375 research outputs found
SST-GATE: A dual mirror telescope for the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) will be the world's first open
observatory for very high energy gamma-rays. Around a hundred telescopes of
different sizes will be used to detect the Cherenkov light that results from
gamma-ray induced air showers in the atmosphere. Amongst them, a large number
of Small Size Telescopes (SST), with a diameter of about 4 m, will assure an
unprecedented coverage of the high energy end of the electromagnetic spectrum
(above ~1TeV to beyond 100 TeV) and will open up a new window on the
non-thermal sky. Several concepts for the SST design are currently being
investigated with the aim of combining a large field of view (~9 degrees) with
a good resolution of the shower images, as well as minimizing costs. These
include a Davies-Cotton configuration with a Geiger-mode avalanche photodiode
(GAPD) based camera, as pioneered by FACT, and a novel and as yet untested
design based on the Schwarzschild-Couder configuration, which uses a secondary
mirror to reduce the plate-scale and to allow for a wide field of view with a
light-weight camera, e.g. using GAPDs or multi-anode photomultipliers. One
objective of the GATE (Gamma-ray Telescope Elements) programme is to build one
of the first Schwarzschild-Couder prototypes and to evaluate its performance.
The construction of the SST-GATE prototype on the campus of the Paris
Observatory in Meudon is under way. We report on the current status of the
project and provide details of the opto-mechanical design of the prototype, the
development of its control software, and simulations of its expected
performance.Comment: In Proceedings of the 33rd International Cosmic Ray Conference
(ICRC2013), Rio de Janeiro (Brazil). All CTA contributions at arXiv:1307.223
The first GCT camera for the Cherenkov Telescope Array
The Gamma Cherenkov Telescope (GCT) is proposed to be part of the Small Size
Telescope (SST) array of the Cherenkov Telescope Array (CTA). The GCT
dual-mirror optical design allows the use of a compact camera of diameter
roughly 0.4 m. The curved focal plane is equipped with 2048 pixels of
~0.2{\deg} angular size, resulting in a field of view of ~9{\deg}. The GCT
camera is designed to record the flashes of Cherenkov light from
electromagnetic cascades, which last only a few tens of nanoseconds. Modules
based on custom ASICs provide the required fast electronics, facilitating
sampling and digitisation as well as first level of triggering. The first GCT
camera prototype is currently being commissioned in the UK. On-telescope tests
are planned later this year. Here we give a detailed description of the camera
prototype and present recent progress with testing and commissioning.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
Gamma-ray Observations Under Bright Moonlight with VERITAS
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive
photomultiplier tube (PMT) cameras. Exposure to high levels of background
illumination degrades the efficiency of and potentially destroys these
photo-detectors over time, so IACTs cannot be operated in the same
configuration in the presence of bright moonlight as under dark skies. Since
September 2012, observations have been carried out with the VERITAS IACTs under
bright moonlight (defined as about three times the night-sky-background (NSB)
of a dark extragalactic field, typically occurring when Moon illumination >
35%) in two observing modes, firstly by reducing the voltage applied to the
PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to
the cameras. This has allowed observations at up to about 30 times previous NSB
levels (around 80% Moon illumination), resulting in 30% more observing time
between the two modes over the course of a year. These additional observations
have already allowed for the detection of a flare from the 1ES 1727+502 and for
an observing program targeting a measurement of the cosmic-ray positron
fraction. We provide details of these new observing modes and their performance
relative to the standard VERITAS observations
Dark Matter Constraints from a Joint Analysis of Dwarf Spheroidal Galaxy Observations with VERITAS
We present constraints on the annihilation cross section of WIMP dark matter
based on the joint statistical analysis of four dwarf galaxies with VERITAS.
These results are derived from an optimized photon weighting statistical
technique that improves on standard imaging atmospheric Cherenkov telescope
(IACT) analyses by utilizing the spectral and spatial properties of individual
photon events. We report on the results of 230 hours of observations of
five dwarf galaxies and the joint statistical analysis of four of the dwarf
galaxies. We find no evidence of gamma-ray emission from any individual dwarf
nor in the joint analysis. The derived upper limit on the dark matter
annihilation cross section from the joint analysis is at 1 TeV for the bottom quark () final state,
at 1 TeV for the tau lepton
() final state and at 1 TeV for the gauge boson () final state.Comment: 14 pages, 9 figures, published in PRD, Ascii tables containing
annihilation cross sections limits are available for download as ancillary
files with readme.txt file description of limit
VERITAS and Multiwavelength Observations of the BL Lacertae Object 1ES 1741+196
We present results from multiwavelength observations of the BL Lacertae
object 1ES 1741+196, including results in the very-high-energy -ray
regime using the Very Energetic Radiation Imaging Telescope Array System
(VERITAS). The VERITAS time-averaged spectrum, measured above 180 GeV, is
well-modelled by a power law with a spectral index of
. The integral flux above 180
GeV is
m s, corresponding to 1.6% of the Crab Nebula flux on average.
The multiwavelength spectral energy distribution of the source suggests that
1ES 1741+196 is an extreme-high-frequency-peaked BL Lacertae object. The
observations analysed in this paper extend over a period of six years, during
which time no strong flares were observed in any band. This analysis is
therefore one of the few characterizations of a blazar in a non-flaring state.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
H.E.S.S. observations of gamma-ray bursts in 2003-2007
Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray
bursts (GRBs) in some scenarios. Exploring this photon energy regime is
necessary for understanding the energetics and properties of GRBs. GRBs have
been one of the prime targets for the H.E.S.S. experiment, which makes use of
four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays.
Dedicated observations of 32 GRB positions were made in the years 2003-2007 and
a search for VHE gamma-ray counterparts of these GRBs was made. Depending on
the visibility and observing conditions, the observations mostly start minutes
to hours after the burst and typically last two hours. Results from
observations of 22 GRB positions are presented and evidence of a VHE signal was
found neither in observations of any individual GRBs, nor from stacking data
from subsets of GRBs with higher expected VHE flux according to a
model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from
the GRB positions were derived. For those GRBs with measured redshifts,
differential upper limits at the energy threshold after correcting for
absorption due to extra-galactic background light are also presented.Comment: 9 pages, 4 tables, 3 figure
Measurement of Cosmic-ray Electrons at TeV Energies by VERITAS
Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique
probe of our local Galactic neighborhood. CREs lose energy rapidly via
synchrotron radiation and inverse-Compton scattering processes while
propagating within the Galaxy and these losses limit their propagation
distance. For electrons with TeV energies, the limit is on the order of a
kiloparsec. Within that distance there are only a few known astrophysical
objects capable of accelerating electrons to such high energies. It is also
possible that the CREs are the products of the annihilation or decay of heavy
dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov
telescopes in southern Arizona, USA, is primarily utilized for gamma-ray
astronomy, but also simultaneously collects CREs during all observations. We
describe our methods of identifying CREs in VERITAS data and present an energy
spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300
hours of observations. A single power-law fit is ruled out in VERITAS data. We
find that the spectrum of CREs is consistent with a broken power law, with a
break energy at 710 40 140 GeV.Comment: 17 pages, 2 figures, accepted for publication in PR
Very-High-Energy -Ray Observations of the Blazar 1ES 2344+514 with VERITAS
We present very-high-energy -ray observations of the BL Lac object
1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array
System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a
statistical significance above background of in hours
(livetime) of observations, making this the most comprehensive very-high-energy
study of 1ES 2344+514 to date. Using these observations the temporal properties
of 1ES 2344+514 are studied on short and long times scales. We fit a constant
flux model to nightly- and seasonally-binned light curves and apply a
fractional variability test, to determine the stability of the source on
different timescales. We reject the constant-flux model for the 2007-2008 and
2014-2015 nightly-binned light curves and for the long-term seasonally-binned
light curve at the level. The spectra of the time-averaged emission
before and after correction for attenuation by the extragalactic background
light are obtained. The observed time-averaged spectrum above 200 GeV is
satisfactorily fitted () by a power-law function with
index and extends to at least 8
TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit
() by a power-law function with index while an F-test indicates that the power-law with
exponential cutoff function provides a marginally-better fit ( =
) at the 2.1 level. The source location is found to be
consistent with the published radio location and its spatial extent is
consistent with a point source.Comment: 7 pages, 2 figures. Published in Monthly Notices of the Royal
Astronomical Societ
A Search for Very High-Energy Gamma Rays from the Missing Link Binary Pulsar J1023+0038 with VERITAS
The binary millisecond radio pulsar PSR J1023+0038 exhibits many
characteristics similar to the gamma-ray binary system PSR B1259--63/LS 2883,
making it an ideal candidate for the study of high-energy non-thermal emission.
It has been the subject of multi-wavelength campaigns following the
disappearance of the pulsed radio emission in 2013 June, which revealed the
appearance of an accretion disk around the neutron star. We present the results
of very high-energy gamma-ray observations carried out by VERITAS before and
after this change of state. Searches for steady and pulsed emission of both
data sets yield no significant gamma-ray signal above 100 GeV, and upper limits
are given for both a steady and pulsed gamma-ray flux. These upper limits are
used to constrain the magnetic field strength in the shock region of the PSR
J1023+0038 system. Assuming that very high-energy gamma rays are produced via
an inverse-Compton mechanism in the shock region, we constrain the shock
magnetic field to be greater than 2 G before the disappearance of the
radio pulsar and greater than 10 G afterwards.Comment: 7 pages, 3 figures, accepted for publication in Ap
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