-
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.
-
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;
-
A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini,
J. Becker, [......],
N. P. Lee,
C. Ly,
J. Madrid,
F. Massaro,
C. G. Mundell,
H. Nagai,
E. S. Perlman,
I. A. Steele,
R. C. Walker,
and D. L. Wood
[show abstract]
[hide abstract]
ABSTRACT: The giant radio galaxy M 87 with its proximity (16 Mpc), famous jet, and very massive black hole ((3 – 6) × 109 M ☉) provides a unique opportunity to investigate the origin of very high energy (VHE; E > 100 GeV) γ-ray emission generated in relativistic outflows and the surroundings of supermassive black holes. M 87 has been established as a VHE γ-ray emitter since 2006. The VHE γ-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz Very Long Baseline Array, VLBA). The excellent sampling of the VHE γ-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times of τrise d = (1.69 ± 0.30) days and τdecay d = (0.611 ± 0.080) days, respectively. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Φ>0.35 TeV (1-3) × 10–11 photons cm–2 s–1), and VHE spectra. VLBA radio observations of 43 GHz of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE γ-ray emission reveal an enhanced flux from the core (flux increased by factor ~2; variability timescale <2 days). The long-term (2001-2010) multi-wavelength (MWL) light curve of M 87, spanning from radio to VHE and including data from Hubble Space Telescope, Liverpool Telescope, Very Large Array, and European VLBI Network, is used to further investigate the origin of the VHE γ-ray emission. No unique, common MWL signature of the three VHE flares has been identified. In the outer kiloparsec jet region, in particular in HST-1, no enhanced MWL activity was detected in 2008 and 2010, disfavoring it as the origin of the VHE flares during these years. Shortly after two of the three flares (2008 and 2010), the X-ray core was observed to be at a higher flux level than its characteristic range (determined from more than 60 monitoring observations: 2002-2009). In 2005, the strong flux dominance of HST-1 could have suppressed the detection of such a feature. Published models for VHE γ-ray emission from M 87 are reviewed in the light of the new data.
The Astrophysical Journal 02/2012; 746(2):151. · 6.02 Impact Factor
-
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 H. E. S. S. Collaboration: A. Abramowski,
F. Acero,
F. Aharonian,
A. G. Akhperjanian,
G. Anton,
A. Balzer,
A. Barnacka,
U. Barres de Almeida,
Y. Becherini,
J. Becker, [......],
N. P. Lee,
C. Ly,
J. Madrid,
F. Massaro,
C. G. Mundell,
H. Nagai,
E. S. Perlman,
I. A. Steele,
R. C. Walker,
D. L. Wood
[show abstract]
[hide abstract]
ABSTRACT: Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and
very massive black hole provides a unique opportunity to investigate the origin
of very high energy (VHE; E>100 GeV) gamma-ray emission generated in
relativistic outflows and the surroundings of super-massive black holes. M 87
has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray
emission displays strong variability on timescales as short as a day. In this
paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and
VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE
was detected triggering further observations at VHE (H.E.S.S.), X-rays
(Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray
light curve enables one to derive a precise temporal characterization of the
flare: the single, isolated flare is well described by a two-sided exponential
function with significantly different flux rise and decay times. While the
overall variability pattern of the 2010 flare appears somewhat different from
that of previous VHE flares in 2005 and 2008, they share very similar
timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2
s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions
indicate no enhanced flux in 2010 in contrast to observations in 2008, where an
increase of the radio flux of the innermost core regions coincided with a VHE
flare. On the other hand, Chandra X-ray observations taken ~3 days after the
peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The
long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio
to VHE and including data from HST, LT, VLA and EVN, is used to further
investigate the origin of the VHE gamma-ray emission. No unique, common MWL
signature of the three VHE flares has been identified.
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;
-
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, 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;
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V. A. Acciari,
E. Aliu,
T. Arlen,
T. Aune,
M. Bautista,
M. Beilicke,
W. Benbow,
D. Boltuch, S. M. Bradbury,
J. H. Buckley, [......],
S. Vincent,
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: We report the discovery of very high energy (VHE) gamma-ray emission from the direction of the SNR G54.1+0.3 using the VERITAS ground-based gamma-ray observatory. The TeV signal has an overall significance of 6.8σ and appears pointlike given the resolution of the instrument. The integral flux above 1 TeV is 2.5% of the Crab Nebula flux and significant emission is measured between 250 GeV and 4 TeV, well described by a power-law energy spectrum dN/dE ~ E –Γ with a photon index Γ = 2.39 ± 0.23stat ± 0.30sys. We find no evidence of time variability among observations spanning almost two years. Based on the location, the morphology, the measured spectrum, the lack of variability, and a comparison with similar systems previously detected in the TeV band, the most likely counterpart of this new VHE gamma-ray source is the pulsar wind nebula (PWN) in the SNR G54.1+0.3. The measured X-ray to VHE gamma-ray luminosity ratio is the lowest among all the nebulae supposedly driven by young rotation-powered pulsars, which could indicate a particle-dominated PWN.
The Astrophysical Journal Letters 07/2010; 719(1):L69. · 5.53 Impact Factor
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The VERITAS Collaboration,
V. A. Acciari,
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. D. Wood,
B. Zitzer
[show abstract]
[hide abstract]
ABSTRACT: Indirect dark matter searches with ground-based gamma-ray observatories provide an alternative for identifying the particle nature of dark matter that is complementary to that of direct search or accelerator production experiments. We present the results of observations of the dwarf spheroidal galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by VERITAS. These galaxies are nearby dark matter dominated objects located at a typical distance of several tens of kiloparsecs for which there are good measurements of the dark matter density profile from stellar velocity measurements. Since the conventional astrophysical background of very high energy gamma rays from these objects appears to be negligible, they are good targets to search for the secondary gamma-ray photons produced by interacting or decaying dark matter particles. No significant gamma-ray flux above 200 GeV was detected from these four dwarf galaxies for a typical exposure of ~20 hours. The 95% confidence upper limits on the integral gamma-ray flux are in the range 0.4-2.2x10^-12 photons cm^-2s^-1. We interpret this limiting flux in the context of pair annihilation of weakly interacting massive particles and derive constraints on the thermally averaged product of the total self-annihilation cross section and the relative velocity of the WIMPs. The limits are obtained under conservative assumptions regarding the dark matter distribution in dwarf galaxies and are approximately three orders of magnitude above the generic theoretical prediction for WIMPs in the minimal supersymmetric standard model framework. However significant uncertainty exists in the dark matter distribution as well as the neutralino cross sections which under favorable assumptions could further lower the limits. Comment: 21 pages, 2 figures, updated to reflect version published in ApJ. NOTE: M.D. Wood added as author
06/2010;
