-
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
-
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
-
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
-
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
-
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.
The Astrophysical Journal 11/2011; 746:151. · 6.02 Impact Factor
-
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
-
[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;
-
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;
-
[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;
