telescopes provide further improvement offline.
The optimum angular resolution and sensitivity to
weak sources is achieved when relatively strict gamma-
ray selection cuts are applied. The results shown here re-
quire that all four telescopes are involved in the shower
reconstruction, and that each image has an integrated
charge (size) per telescope of> 75 photo-electrons. With
these requirements we obtain the energy dependent an-
gular resolution illustrated in Figure 2. Averaged over
all energies, the angular resolution for a single event is
∼ 0.1◦ (68% containment radius). This source localiza-
tion capability allows us to distinguish between point
sources and sources with a small angular extent, as il-
lustrated by VERITAS observations of the extended TeV
source in the IC443 supernova remnant [8].
Sensitivity
Using the same strict cuts, with the addition of an an-
gular cut ofθ < 0.12◦, Figure 3 illustrates the current
sensitivity of the array as measured using Crab Nebula
observations. An unresolved source with a flux of 1%
that of the Crab Nebula is detected in 47 hours of obser-
vations, a 5% Crab source requires 2.5 hours. The TeV
blazar 1ES 0806+524 has been detected by VERITAS
at this flux level [9]. Figure 3 also shows the gamma-
ray rate as a function of the source position in the field
of view. The array has useful sensitivity out to at least
2.0◦, which has allowed the measurement of a gamma-
ray flux from 1ES 1218+304 during observations cen-
tered around WComae, separated by 1.94◦ [2]
Energy Reconstruction
The energy of incident gamma rays is calculated from
the measured impact parameter, imagesiz , and azimuth
and elevation angle using lookup tables filled from sim-
ulated gamma-ray events. The energy scale of the sim-
ulations is calibrated using a variety of complementary
methods, such asin situmeasurements of the PMT single
photo-electron response, studies of the light yield from
local muons, and measurements of the Rayleigh scattered
light from a nearby laser pulse [10]. In order to provide
a wide dynamic range for the energy spectra, gamma-
ray selection cuts are loosened to allow any event with
at least two telescopes recording an image with >4 pix-
 energy [TeV]10log
-1 -0.5 0 0.5 1
an
g
u
la
r 
re
so
lu
ti
o
n
 [
d
eg
] 
(6
8%
)
0
0.05
0.1
0.15
0.2
]2 [deg2θ
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
N
u
m
b
er
 o
f 
ev
en
ts
0
100
200
300
400
FIGURE 2. Left: The angular resolution as a function of energy, measured using simulated gamma-ray events.Right: Theθ 2
distribution for the gamma-ray excess measured during observations of the Crab Nebula. The fit is a combination of two Gaussian
functions, as described in [7]
flux [Crab Units]
−310 −210 −110 1 10
ob
se
rv
at
io
n 
tim
e 
[h
ou
rs
]
−210
−110
1
10
210
310
1% Crab in 47 hours
5% Crab in 2.5 hours
Source Offset (degrees)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
R
el
at
iv
e 
ga
m
m
a−
ra
y 
ra
te
0
0.2
0.4
0.6
0.8
1
1.2
1.4
FIGURE 3. Left: The time required for a 5σ detection for different source strengths, assuming a Crab-like gamma-ray spectrum.
Right: The gamma-ray rate as a function of source position, measured using observations of Crab Nebula. The measurements are
normalized to the measured rate at an offset of 0.5◦.
FIGURE 4. The VERITAS source catalogue as of summer 2008 (in galactic coordinates). Extragalactic sources are labelled in
red, those within our galaxy are labelled in black. The shaded ar a shows the region of the sky visible to VERITAS at elevations
greater than 55◦. Figure provided courtesy of TevCat (http://tevcat.uchicago.edu/)
els, excluding the telescope combination with the small-
est baseline (Telescopes 1 and 4). Spectral reconstruction
is possible from a minimum energy of∼ 150 GeV and
with an energy resolution of∼ 15% at high energies.
SUMMARY
We have completed the first full season of observa-
tions with the four-telescope VERITAS array. The tech-
nical performance, site conditions and mechanical re-
liability are good, providing∼ 800 hours of clear sky
observations each year. Highlight results from this first
season include the detection of two new TeV sources,
1ES 0806+524 and WComae, and detailed studies of the
supernova remnants IC443 and Cas A, including a mea-
surement of an extended TeV source in IC443 [8]. Ex-
tensive multiwavelength observations have been made
of known extragalactic TeV sources, including a com-
prehensive multi-instrument campaign on M87 [11] the
detection of extreme flaring from Markarian 421 and
the highest recorded flux from 1ES 2344+514 [9]. Ad-
ditional observations include uniquely interesting galac-
tic sources such as the TeV binary LS I+61◦303 [12]
and HESS J1908+063 [13], identified with the Milagro
source MGRO J1908+06.
We conclude by presenting the VERITAS source cat-
alogue at the close of this first season (Figure 4), con-
taining 13 northern hemisphere TeV sources. Ongoing
projects include a survey of the Cygnus region of the
galactic plane, and a search for gamma-ray signatures
from objects predicted to have an enhanced dark matter
component.
ACKNOWLEDGMENTS
This research is supported by grants from the U.S. De-
partment of Energy, the U.S. National Science Founda-
tion, the Smithsonian Institution, by NSERC in Canada,
PPARC in the U.K. and by Science Foundation, Ireland.
REFERENCES
1. Holder, J., et al.Astropart. Phys., 25, 391 (2006).
2. Acciari, V.A. et al (The VERITAS Collaboration),ApJ,
648, L73 (2008).
3. Swordy, S. et al (The VERITAS Collaboration),
Astronomer’s Telegram #1565(2008).
4. Reyes, L. et al (The VERITAS Collaboration),These
proceedings(2008).
5. Buckley, J.H. et al.Proc. 28th ICRC, Tsukuba, Eds: T.
Kajita, Y. Asaoka, A. Kawachi, Y. Matsubara and M.
Sasaki, 2827 (2003).
6. Acciari, V.A. et al (The VERITAS Collaboration),ApJ,
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7. Lemoine-Goumard, M., Degrange, B. & Tluczykont, M.,
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proceedings(2008).
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