VERITAS: Status and Highlights

Abstract

The VERITAS telescope array has been operating smoothly since 2007, and has
detected gamma-ray emission above 100 GeV from 40 astrophysical sources. These
include blazars, pulsar wind nebulae, supernova remnants, gamma-ray binary
systems, a starburst galaxy, a radio galaxy, the Crab pulsar, and gamma-ray
sources whose origin remains unidentified. In 2009, the array was reconfigured,
greatly improving the sensitivity. We summarize the current status of the
observatory, describe some of the scientific highlights since 2009, and outline
plans for the future.

Full text

Page 1

arXiv:1111.1225v1 [astro-ph.HE] 4 Nov 2011
32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
VERITAS: Status and Highlights
J. HOLDER1FOR THE VERITAS COLLABORATION2: E. ALIU, T. ARLEN, T. AUNE, M. BEILICKE, W. BEN-
BOW, M. B¨ OTTCHER, A. BOUVIER, J. H. BUCKLEY, V. BUGAEV, K. BYRUM, A. CANNON, A. CESARINI,
J. L. CHRISTIANSEN, L. CIUPIK, E. COLLINS-HUGHES, M. P. CONNOLLY, W. CUI, R. DICKHERBER, C. DUKE,
V. V. DWARKADAS, M. ERRANDO, A. FALCONE, J. P. FINLEY, G. FINNEGAN, L. FORTSON, A. FURNISS,
N. GALANTE, D. GALL, K. GIBBS, G. H. GILLANDERS, S. GODAMBE, S. GRIFFIN, J. GRUBE, R. GUENETTE,
G. GYUK, D. HANNA, J. HOLDER, H. HUAN, G. HUGHES, C. M. HUI, T. B. HUMENSKY, A. IMRAN, P. KAARET,
N. KARLSSON, M. KERTZMAN, Y. KHASSEN, D. KIEDA, H. KRAWCZYNSKI, F. KRENNRICH, M. J. LANG,
M. LYUTIKOV, A. S MADHAVAN, G. MAIER, P. MAJUMDAR, S. MCARTHUR, A. MCCANN, M. MCCUTCHEON,
P. MORIARTY, R. MUKHERJEE, P. D NU˜ NEZ, R. A. ONG, M. ORR, A. N. OTTE, N. PARK, J. S. PERKINS, A. PICHEL,
F. PIZLO, M. POHL, H. PROKOPH, J. QUINN, K. RAGAN, L. C. REYES, P. T. REYNOLDS, A. C. ROVERO, J. RUPPEL,
A. C. SADUN, D. B. SAXON, M. SCHROEDTER, G. H. SEMBROSKI, G. D. S ¸ ENT¨ URK, A. W. SMITH, D. STASZAK,
G. TEˇ SI´ C, M. THEILING, S. THIBADEAU, K. TSURUSAKI, J. TYLER, A. VARLOTTA, V. V. VASSILIEV, S. VINCENT,
M. VIVIER, S. P. WAKELY, J. E. WARD, T. C. WEEKES, A. WEINSTEIN, T. WEISGARBER, D. A. WILLIAMS,
B. ZITZER
1Department of Physics and Astronomy and the Bartol Research Institute, University of Delaware, Newark, DE 19711.
2see http://veritas.sao.arizona.edu/conferences/authors?icrc2011for a full list of affiliations
jholder@physics.udel.edu
Abstract: The VERITAS telescope array has been operating smoothly since 2007, and has detected gamma-ray emission
above 100 GeV from 40 astrophysical sources. These include blazars, pulsar wind nebulae, supernova remnants, gamma-
ray binary systems, a starburst galaxy, a radio galaxy, the Crab pulsar, and gamma-ray sources whose origin remains
unidentified. In 2009, the array was reconfigured, greatly improving the sensitivity. We summarize the current status of
the observatory, describe some of the scientific highlights since 2009, and outline plans for the future.
Keywords: Gamma-ray astronomy, VERITAS
1Overview: VERITAS
VERITAS (the Very Energetic Radiation Imaging Tele-
scope Array System) is an array of four atmospheric
Cherenkov telescopes located near Tucson in southern Ari-
zona (31◦40′N 110◦57′W, 1268 m a.s.l) [1]. It is used
to study astrophysical sources of gamma-ray emission in
the 100 GeV-30 TeV range via the imaging atmospheric
Cherenkov technique.The array was commissioned in
2007, and the source catalogue now contains 40 objects,
includingpulsarwind nebulae (PWN), supernovaremnants
(SNR), active galaxies, binary systems, a starburst galaxy
andapulsar,alongwithotherobjectswhosenatureremains
unclear.
Eachofthefourtelescopesconsistsofa12 mdiameterseg-
mented reflector, at the focus of which is a 499-pixel pho-
tomultiplier tube (PMT) camera covering a field of view of
3.5◦[2]. A coincident Cherenkov signal in 2 out of 4 tele-
scopes triggers a read-out of the PMT signals by custom-
built 500 MSPS FADCs, at a typical rate of ∼ 250 Hz.
The resulting images in each camera are parameterized by
their second moments, and these parameters are then used
to discriminate gamma-rayinitiated air showers from those
initiated by cosmic ray particles. The recorded images are
also used to reconstruct the energy and arrival direction of
the primaryphoton. The angular resolutionand energyres-
olution of the reconstruction is energy dependent, reaching
∼ 0.1◦and ∼ 15%, respectively, for gamma-ray primaries
with an energy of 1 TeV.
The sensitivity of the array can be quantified by the observ-
ing time required to detect a typical weak source. The sen-
sitivity of VERITAS has steadily improved over the life-
time of the array, due to improvements in data analysis
techniques, optical alignment, calibration and, most signif-
icantly, following the relocation of the original prototype
telescope to a more favourable location in 2009. Currently,

Page 2

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
Figure 1: The VERITAS array of four gamma-ray telescopes in its initial (Top) and present (Bottom) configuration.
a source with a flux of 1% of the Crab Nebula flux and a
spectrum similar to the Crab Nebula can be detected in ap-
proximately 25 hours of observations: roughly half of the
time requiredwhenthe arraywas originallycommissioned.
Figure 1 shows the array in both its initial (top) and current
(bottom) configurations. Typically, around 1000 hours of
data are collected every year, ∼ 20% of which are taken
when the moon is visible.
2ExtragalacticTeV Gamma-ray Astronomy
Approximatelytwo-thirds of the VERITAS programof ob-
servationsis dedicatedtothestudyofextragalacticgamma-
ray sources and source candidates. This focus has resulted
in the detection by VERITAS of 21 blazars, one radio
galaxy (M 87) and one starburst galaxy (M 82). The ex-
tragalactic program is described in more detail elsewehere
in these proceedings [3, 4]. Here, we highlight some of the
recent results and ongoing studies.
Table 1 lists all of the extragalactic sources detected,
roughly half of which were first identified as > 100 GeV
gamma-ray sources by VERITAS.
2.1Blazars
The most common extragalactic TeV sources are the
blazars: Active Galactic Nuclei (AGN) in which a rel-
ativistic jet is directed along the line-of-sight to the ob-
server. The first few TeV blazars to be identified were
all high-frequency-peaked BL Lac objects. The combined
Source NameClass
z
Mrk 421
Mrk 501
1ES 2344+514
1ES 1959+650
BL Lac
W Comae
RGB J0710+591
H 1426+428
1ES 0806+524
1ES 0229+200
1ES 1440+122
RX J0648.7+1516
1ES 1218+304
RBS 0413
1ES 0414+009
PG 1553+113
1ES0502+675
3C 66A
B2 1215+30
PKS 1424+240
VER J0521+211
M 87
M 82
HBL
HBL
HBL
HBL
LBL
IBL
HBL
HBL
HBL
HBL
IBL
HBL
HBL
HBL
HBL
HBL
HBL
IBL
LBL
I/HBL
HBL
FR I
Starburst
0.030
0.034
0.044
0.047
0.069
0.102
0.125
0.129
0.138
0.139
0.162
0.179
0.182
0.190
0.287
0.43 < z < 0.50
?
?
?
?
?
16.7 Mpc
3.9 Mpc
Table 1: Extragalacticsources of TeV gamma-rayemission
detected by VERITAS.

Page 3

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
Figure 2: The VERITAS source map, in Galactic coordinates, as of July 2011.
Figure 3: Significance map for a single field of view con-
taining three blazars. Each source is point-like; the appar-
ent size differences are due to saturation on the z-scale
.
results of H.E.S.S., MAGIC and VERITAS have dramati-
cally expanded the TeV blazar catalogue over the past few
years, allowing population studies to be carried out and
adding new classes of object. Originally, due mainly to
limited sensitivity and biases in target selection, only high-
frequencypeaked BL Lac objects were detected at TeV en-
ergies. More recently, intermediate- and low-frequency-
peaked BL Lacs have been observed [5], as well as flat-
spectrum radio quasars. The population is also growing as
the catalogue extends to ever larger distances. An illustra-
tion of these developments is provided by Figure 3, which
shows VERITAS observations of three blazars, one high-
frequencyand two intermediate-frequencyBL Lac objects,
all contained within the 3.5◦field-of-view of the array.
VERITAS observations of blazars have, in many cases,
been guided and enhanced by the results of observations
at other wavelengths. In particular, the Fermi-LAT instru-
ment provides a daily view of the entire sky in the en-
ergy range just below that covered by VERITAS, and the
VERITAS blazar discovery program now focuses on ob-
jects detected by Fermi-LAT. Objects are typically selected
from the LAT catalog based either upon a power-law ex-
trapolation of their energy spectra, or upon the detection of
clusters of high energy (> 10 GeV) photons by the LAT.
We have also implementedanalysis pipelines that automat-
ically processandanalyzeFermi-LATdata ona dailybasis,
in order to identify flaring objects [6].
TheWhipple10mtelescopehasplayedanimportantrolein
the blazar program,providingregular monitoringof bright,
known, variableblazars such as Mkn 421 and Mkn 501[7].
Summer 2011 marks the end of operations for the Whipple
10m, which holds a remarkable record as the longest oper-
ating atmospheric Cherenkov telescope (since 1968), and
as the instrument on which the Whipple Collaboration de-
veloped the imaging technique used to detect the first TeV
source, the Crab Nebula [8].
Highlights from the VERITAS blazar programpresented at
this conference include the measurement of an extremely
bright flare from Mkn 421 in February 2010 [9]. Dur-
ing 5 hours of VERITAS observations on February 17,
gamma-ray emission from the source reached a flux level

Page 4

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
hLightCurvehLightCurve
Entries Entries 5900 5900
Mean 5.522e+04Mean 5.522e+04
RMS RMS
41.4 41.4
MJD
551605518055200552205524055260
-1
s
-2
m
0
2
4
6
8
10
12
14
16
18
-6
10
×
VERITAS Mrk 421 lightcurve E>300 GeV
PRELIMINARY
hLightCurve hLightCurve
Entries Entries 11038 11038
Mean 5.524e+04Mean 5.524e+04
RMS RMS 0.0622 0.0622
MJD
55244.3555244.455244.4555244.5
-1
s
-2
m
5
10
15
20
25
-6
10
×
17 February 2010 flare E>300 GeV
Figure 4: Top Nightly lightcurve of Markarian 421 during the VERITAS 2009-2010 observing campaign, with a zoom
of the flare on the night of February 17, 2010 [9]. Bottom Nightly lightcurve of BL Lacertae during the VERITAS
2010-2011 observing campaign, with a zoom of the flare on the night of June 28, 2011 [12].
of 8 times the steady flux from the Crab Nebula, allowing
precise measurement of the light curve with a time reso-
lution of two minutes (Figure 4). We also summarize ob-
servations of PG 1553+113 between May 2010 and May
2011 [10]. This source is one of the most distant TeV
blazars known, making it an important tool for studies of
the extragalactic background light. The VERITAS spectral
measurements presented here are used to place an upper
limit on the source redshift of z < 0.5 at 95% confidence.
The identification of a new high-frequency-peaked blazar,
RBS 0413, with VERITAS and Fermi-LAT was also pre-
sented [11], providing a good example of the impact of the
LATuponTeVtargetselection. Finally,observationsofBL
Lac (the eponymous BL Lacertae object) were shown. BL
Lac is the first blazar classified as ’low-frequency-peaked’
from which VERITAS has detected gamma-ray emission.
Following reports of activity from several observatories at
other wavelengths in May 2011, VERITAS commenced
monitoring the source and, on June 28, 2011, observed the
tail-end of a flare during 40 minutes of observations, with
further observations curtailed by the rising sun. The flux
decreasedrapidlyfroma maximumof ∼ 50% of the steady
Crab Nebula flux, demonstrating variability on a timescale
of ∼ 4 minutes [5](Figure 4).
2.2 Other Extragalactic Sources
In addition to blazars, VERITAS has been used to observe
various other extragalactic gamma-ray source candidates,
including radio galaxies, galaxy clusters, starburst galax-
ies and globular clusters [4]. Highlights from this program
include the study of gamma-ray emission from M 87 and
M 82, which we summarize here.
2.2.1M 87
M 87 is the central giant radio galaxy of the Virgo cluster,
located at a distance of 16.7 Mpc [13]. Its proximity, and
the fact that its jet is not aligned along the line-of-sight,
allows detailed study of the jet structure in radio, optical
and X-ray wavelengths. This provides the possibility of

Page 5

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
Figure 5: VERITAS lightcurve of M 87 in 2010,
.
identifying which structures in the jet are responsible for
the TeV emission, and so M 87 has been a prime target for
gamma-rayobservatoriessince the initial reportof gamma-
ray emission by the HEGRA array in 2003 [14]. A con-
certed multiwavelength monitoring campaign involving all
of the TeV observatories, as well as Chandra, VLBA and
a number of optical observatories, has been ongoing for
several years (e.g. [15]). Initial multiwavelengthresults in-
dicated that the gamma-ray emission may originate from a
region close to the core of the galaxy. In April 2010, the
brightest flaring event to date was observed, with a peak
flux exceeding 10% of the steady Crab Nebula flux. The
VERITAS lightcurve for 2010 is shown in Figure 5, re-
vealing flux variability during the flare on the timescale of
∼ 1 day.
2.2.2M 82
M 82 is a bright galaxy located at a distance of approxi-
mately 3.9 Mpc, with an active starburst region at its cen-
tre. The star formation rate in this region is approximately
10 times that of the Milky Way, with an estimated super-
nova rate of 0.1 to 0.3 per year. High cosmic-ray and
gas densities in the starburst region make it a promising
target for gamma-ray observations, with gamma-ray emis-
sion expected to result from the interactions of hadronic
cosmic rays in the dense gas. A deep VERITAS expo-
sure (137 hours) in 2008-2009 resulted in a detection of
gamma-ray emission from M 82 with a flux of (3.7 ±
0.8stat± 0.7syst) × 10−13cm−2s−1above the 700 GeV
energythresholdofthe analysis, consistent with the predic-
tions of models based on the acceleration and propagation
of cosmic rays in the starburst core [16].
2.2.3 Gamma-ray Bursts
VERITAS has maintained a continuous program to search
for very high-energy emission associated with gamma-ray
bursts (GRBs). The motivation for such searches has been
given added impetus in recent years, due to the detection
high energyemission from GRBs by Fermi-LAT with a de-
layofhundredsofsecondsfromthestartoftheburst. VER-
ITAS results at this conference focused on bursts which
were detected by both the Fermi and Swift satellites [17].
Since VERITAS is a pointed instrument, with a field of
view of 3.5◦, the telescopes must move into position when
a burst alert is received. The average time delay for this re-
point is 240 s, and in many cases significantly shorter. Pre-
dictions based on the brightest bursts observed by the LAT
indicate that the potential for detecting TeV emission asso-
ciated with a GRB with VERITAS is promising, assuming
no intrinsic spectral cut-off of the high energy emission.
2.2.4Dark Matter Searches
Objects outside of our own galaxyprovidesome of the best
locations in which to search for the annihilation signatures
of dark matter particles. Dwarf spheroidal galaxies of the
local group are among the most promising of these, due
to their proximity, their large dark matter content and the
absence of astrophysical background sources (supernova
remnants, pulsar wind nebulae, etc.). Studies of the stellar
kinematics of the dwarf spheroidal galaxy Segue I indicate
that is is the most dark matter dominated dwarf spheroidal
galaxyknown. Theresultsofa 48hourVERITAS exposure
on Segue I were presented at this conference [18], along
with upper limits on the annihilation cross-section for vari-

Page 6

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
[GeV]
χ
m
3
10
4
10
]
-1
s
3
[cm
v>
σ
<
0
-30
10
-28
10
-26
10
-24
10
-22
10
-20
10
b b
W
W
τ
→
→
→
→
χ
χ
χ
χ
χ
χ
χ
χ
-
W
W
+
, Sommerfeld
-
+
-τ
+
Figure 6: Upper limits at the 95% confidence level on the velocity-weighted annihilation cross-section for different
annihilation channels. The dark band represents the typical range of predictions.
ous annihilation channels, illustrated in Figure 6. The lim-
its lie in the range 10−22− 10−24cm3s−1, depending on
the annihilation channel and dark matter particle mass.
3Galactic TeV Gamma-ray Sources
The VERITAS Galactic catalogue contains 17 sources,
listed inTable 2. VERITAS is locatedin the northernhemi-
sphere, and can therefore only view the outer Galaxy at
high elevation angles (l < −30◦and l > 150◦). The rel-
ative scarcity of Galactic TeV sources in this region, com-
pared to the inner Galaxy, is balanced in part by the variety
of source classes which have been identified and studied.
These include pulsars and their nebulae, supernova rem-
nants, gamma-raybinary systems, and a numberof sources
whose identification remains unclear. Some of the recent
highlightsfrom the Galactic programare summarizedhere.
Additional VERITAS results are presented elsewhere in
these proceedings, including observations of CTA 1 [19],
LS I+61◦303 and 1A 0535+262[20], and VER J2019+407
[21].
3.1 HESS J0632+057
HESS J0632+057 was first identified as an unresolved
point source of TeV gamma rays in HESS observations of
the Monoceros Loop SNR [22]. The possibility that this
source might be a new TeV binary system, comprised of a
massive star and a compact companion, was noted in the
original detection paper, based on the fact that the gamma-
Source NameClass
(likely)
PWN
Pulsar
Binary
SNR
SNR
SNR/PWN
PWN
Binary
PWN
SNR
PWN?
PWN
PWN?
PWN?
UID
UID
UID
Crab Nebula
Crab Pulsar
LS I +61◦303
IC 443
Cas A
G106.3+2.7
G54.1+0.3
HESS J0632+057
CTA 1
Tycho’s SNR
HESS J1857+026
CTB 87
MGRO J1908+06
TeV J2032+4130
Galactic Centre
VER J2019+407
Cyg OB1 TeV complex
Table 2: Galactic sources of TeV gamma-ray emission
detected by VERITAS. Some of the designations are not
definitive; we list here the most likely counterpart. The
unidentified sources (UID) are presumed Galactic due to
their location in the Galactic plane or their angular extent.
ray emission was centered on the location of a massive
emission-linestar (MWC148). Furthersupportforthis idea
came from the detection of variable radio [23] and X-ray

Page 7

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
phase
00.1 0.2 0.30.40.50.60.70.80.91
XRT rate (1.5-10 keV)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
phase
00.10.20.30.4 0.50.6 0.70.8 0.91
]
-1
s
-2
F(E>1.0 TeV) [cm
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-12
10
×
Figure 7: The Swift (Top) and VERITAS/ H.E.S.S. (Bottom) light curves for HESS J0632+057, folded by the 321-day
X-ray period [27].
[24] sources at the same location, and with the discovery
of gamma-ray flux variability by VERITAS [25].
Recently, a long-term monitoring campaign with Swift has
been used to identify a 321-day period in the X-ray emis-
sion, presumablyassociated with the binary orbit [26]. The
X-ray emission is characterized by a bright flare lasting for
∼ 20 days, which may be associated with a periastron pas-
sage. VERITAS and H.E.S.S. observations are presented
at this conference [27] which total 150 hours over the past
seven years, including a deep VERITAS exposure around
the X-ray flare in February2011. A bright gamma-rayflare
is also observed, with the peak slightly offset from the X-
ray flare, suggesting that the gamma-ray emission fades
away at the onset of the X-ray high state. Figure 7 shows
the VERITAS/H.E.S.S. lightcurve, folded by the X-ray pe-
riod.
3.2The Crab Pulsar
Steady emission from the Crab Nebula has provided a
standard candle for ground-based gamma-ray observato-
ries since its detection in 1989 [8]. The Crab pulsar, which
powers the Nebula, is among the most energetic pulsars in
our galaxy, with a spin-down power of 4.6×1038erg s−1.
Existing measurements of the pulsar spectrum can be fit by
a power-law with an exponential cut-off, consistent with
theoretical predictions based on curvature radiation as the
dominant gamma-ray production mechanism [28, 29].

Page 8

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
PhasePhase
-1-1-0.5-0.5000.50.511
Counts per Bin
31003100
2000
32003200
33003300
34003400
3500 3500
3600 3600
3700 3700
Counts per Bin
VERITAS > 120 GeV
Phase
-1 -0.500.51
0
500
1000
1500
Fermi > 100 MeV
Counts per Bin
Energy (MeV)
3
10
4
10
5
10
6
10
)
-1
s
-2
dF/dE (MeV cm
2
E
-7
10
-6
10
-5
10
-4
10
-3
10
VERITAS, this work
Fermi (Abdo et al, 2010)
MAGIC (Aliu et al. 2008)
MAGIC (Albert et al. 2008)
CELESTE (De Naurois et al. 2002)
STACEE (Oser et al. 2001)
HEGRA (Aharonian et al. 2004)
Whipple (Lessard et al. 2000)
Broken power law fit
Exponential cutoff fit
Energy (MeV)
3
10
4
10
5
10
6
10
2
χ
0
5
10
15
20
Power law with exponential cutoff
Broken power law
Figure 8: Top: VERITAS measured pulse profile of the Crab pulsar at energies above 120 GeV [30, 31]. The pulse
profile above 100 MeV from Fermi-LAT is shown underneath for comparison. Bottom: Spectral energy distribution of
the Crab Pulsar in gamma-rays. The solid line shows the result of a fit to the VERITAS and Fermi-LAT data with a
broken power-law. See [30] for details.
A deep, 107 hour, VERITAS exposure of the Crab pul-
sar region now reveals that the pulsar spectrum extends to
much higher energies than previously expected [30, 31].
The emission is fit by a power-law spectrum between 100
and400 GeV withanindexofα = −3.8±0.5stat±0.2sys.
The pulse profile (Figure 8) exhibits two peaks, 2-3 times
narrower than those measured at 100 MeV. The dominant
pulse is observed at phase 0.4 and a smaller pulse at phase
0.1, again, in contrast to the measured profile at 100 MeV.
These results likely require a substantial revision of our
understanding of the high energy emission from pulsars,
both in terms of the location of the emission region, and
the mechanisms responsible.
3.3The Galactic Centre
The Galactic Centre is a complex region at TeV energies,
comprised of a bright source in the direction of SgrA*,
and extended diffuse emission along the Galactic ridge
[32, 33, 34].For VERITAS, the source culminates at
an elevation angle of below 30◦. The path length to the
Cherenkov emission from the air shower is therefore larger
than for observations at high elevation angles, resulting in
an increase in the energy threshold above which gamma-
ray initiated showers can be detected. This is compensated
by a corresponding increase in the collection area for high
energyprimaries,dueto thelargerlight poolon theground.
Usingan analysisspecificallytailoredto suchobservations,

Page 9

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
a 25 hour VERITAS exposure has been used to clearly de-
tect the Galactic Centre gamma-raysource, and to measure
its spectrum above 2 TeV [35] (Figure 9). Future observa-
tions using this method offer the possibility of improving
measurements of the high energy cut-off of the emission
spectrum.
Figure 9: A significancemap of the Galactic Center region,
measured with VERITAS
3.4The Cygnus OB1 Region
The Cygnus region hosts some of the closest and most
active areas of massive star formation and destruction
in the Galaxy, providing numerous potential sources for
TeV gamma-ray production.
extended source in the Milagro Galactic Plane survey,
MGRO J2019+37 overlaps with the Cyg OB1 association,
with a total flux of ∼ 80% of the steady Crab Nebula flux
above 12 TeV, spread over an area of 0.6◦× 1.0◦[36]. At
this conference we present 75 hours of VERITAS observa-
tions centered on this region[19], revealingtwo distinct ar-
eas of gamma-ray excess. One of these, VER J2016+372,
is point-like, within the angular resolution of the instru-
ment, and consistent with the location of a pulsar wind
nebula, CTB 87, which is the likely counterpart. The other
gamma-ray excess is extended over a region which encom-
passes multiple possible counterparts,includingthe power-
fulpulsarPSR J2021+3651andits nebula. Figure10shows
the VERITAS skymap of this region above 650 GeV.
The brightest and most
3.5 Tycho’s SNR
Tycho’s SNR is the remnant of a historical type Ia super-
nova, recorded by Tycho Brahe in 1572. The SNR envi-
ronment and morphologyare relatively simple, making Ty-
cho’s SNR a favored system for theorists seeking to model
particle acceleration processes in SNR blast waves (e.g.
[37]). Evidence for nuclear particle acceleration has al-
ready been claimed, based on detailed studies of the shock
front and contact discontinuity locations [38]. VERITAS
observations of Tycho’s SNR comprise a 68 hour expo-
sure, taken between 2008 and 2010, and reveal a faint, un-
resolved gamma-ray source (∼ 0.9% Crab), significant at
the level of 5.0 standard deviations [39]. Figure 11 shows
the gamma-ray excess map, along with X-ray and12CO
emission. Dense molecular cloud/ SNR interactions pro-
vide both the high energy particle population and interac-
tiontargetmaterialidealfortheproductionofahighenergy
gamma-ray signal (e.g. [40]). However, no direct evidence
of such an interaction exists, and the molecular emission
may simply be a chance association.
The VERITAS result alone is consistent with both leptons
and hadrons as the dominant particle population respon-
sible for the gamma-ray emission, although in both cases
the models provide evidence for magnetic field amplica-
tion,andhencenuclearparticleacceleration. Recentresults
from Fermi-LAT provide further constraints and, com-
bined with the VERITAS results, have been used to argue
strongly for a hadronic origin [41, 42].
Figure 11: A map of excess gamma-ray events for VER-
ITAS observations of Tycho’s SNR. black contours show
the Chandra X-ray emission, magenta contours are12CO.
TheinstrumentalPSF is alsoindicated. See [39] fordetails.
4The VERITAS Upgrade
As described in the introduction, the sensitivity of VER-
ITAS has steadily increased since the array was commis-
sioned. Ongoinganalysis developmentspromiseincremen-
tal improvementsin the future, but a significant furtherstep
requires new hardware. VERITAS is currently in the pro-
cess of implementing a series of fully-funded upgrades to
the instrument [43]. These consist of:
• Replacing all of the existing photomultiplier tubes
in the telescope cameras with more sensitive, super-
bialkali devices.

Page 10

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
Figure 10: A map of excess gamma-ray events for VERITAS observations of Cygnus OB1 (indicated by the large yellow
partial circle), above an energy of 650 GeV. See [19] for details.
• Replacing the telescope-level trigger system with a
higher speed, FGPA-based device.
• Upgrading inter-telescope networking and commu-
nications.
• Adding instrumentation to the central pixels of each
camera to allow high-speed optical monitoring and
stellar intensity interferometry.
Figure 12: Measured pulse shapes for the existing (Photo-
nis XP2970) and replacement (Hamamatsu R10560) pho-
tomultiplier tubes, after signal cable dispersion.
The first of these, the photomultiplier replacement, is ex-
pected to have the most significant impact on the ar-
ray performance. The new photomultipliers (Hamamatsu
R10560-100-20) are currently being fabricated and tested.
Initial results, both in the laboratory and in the tele-
scope cameras, indicate at least a 35% improvement in
Cherenkov photon collection efficiency [44]. The Hama-
matsu PMTs are also significantly faster: Figure 12 shows
the measured pulse shape, in comparison with the existing
Photonis tubes. Installation of the new PMT pixels will
take place during the regular summer shutdown in 2012.
All four of the upgraded trigger systems have also been
constructed and tested, and will be installed in Fall 2011.
No loss of observing time is foreseen for any of the up-
grade tasks. Figure 13 illustrates the expected increase in
the differentialgamma-rayevent rate at the trigger level for
a source with a spectrum similar to that of the Crab Neb-
ula. The predicted energy threshold at the hardware level
is defined as the energy giving the peak rate in this figure,
corresponding to 75 GeV.
5 Summary
The results presented here constitute a brief summary of
the status the VERITAS project after ∼ 4 years of oper-
ation. Observations are expected to continue for at least
the next ∼ 5 years, which will allow us to fully exploit
the VERITAS upgrade, and to complement the Fermi-LAT
effectively until the next generation of ground-based
instruments come online.

Page 11

32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011
Figure 13: Monte Carlo predictions of the differential event rate for gamma-ray events from a Crab-Nebula-like source at
the trigger level, both before and after the upgrade.
Acknowledgements
This research is supported by grants from the US Depart-
ment of EnergyOffice of Science, the US National Science
Foundation, and the Smithsonian Institution, by NSERC in
Canada, by Science Foundation Ireland, and by STFC in
the UK. We acknowledge the excellent work of the techni-
cal support staff at the FLWO and the collaborating institu-
tions in the construction and operation of the instrument.
References
[1] T. C. Weekes et al. , Astropart. Phys., 2002 17 221
[2] J. Holder et al. , Astropart. Phys., 2006 25, 391
[3] W. Benbow et al. , These Proceedings
[4] N. Galante et al. , These Proceedings
(arXiv:1109.6057)
[5] P. Majumdar et al. , These Proceedings
(arXiv:1109.6000)
[6] M. Errando, M. Orr & E. Kara, These Proceedings
[7] A. Pichel et al. , These Proceedings
(arXiv:1110.2549)
[8] T.C. Weekes et al. , ApJ, 1989, 342,379
[9] N. Galante et al. , These Proceedings
(arXiv:1109.6059)
[10] M. Orr et al. , These Proceedings
[11] G. Senturk et al. , These Proceedings
(arXiv:1109.6035)
[12] R.A. Ong et al. , Astronomer’s Telegram #3459
[13] S. Mei et al. , ApJ, 2007, 655, 144
[14] F.A. Aharonian et al. , A&A, 2003, 403, L1
[15] V.A. Acciari et al. , Science, 2009, 325, 444
[16] V.A. Acciari et al. , Nature, 2009, 462, 770
[17] T. Aune et al. , These Proceedings
[18] M. Vivier et al. , These Proceedings
(arXiv:1110.4358)
[19] E. Aliu et al. , These Proceedings
[20] G. Maier et al. , These Proceedings
[21] A. Weinstein et al. , These Proceedings
[22] F. Aharonian et al. , 2007, A&A, 469, L1
[23] J. Skilton, et al. , 2009,MNRAS 399, 317
[24] J. Hinton,et al. , 2009, ApJ, 690, L101
[25] V.A. Acciari et al. , 2009, ApJ, 698, L94
[26] S.D. Bongiorno et al. , 2009, ApJ, 737, L11
[27] G. Maier et al. , These Proceedings
[28] A.A. Abdo, 2010, ApJ, 708 1254
[29] E. Aliu et al. , 2008, Science, 322 1221
[30] A. McCann et al. , These Proceedings
(arXiv:1110.4352)
[31] E. Aliu et al. , 2011, Science, 334, 69
[32] F.A. Aharonian et al. , 2004, A&A, bf 425 L13
[33] K. Kosack et al. , 2004, ApJ, 608 L97
[34] F.A. Aharonian et al. , 2006, Nature, 439 695
[35] M. Beilicke et al. , 2011, 3rdFermi Symposium,
Rome, 2011
[36] A. Abdo, A. et al. , 2007, ApJ, 658, L33
[37] H.J. V¨ olk, E.G. Berezhko, L.T. Ksenofontov, 2008,
A&A, 483, 529
[38] J.S. Warren, et al. , 2005, ApJ, 634, 376
[39] V.A. Acciari, et al. , 2011, ApJ, 730, 20
[40] J.W. Hewitt, F. Yusef-Zadeh& M.Wardle, 2009,ApJ,
706, L270

Page 12

J. HOLDER et al. VERITAS: STATUS AND HIGHLIGHTS
[41] F. Giordano, et al. , 2011, arXiv:1108.0265
[42] G. Morlino, et al. , 2011, arXiv1105.6342
[43] D. B. Kieda et al. , These Proceedings
(arXiv:1110.4360)
[44] A.N. Otte et al. , These Proceedings