-
V A Acciari,
E Aliu,
T Aune,
M Beilicke,
W Benbow,
M Böttcher,
S M Bradbury,
J H Buckley,
V Bugaev,
Y Butt, [......],
F De Sabata,
A Dominguez,
D Dorner,
M Doro,
D Elsaesser,
M Errando,
D Ferenc,
E Fer Andez,
R Firpo,
M Mariotti
[show abstract]
[hide abstract]
ABSTRACT: We report on the results of two coordinated multiwavelength campaigns that focused on the blazar Markarian 421 during its 2006 and 2008 outbursts. These campaigns obtained UV and X-ray data us-ing the XMM-Newton satellite, while the gamma-ray data were obtained utilizing three imaging atmo-spheric Cerenkov telescopes, the Whipple 10 m telescope and VERITAS, both based in Arizona, as well as the MAGIC telescope, based on La Palma in the Canary Islands. The coordinated effort between the gamma-ray groups allowed for truly simultaneous data in UV/X-ray/gamma-ray wavelengths during a sig-nificant portion of the XMM-Newton observations. This simultaneous coverage allowed for a reliable search for correlations between UV, X-ray, and gamma-ray variability over the course of the observations. In-vestigations of spectral hysteresis and modeling of the spectral energy distributions are also presented.
The Astrophysical Journal J. Moldón J. Ninkovic E. Prandini N. Puchades I. Reichardt J. Rico T. Y. Saito V. Scalzotto S. N. Shore N. Sidro A. Sierpowska-Bartosik J. Sitarek J. Zapatero. 03/2037; 703455657454052443847(35):169-178.
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
A. Cesarini, [......],
A. Varlotta,
V. V. Vassiliev,
S. Vincent,
M. Vivier,
S. P. Wakely,
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery of TeV gamma-ray emission coincident with the
shell-type radio supernova remnant (SNR) CTA 1 using the VERITAS gamma-ray
observatory. The source, VER J0006+729, was detected as a 6.5 standard
deviation excess over background and shows an extended morphology, approximated
by a two-dimensional Gaussian of semi-major (semi-minor) axis 0.30 degree (0.24
degree) and a centroid 5' from the Fermi gamma-ray pulsar PSR J0007+7303 and
its X-ray pulsar wind nebula (PWN). The photon spectrum is well described by a
power-law dN/dE = N_0 (E/3 TeV)^(-\Gamma), with a differential spectral index
of \Gamma = 2.2 +- 0.2_stat +- 0.3_sys, and normalization N_0 = (9.1 +-
1.3_stat +- 1.7_sys) x 10^(-14) cm^(-2) s^(-1) TeV^(-1). The integral flux,
F_\gamma = 4.0 x 10^(-12) erg cm^(-2) s^(-1) above 1 TeV, corresponds to 0.2%
of the pulsar spin-down power at 1.4 kpc. The energetics, co-location with the
SNR, and the relatively small extent of the TeV emission strongly argue for the
PWN origin of the TeV photons. We consider the origin of the TeV emission in
CTA 1.
The Astrophysical Journal 12/2012; 764(1):38. · 6.02 Impact Factor
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T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
A. Cesarini,
L. Ciupik,
M. P. Connolly, [......],
W. Max-Moerbeck,
A. Readhead,
M. L. Lister,
Y. Y. Kovalev,
A. B. Pushkarev,
M. A. Gurwell,
A. Lahteenmaki,
E. Nieppola,
M. Tornikoski,
E. Jarvela
[show abstract]
[hide abstract]
ABSTRACT: We report on the detection of a very rapid TeV gamma-ray flare from BL
Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope
Array System (VERITAS). The flaring activity was observed during a 34.6-minute
exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times
10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab
Nebula flux measured by VERITAS. The light curve indicates that the
observations missed the rising phase of the flare but covered a significant
portion of the decaying phase. The exponential decay time was determined to be
$13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a
TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft,
with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement
made previously by MAGIC in a lower flaring state. Contemporaneous radio
observations of the source with the Very Long Baseline Array (VLBA) revealed
the emergence of a new, superluminal component from the core around the time of
the TeV gamma-ray flare, accompanied by changes in the optical polarization
angle. Changes in flux also appear to have occurred at optical, UV, and GeV
gamma-ray wavelengths at the time of the flare, although they are difficult to
quantify precisely due to sparse coverage. A strong flare was seen at radio
wavelengths roughly four months later, which might be related to the gamma-ray
flaring activities. We discuss the implications of these multiwavelength
results.
The Astrophysical Journal 11/2012; 762(2):92. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Bottcher,
A. Bouvier,
J. H. Buckley,
V. Bugaev, [......],
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer,
P. Fortin,
D. Horan,
M. Fumagalli,
K. Kaplan,
J. X. Prochaska
[show abstract]
[hide abstract]
ABSTRACT: We report on VERITAS very-high-energy (VHE; E>100 GeV) observations of six
blazars selected from the Fermi Large Area Telescope First Source Catalog
(1FGL). The gamma-ray emission from 1FGL sources was extrapolated up to the VHE
band, taking gamma-ray absorption by the extragalactic background light into
account. This allowed the selection of six bright, hard-spectrum blazars that
were good candidate TeV emitters. Spectroscopic redshift measurements were
attempted with the Keck Telescope for the targets without Sloan Digital Sky
Survey (SDSS) spectroscopic data. No VHE emission is detected during the
observations of the six sources described here. Corresponding TeV upper limits
are presented, along with contemporaneous Fermi observations and non-concurrent
Swift UVOT and XRT data. The blazar broadband spectral energy distributions
(SEDs) are assembled and modeled with a single-zone synchrotron self-Compton
model. The SED built for each of the six blazars show a synchrotron peak
bordering between the intermediate- and high-spectrum-peak classifications,
with four of the six resulting in particle-dominated emission regions.
The Astrophysical Journal 10/2012; 759(2):102. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
K. Byrum, [......],
S. Vincent,
M. Vivier,
R. G. Wagner,
S. P. Wakely,
T. C. Weekes,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer,
V. Kondratiev
[show abstract]
[hide abstract]
ABSTRACT: We present the results of a joint observational campaign between the Green
Bank radio telescope and the VERITAS gamma-ray telescope, which searched for a
correlation between the emission of very-high-energy (VHE) gamma rays
($E_{\gamma} >$ 150 GeV) and Giant Radio Pulses (GRPs) from the Crab pulsar at
8.9 GHz. A total of 15366 GRPs were recorded during 11.6 hours of simultaneous
observations, which were made across four nights in December 2008 and in
November and December 2009. We searched for an enhancement of the pulsed
gamma-ray emission within time windows placed around the arrival time of the
GRP events. In total, 8 different time windows with durations ranging from
0.033 ms to 72 s were positioned at three different locations relative to the
GRP to search for enhanced gamma-ray emission which lagged, led, or was
concurrent with, the GRP event. Further, we performed separate searches on main
pulse GRPs and interpulse GRPs and on the most energetic GRPs in our data
sample. No significant enhancement of pulsed VHE emission was found in any of
the preformed searches. We set upper limits of 5-10 times the average VHE flux
of the Crab pulsar on the flux simultaneous with interpulse GRPs on
single-rotation-period time scales. On $\sim$8-second time scales around
interpulse GRPs, we set an upper limit of 2-3 times the average VHE flux.
Within the framework of recent models for pulsed VHE emission from the Crab
pulsar, the expected VHE-GRP emission correlations are below the derived
limits.
The Astrophysical Journal 10/2012; 760(2):136. · 6.02 Impact Factor
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The Veritas Collaboration with C. Pfrommer,
A. Pinzke: T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
J. H. Buckley,
V. Bugaev,
K. Byrum,
A. Cannon, [......],
S. Thibadeau,
K. Tsurusaki,
A. Varlotta,
M. Vivier,
S. P. Wakely,
J. E. Ward,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Observations of radio halos and relics in galaxy clusters indicate efficient
electron acceleration. Protons should likewise be accelerated, suggesting that
clusters may also be sources of very high-energy (VHE; E>100 GeV) gamma-ray
emission. We report here on VHE gamma-ray observations of the Coma galaxy
cluster with the VERITAS array of imaging Cherenkov telescopes, with
complementing Fermi-LAT observations at GeV energies. No significant gamma-ray
emission from the Coma cluster was detected. Integral flux upper limits at the
99% confidence level were measured to be on the order of (2-5)*10^-8\ ph. m^-2
s^-1 (VERITAS, >220 GeV} and ~2*10^-6 ph. m^-2 s^-1 (Fermi, 1-3 GeV),
respectively. We use the gamma-ray upper limits to constrain CRs and magnetic
fields in Coma. Using an analytical approach, the CR-to-thermal pressure ratio
is constrained to be < 16% from VERITAS data and < 1.7% from Fermi data
(averaged within the virial radius). These upper limits are starting to
constrain the CR physics in self-consistent cosmological cluster simulations
and cap the maximum CR acceleration efficiency at structure formation shocks to
be <50%. Assuming that the radio-emitting electrons of the Coma halo result
from hadronic CR interactions, the observations imply a lower limit on the
central magnetic field in Coma of (2 - 5.5) muG, depending on the radial
magnetic-field profile and on the gamma-ray spectral index. Since these values
are below those inferred by Faraday rotation measurements in Coma (for most of
the parameter space), this {renders} the hadronic model a very plausible
explanation of the Coma radio halo. Finally, since galaxy clusters are
dark-matter (DM) dominated, the VERITAS upper limits have been used to place
constraints on the thermally-averaged product of the total self-annihilation
cross section and the relative velocity of the DM particles, <\sigma v>. (abr.)
The Astrophysical Journal 08/2012; 757(2):123. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Bottcher,
A. Bouvier,
V. Bugaev,
A. Cannon, [......],
K. Tsurusaki,
A. Varlotta,
V. V. Vassiliev,
M. Vivier,
S. P. Wakely,
J. E. Ward,
A. Weinstein,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We present observations of the BL Lac object 1ES 0414+009 in the >200 GeV
gamma-ray band by the VERITAS array of Cherenkov telescopes. 1ES 0414+009 was
observed by VERITAS between January 2008 and February 2011, resulting in 56.2
hours of good quality pointed observations. These observations resulted in a
detection of 822 events from the source corresponding to a statistical
significance of 6.4 standard deviations (6.4 sigma) above the background. The
source flux, showing no evidence for variability, is measured as 5.2 +/-
1.1_stat +/- 2.6_sys * 10^-12 photons cm^-2 s^-1 above 200 GeV, equivalent to
approximately 2% of the Crab Nebula flux above this energy. The differential
photon spectrum from 230 GeV to 850 GeV is well fit by a power law with an
photon index of Gamma 3.4 +/- 0.5_stat +/- 0.3_sys and a flux normalization of
1.6 +/- 0.3_stat +/- 0.8_sys * 10^-11 photons cm^-2 s^-1 at 300 GeV. We also
present multiwavelength results taken in the optical (MDM), X-ray (Swift-XRT),
and GeV (Fermi-LAT) bands and use these results to construct a broadband
spectral energy distribution (SED). Modeling of this SED indicates that
homogenous one-zone leptonic scenarios are not adequate to describe emission
from the system, with a lepto-hadronic model providing a better fit to the
data.
The Astrophysical Journal 06/2012; 755(2):118. · 6.02 Impact Factor
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E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
R. Welsing,
D. A. Williams,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report on very high energy (E > 100 GeV) gamma-ray observations of V407
Cygni, a symbiotic binary that underwent a nova outburst producing 0.1-10 GeV
gamma rays during 2010 March 10-26. Observations were made with the Very
Energetic Radiation Imaging Telescope Array System during 2010 March 19-26 at
relatively large zenith angles, due to the position of V407 Cyg. An improved
reconstruction technique for large zenith angle observations is presented and
used to analyze the data. We do not detect V407 Cygni and place a differential
upper limit on the flux at 1.6 TeV of 2.3 \times 10^(-12) erg cm^(-2) s^(-1)
(at the 95% confidence level). When considered jointly with data from
Fermi-LAT, this result places limits on the acceleration of very high energy
particles in the nova.
05/2012;
-
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley, [......],
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
P. Fortin,
and D. Horan
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery of high-energy (HE; E > 0.1 GeV) and very high energy (VHE; E > 100 GeV) γ-ray emission from the high-frequency-peaked BL Lac object RBS 0413. VERITAS, a ground-based γ-ray observatory, detected VHE γ rays from RBS 0413 with a statistical significance of 5.5 standard deviations (σ) and a γ-ray flux of (1.5 ± 0.6stat ± 0.7syst) × 10–8 photons m–2 s–1 (~1% of the Crab Nebula flux) above 250 GeV. The observed spectrum can be described by a power law with a photon index of 3.18 ± 0.68stat ± 0.30syst. Contemporaneous observations with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope detected HE γ rays from RBS 0413 with a statistical significance of more than 9σ, a power-law photon index of 1.57 ± 0.12stat +0.11 – 0.12sys, and a γ-ray flux between 300 MeV and 300 GeV of (1.64 ± 0.43stat +0.31 – 0.22sys) × 10–5 photons m–2 s–1. We present the results from Fermi-LAT and VERITAS, including a spectral energy distribution modeling of the γ-ray, quasi-simultaneous X-ray (Swift-XRT), ultraviolet (Swift-UVOT), and R-band optical (MDM) data. We find that, if conditions close to equipartition are required, both the combined synchrotron self-Compton/external-Compton and the lepto-hadronic models are preferred over a pure synchrotron self-Compton model.
The Astrophysical Journal 04/2012; 750(2):94. · 6.02 Impact Factor
-
VERITAS Collaboration,
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Boettcher,
A. Bouvier,
S. M. Bradbury, [......],
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
P. Fortin,
D. Horan
[show abstract]
[hide abstract]
ABSTRACT: We report on the discovery of high-energy (HE; E > 0.1 GeV) and very
high-energy (VHE; E > 100 GeV) gamma-ray emission from the
high-frequency-peaked BL Lac object RBS 0413. VERITAS, a ground-based gamma-ray
observatory, detected VHE gamma rays from RBS 0413 with a statistical
significance of 5.5 standard deviations (sigma) and a gamma-ray flux of (1.5
\pm 0.6stat \pm 0.7syst) \times 10^(-8) photons m^(-2) s^(-1) (\sim 1% of the
Crab Nebula flux) above 250 GeV. The observed spectrum can be described by a
power law with a photon index of 3.18 \pm 0.68stat \pm 0.30syst.
Contemporaneous observations with the Large Area Telescope (LAT) on the Fermi
Gamma-ray Space Telescope detected HE gamma rays from RBS 0413 with a
statistical significance of more than 9 sigma, a power-law photon index of 1.57
\pm 0.12stat +0.11sys -0.12sys and a gamma-ray flux between 300 MeV and 300 GeV
of (1.64 \pm 0.43stat +0.31sys -0.22sys) \times 10^(-5) photons m^(-2) s^(-1).
We present the results from Fermi-LAT and VERITAS, including a spectral energy
distribution modeling of the gamma-ray, quasi-simultaneous X-ray (Swift-XRT),
ultraviolet (Swift-UVOT) and R-band optical (MDM) data. We find that, if
conditions close to equipartition are required, both the combined synchrotron
self-Compton/external-Compton and the lepto-hadronic models are preferred over
a pure synchrotron self-Compton model.
04/2012;
-
E. Aliu,
S. Archambault,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
M. Vivier,
R. G. Wagner,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
B. Zitzer,
for the VERITAS Collaboration
[show abstract]
[hide abstract]
ABSTRACT: The VERITAS array of Cherenkov telescopes has carried out a deep
observational program on the nearby dwarf spheroidal galaxy Segue 1. We report
on the results of nearly 48 hours of good quality selected data, taken between
January 2010 and May 2011. No significant $\gamma$-ray emission is detected at
the nominal position of Segue 1, and upper limits on the integrated flux are
derived. According to recent studies, Segue 1 is the most dark matter-dominated
dwarf spheroidal galaxy currently known. We derive stringent bounds on various
annihilating and decaying dark matter particle models. The upper limits on the
velocity-weighted annihilation cross-section are $\mathrm{<\sigma v >^{95% CL}
\lesssim 10^{-23} cm^{3} s^{-1}}$, improving our limits from previous
observations of dwarf spheroidal galaxies by at least a factor of two for dark
matter particle masses $\mathrm{m_{\chi}\gtrsim 300 GeV}$. The lower limits on
the decay lifetime are at the level of $\mathrm{\tau^{95% CL} \gtrsim 10^{24}
s}$. Finally, we address the interpretation of the cosmic ray lepton anomalies
measured by ATIC and PAMELA in terms of dark matter annihilation, and show that
the VERITAS observations of Segue 1 disfavor such a scenario.
02/2012;
-
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev,
K. Byrum, [......],
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
[show abstract]
[hide abstract]
ABSTRACT: VERITAS has been monitoring the very-high-energy (VHE; >100GeV) gamma-ray
activity of the radio galaxy M87 since 2007. During 2008, flaring activity on a
timescale of a few days was observed with a peak flux of (0.70 +- 0.16) X
10^{-11} cm^{-2} s^{-1} at energies above 350GeV. In 2010 April, VERITAS
detected a flare from M87 with peak flux of (2.71 +- 0.68) X 10^{-11} cm^{-2}
s^{-1} for E>350GeV. The source was observed for six consecutive nights during
the flare, resulting in a total of 21 hr of good quality data. The most rapid
flux variation occurred on the trailing edge of the flare with an exponential
flux decay time of 0.90^{+0.22}_{-0.15} days. The shortest detected exponential
rise time is three times as long, at 2.87^{+1.65}_{-0.99} days. The quality of
the data sample is such that spectral analysis can be performed for three
periods: rising flux, peak flux, and falling flux. The spectra obtained are
consistent with power-law forms. The spectral index at the peak of the flare is
equal to 2.19 +- 0.07. There is some indication that the spectrum is softer in
the falling phase of the flare than the peak phase, with a confidence level
corresponding to 3.6 standard deviations. We discuss the implications of these
results for the acceleration and cooling rates of VHE electrons in M87 and the
constraints they provide on the physical size of the emitting region.
12/2011;
-
The VERITAS Collaboration,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
J. H. Buckley,
V. Bugaev, [......],
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
[show abstract]
[hide abstract]
ABSTRACT: Compilation of papers contributed by the VERITAS Collaboration to the 32nd
International Cosmic Ray Conference, held 11-18 August 2011 in Beijing, China.
11/2011;
-
J. Holder,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
J. H. Buckley,
V. Bugaev, [......],
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
[show abstract]
[hide abstract]
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.
11/2011;
-
E. Aliu,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev,
A. Cannon, [......],
S. P. Wakely,
T. C. Weekes,
A Weinstein,
D A Williams,
B. Zitzer,
S. Ciprini,
M. Fumagalli,
K. Kaplan,
D. Paneque,
J. X. Prochaska
[show abstract]
[hide abstract]
ABSTRACT: We report on the VERITAS discovery of very-high-energy (VHE) gamma- ray
emission above 200 GeV from the high-frequency-peaked BL Lac object
RXJ0648.7+1516 (GBJ0648+1516), associated with 1FGLJ0648.8+1516. The photon
spectrum above 200 GeV is fit by a power law dN/dE = F0(E/E0)-{\Gamma} with a
photon index {\Gamma} of 4.4 {\pm} 0.8stat {\pm}0.3syst and a flux
normalization F0 of (2.3 {\pm}0.5stat {\pm} 1.2sys) {\times} 10-11 TeV-1cm-2s-1
with E0 = 300 GeV. No VHE vari- ability is detected during VERITAS observations
of RXJ0648.7+1516 between 2010 March 4 and April 15. Following the VHE
discovery, the optical identifica- tion and spectroscopic redshift were
obtained using the Shane 3-m Telescope at the Lick Observatory, showing the
unidentified object to be a BL Lac type with a redshift of z = 0.179. Broadband
multiwavelength observations contemporaneous with the VERITAS exposure period
can be used to sub-classify the blazar as a high-frequency-peaked BL Lac (HBL)
object, including data from the MDM ob- servatory, Swift-UVOT and XRT, and
continuous monitoring at photon energies above 1 GeV from the Fermi Large Area
Telescope (LAT). We find that in the absence of undetected, high-energy rapid
variability, the one-zone synchrotron self-Compton model (SSC) overproduces the
high-energy gamma-ray emission measured by the Fermi-LAT over 2.3 years. The
SED can be parameterized sat- isfactorily with an external-Compton or
lepto-hadronic model, which have two and six additional free parameters,
respectively, compared to the one-zone SSC model.
10/2011;
-
E Aliu,
T Arlen,
T Aune,
M Beilicke,
W Benbow,
A Bouvier,
S M Bradbury,
J H Buckley,
V Bugaev,
K Byrum, [......],
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
[show abstract]
[hide abstract]
ABSTRACT: We report the detection of pulsed gamma rays from the Crab pulsar at energies above 100 giga-electron volts (GeV) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) array of atmospheric Cherenkov telescopes. The detection cannot be explained on the basis of current pulsar models. The photon spectrum of pulsed emission between 100 mega-electron volts and 400 GeV is described by a broken power law that is statistically preferred over a power law with an exponential cutoff. It is unlikely that the observation can be explained by invoking curvature radiation as the origin of the observed gamma rays above 100 GeV. Our findings require that these gamma rays be produced more than 10 stellar radii from the neutron star.
Science 10/2011; 334(6052):69-72. · 31.20 Impact Factor
-
VERITAS Collaboration,
V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
V. V. Vassiliev,
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein,
T. Weisgarber,
D A Williams,
M. Wood
[show abstract]
[hide abstract]
ABSTRACT: We present the results of sixteen Swift-triggered GRB follow-up observations
taken with the VERITAS telescope array from January, 2007 to June, 2009. The
median energy threshold and response time of these observations was 260 GeV and
320 s, respectively. Observations had an average duration of 90 minutes. Each
burst is analyzed independently in two modes: over the whole duration of the
observations and again over a shorter time scale determined by the maximum
VERITAS sensitivity to a burst with a t^-1.5 time profile. This temporal model
is characteristic of GRB afterglows with high-energy, long-lived emission that
have been detected by the Large Area Telescope (LAT) on-board the Fermi
satellite. No significant VHE gamma-ray emission was detected and upper limits
above the VERITAS threshold energy are calculated. The VERITAS upper limits are
corrected for gamma-ray extinction by the extragalactic background light (EBL)
and interpreted in the context of the keV emission detected by Swift. For some
bursts the VHE emission must have less power than the keV emission, placing
constraints on inverse Compton models of VHE emission.
08/2011;
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V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
D. Boltuch,
V. Bugaev,
A. Cannon,
L. Ciupik, [......],
R. Wagner,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A. Weinstein,
T. Weisgarber,
D. A. Williams,
S. Wissel,
M. Wood,
and B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Multiwavelength observations of the high-frequency-peaked blazar 1ES 2344+514 were performed from 2007 October to 2008 January. The campaign represents the first contemporaneous data on the object at very high energy (VHE, E >100 GeV) γ-ray, X-ray, and UV energies. Observations with VERITAS in VHE γ-rays yield a strong detection of 20σ with 633 excess events in a total exposure of 18.1 hr live time. A strong VHE γ-ray flare on 2007 December 7 is measured at F(>300 GeV) = (6.76 ± 0.62) × 10–11 photons cm–2 s–1, corresponding to 48% of the Crab Nebula flux. Excluding this flaring episode, nightly variability at lower fluxes is observed with a time-averaged mean of F(>300 GeV) = (1.06 ± 0.09) × 10–11 photons cm–2 s–1 (7.6% of the Crab Nebula flux). The differential photon spectrum between 390 GeV and 8.3 TeV for the time-averaged observations excluding 2007 December 7 is well described by a power law with a photon index of Γ = 2.78 ± 0.09stat ± 0.15syst. On the flaring night of 2007 December 7 the measured VHE γ-ray photon index was Γ = 2.43 ± 0.22stat ± 0.15syst. Over the full period of VERITAS observations contemporaneous X-ray and UV data were taken with Swift and RXTE. The measured 2-10 keV flux ranged by a factor of ~7 during the campaign. On 2007 December 8 the highest ever observed X-ray flux from 1ES 2344+514 was measured by Swift X-ray Telescope at a flux of F(2-10 keV) = (6.28 ± 0.31) × 10–11 erg cm–2 s–1. Evidence for a correlation between the X-ray flux and VHE γ-ray flux on nightly timescales is indicated with a Pearson correlation coefficient of r = 0.60 ± 0.11. Contemporaneous spectral energy distributions (SEDs) of 1ES 2344+514 are presented for two distinct flux states. A one-zone synchrotron self-Compton (SSC) model describes both SEDs using parameters consistent with previous SSC modeling of 1ES 2344+514 from non-contemporaneous observations.
The Astrophysical Journal 08/2011; 738(2):169. · 6.02 Impact Factor
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V A Acciari,
E Aliu,
T Arlen,
T Aune,
M Beilicke,
W Benbow,
D Boltuch,
S M Bradbury,
J H Buckley,
V Bugaev, [......],
R G Wagner,
S P Wakely,
J E Ward,
T C Weekes,
A Weinstein,
T Weisgarber,
D A Williams,
S Wissel,
M Wood,
B Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report on TeV γ-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10 m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV γ-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10 m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV γ-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV γ-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV γ-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV γ-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model.
J. Sainio. 07/2011; 103(103).
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E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
A. Bouvier,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
K. Tsurusaki,
J. Tyler,
A. Varlotta,
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
A Weinstein,
T. Weisgarber,
D A Williams
[show abstract]
[hide abstract]
ABSTRACT: We report on very-high-energy ($>$100 GeV) gamma-ray observations of Swift
J164449.3+573451, an unusual transient object first detected by the {\it Swift}
Observatory and later detected by multiple radio, optical and X-ray
observatories. A total exposure of 28 hours was obtained on Swift
J164449.3+573451 with VERITAS during 2011 March 28 -- April 15. We do not
detect the source and place a differential upper limit on the emission at 500
GeV during these observations of $1.4 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$
(99% confidence level). We also present time-resolved upper limits and use a
flux limit averaged over the X-ray flaring period to constrain various emission
scenarios that can accommodate both the radio-through-X-ray emission detected
from the source and the lack of detection by VERITAS.
07/2011;
