-
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 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
-
[show abstract]
[hide abstract]
ABSTRACT: Primary gamma rays emitted by extragalactic sources, such as blazars, will
generate electromagnetic cascades in intergalactic space. These cascades
proceed via electron-positron pair production and inverse Compton scattering on
cosmic background radiation, mainly the cosmic microwave background (CMB) and
extragalactic background light (EBL) fields. The existence of an extragalactic
magnetic field (EGMF) could deflect electron-positron pair trajectories and
scatter the cascade photons, possibly creating a halo around the source while
suppressing the cascade flux collected by a detector. We develop a
semi-analytic model for the cascade process and apply it to combine GeV-TeV
data on high-frequency-peaked Bl Lacertae objects (HBLs) from the Fermi Large
Area Telescope (LAT) and ground-based Cherenkov telescopes, comparing
observation results with model predictions using a robust statistical
framework. Lower limits with different confidence levels on the field strength
of the EGMF derived from this procedure are discussed under various assumptions
about the source livetime.
09/2011;
-
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;
-
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
-
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).
-
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;
-
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,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Multiwavelength observations of the high-frequency-peaked blazar 1ES2344+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)
{\gamma}-ray, X-ray, and UV energies. Observations with VERITAS in VHE
{\gamma}-rays yield a strong detection of 20 {\sigma} with 633 excess events in
a total exposure of 18.1 hours live-time. A strong VHE {\gamma}-ray flare on
2007 December 7 is measured at F(>300 GeV) = (6.76 \pm 0.62) \times 10-11 ph
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 \pm 0.09) \times 10-11 ph 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 {\Gamma} = 2.78 \pm 0.09stat \pm
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 XRT at a flux of
F(2-10 keV) = (6.28 \pm 0.31) \times 10-11 erg cm-2 s-1. Evidence for a
correlation between the X-ray flux and VHE {\gamma}-ray flux on nightly
time-scales is indicated with a Pearson correlation coefficient of r = 0.60 \pm
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.
06/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Very-high-energy (VHE, E \gtrsim 100 GeV) gamma rays emitted by extragalactic
sources, such as blazars, initiate electromagnetic cascades in the
intergalactic medium. The cascade photons arrive at the earth with angular and
temporal distributions correlated with the extragalactic magnetic field (EGMF).
We have developed a new semi-analytical model of the cascade properties which
is more accurate than previous analytic approaches and faster than full Monte
Carlo simulations. Within its range of applicability, our model can quickly
generate cascade spectra for a variety of source emission models, EGMF
strengths, and assumptions about the source livetime. In this Letter, we
describe the properties of the model and demonstrate its utility by exploring
the gamma-ray emission from the blazar RGB J0710+591. In particular, we
predict, under various scenarios, the VHE and high-energy (HE, 100 MeV \lesssim
E \lesssim 300 GeV) fluxes detectable with the VERITAS and Fermi Large Area
Telescope (LAT) observatories. We then develop a systematic framework for
comparing the predictions to published results, obtaining constraints on the
EGMF strength. At a confidence level of 95%, we find the lower limit on the
EGMF strength to be ~ 2 \times 10^{-16} Gauss if no limit is placed on the
livetime of the source or ~ 3 \times 10^{-18} Gauss if the source livetime is
limited to the past ~ 3 years during which Fermi observations have taken place.
06/2011;
-
V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
D. Boltuch,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev, [......],
L. O. Takalo,
C. Villforth,
T. Montaruli,
M Baker,
A. Lahteenmaki,
M. Tornikoski,
T. Hovatta,
E. Nieppola,
H. D. Aller,
M. F. Aller
[show abstract]
[hide abstract]
ABSTRACT: We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of
0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope.
The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-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 10m data were acquired between January 2006 and June 2008. We present
the results of a study of the TeV gamma-ray energy spectra as a function of
time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV
gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The
TeV gamma-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 gamma-rays. Contemporaneous spectral energy
distributions were generated for 18 nights, each of which are reasonably
described by a one-zone SSC model.
06/2011;
-
V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
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 present the results of observations of the TeV binary LS I +61 303 with
the VERITAS telescope array between 2008 and 2010, at energies above 300 GeV.
In the past, both ground-based gamma-ray telescopes VERITAS and MAGIC have
reported detections of TeV emission near the apastron phases of the binary
orbit. The observations presented here show no strong evidence for TeV emission
during these orbital phases; however, during observations taken in late 2010,
significant emission was detected from the source close to the phase of
superior conjunction (much closer to periastron passage) at a 5.6 standard
deviation (5.6 sigma) post-trials significance. In total, between October 2008
and December 2010 a total exposure of 64.5 hours was accumulated with VERITAS
on LS I +61 303, resulting in an excess at the 3.3 sigma significance level for
constant emission over the entire integrated dataset. The flux upper limits
derived for emission during the previously reliably active TeV phases (i.e.
close to apastron) are less than 5% of the Crab Nebula flux in the same energy
range. This result stands in apparent contrast to previous observations by both
MAGIC and VERITAS which detected the source during these phases at >10% of the
Crab Nebula flux. During the two year span of observations, a large amount of
X-ray data were also accrued on LS I +61 303 by the Swift X-ray Telescope (XRT)
and the Rossi X-ray Timing Explorer Timing (RXTE) Proportional Counter Array
(PCA). We find no evidence for a correlation between emission in the X-ray and
TeV regimes during 20 directly overlapping observations. We also comment on
data obtained contemporaneously by the Fermi Large Area Telescope (LAT).
05/2011;
-
VERITAS Collaboration,
V. A. Acciari,
E. Aliu,
M Araya,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
S. M. Bradbury,
J. H. Buckley, [......],
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein, T. Weisgarber,
S. Weng,
D A Williams,
M. Wood,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Giant X-ray outbursts, with luminosities of about $ 10^{37}$ erg s$^{-1}$,
are observed roughly every 5 years from the nearby Be/pulsar binary 1A
0535+262. In this article, we present observations of the source with VERITAS
at very-high energies (VHE; E$>$100 GeV) triggered by the X-ray outburst in
December 2009. The observations started shortly after the onset of the
outburst, and they provided comprehensive coverage of the episode, as well as
the 111-day binary orbit. No VHE emission is evident at any time. We also
examined data from the contemporaneous observations of 1A 0535+262 with the
Fermi/LAT at high energy photons (HE; E$>$0.1 GeV) and failed to detect the
source at GeV energies. The X-ray continua measured with the Swift/XRT and the
RXTE/PCA can be well described by the combination of blackbody and Comptonized
emission from thermal electrons. Therefore, the gamma-ray and X-ray
observations suggest the absence of a significant population of non-thermal
particles in the system. This distinguishes 1A~0535+262 from those Be X-ray
binaries (such as PSR B1259--63 and LS I +61$^{\circ}$303) that have been
detected at GeV--TeV energies. We discuss the implications of the results on
theoretical models.
03/2011;
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V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
S. M. Bradbury,
J. H. Buckley,
V. Bugaev,
K. Byrum, [......],
S. Vincent,
M. Vivier,
S. P. Wakely,
J. E. Ward,
T. C. Weekes,
A Weinstein, T. Weisgarber,
D A Williams,
M. Wood,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: We report the discovery of TeV gamma-ray emission from the Type Ia supernova
remnant (SNR) G120.1+1.4, known as Tycho's supernova remnant. Observations
performed in the period 2008-2010 with the VERITAS ground-based gamma-ray
observatory reveal weak emission coming from the direction of the remnant,
compatible with a point source located at $00^{\rm h} \ 25^{\rm m} \ 27.0^{\rm
s},\ +64^{\circ} \ 10^{\prime} \ 50^{\prime\prime}$ (J2000). The TeV photon
spectrum measured by VERITAS can be described with a power-law $dN/dE =
C(E/3.42\;\textrm{TeV})^{-\Gamma}$ with $\Gamma = 1.95 \pm 0.51_{stat} \pm
0.30_{sys}$ and $C = (1.55 \pm 0.43_{stat} \pm 0.47_{sys}) \times 10^{-14}$
cm$^{-2}$s$^{-1}$TeV$^{-1}$. The integral flux above 1 TeV corresponds to $\sim
0.9%$ percent of the steady Crab Nebula emission above the same energy, making
it one of the weakest sources yet detected in TeV gamma rays. We present both
leptonic and hadronic models which can describe the data. The lowest magnetic
field allowed in these models is $\sim 80 \mu$G, which may be interpreted as
evidence for magnetic field amplification.
02/2011;
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The VERITAS Collaboration,
V. A. Acciari,
T. Arlen,
T. Aune,
M. Beilicke,
W. Benbow,
M. Böttcher,
D. Boltuch,
S. M. Bradbury,
J. H. Buckley, [......],
M. Teshima,
D. F. Torres,
H. Vankov,
R. M. Wagner,
Q. Weitzel,
V. Zabalza,
F. Zandanel,
R. Zanin,
D. Paneque,
M Hayashida
[show abstract]
[hide abstract]
ABSTRACT: The very high energy (VHE; E > 100 GeV) blazar Markarian 501 has a
well-studied history of extreme spectral variability and is an excellent
laboratory for studying the physical processes within the jets of active
galactic nuclei. However, there are few detailed multiwavelength studies of
Markarian 501 during its quiescent state, due to its low luminosity. A
short-term multiwavelength study of Markarian 501 was coordinated in March
2009, focusing around a multi-day observation with the Suzaku X-ray satellite
and including {\gamma}-ray data from VERITAS, MAGIC, and the Fermi Gamma-ray
Space Telescope with the goal of providing a well-sampled multiwavelength
baseline measurement of Markarian 501 in the quiescent state. The results of
these quiescent-state observations are compared to the historically extreme
outburst of April 16, 1997, with the goal of examining variability of the
spectral energy distribution between the two states. The derived broadband
spectral energy distribution shows the characteristic double-peaked profile. We
find that the X-ray peak shifts by over two orders of magnitude in photon
energy between the two flux states while the VHE peak varies little. The
limited shift in the VHE peak can be explained by the transition to the
Klein-Nishina regime. Synchrotron self-Compton models are matched to the data
and the implied Klein-Nishina effects are explored.
12/2010;
