45 research outputs found
Geomagnetic field and altitude effects on the performance of future IACT arrays
The performance of IACT's arrays is sensitive to the altitude and geomagnetic
field (GF) of the observatory site. Both effects play important role in the
region of the sub-TeV gamma-ray measurements. We investigate the influence of
GF on detection rates and the energy thresholds for five possible locations of
the future CTA observatory using the Monte Carlo simulations. We conclude that
the detection rates of gamma rays and the energy thresholds of the arrays can
be fitted with linear functions of the altitude and the component of the GF
perpendicular to the shower axis core. These results can be directly
extrapolated for any possible localization of the CTA. In this paper we also
show the influence of both geophysical effects on the images of shower and
gamma/hadron separation.Comment: 4 pages, 6 figures, two-column. Contribution to ICRC 2013 proceeding
The background from single electromagnetic subcascades for a stereo system of air Cherenkov telescopes
The MAGIC experiment, a very large Imaging Air Cherenkov Telescope (IACT)
with sensitivity to low energy (E < 100 GeV) VHE gamma rays, has been operated
since 2004. It has been found that the gamma/hadron separation in IACTs becomes
much more difficult below 100 GeV [Albert et al 2008] A system of two large
telescopes may eventually be triggered by hadronic events containing Cherenkov
light from only one electromagnetic subcascade or two gamma subcascades, which
are products of the single pi^0 decay. This is a possible reason for the
deterioration of the experiment's sensitivity below 100 GeV. In this paper a
system of two MAGIC telescopes working in stereoscopic mode is studied using
Monte Carlo simulations. The detected images have similar shapes to that of
primary gamma-rays and they have small sizes (mainly below 400 photoelectrons
(p.e.)) which correspond to an energy of primary gamma-rays below 100 GeV. The
background from single or two electromagnetic subcascdes is concentrated at
energies below 200 GeV. Finally the number of background events is compared to
the number of VHE gamma-ray excess events from the Crab Nebula. The
investigated background survives simple cuts for sizes below 250 p.e. and thus
the experiment's sensitivity deteriorates at lower energies.Comment: 15 pages, 7 figures, published in Journ.of Phys.
Limits to the energy resolution of a single Air Cherenkov Telescope at low energies
The photon density on the ground is a fundamental quantity in all experiments
based on Cherenkov light measurements, e.g. in the Imaging Air Cherenkov
Telescopes (IACT). IACT's are commonly and successfully used in order to search
and study Very High Energy (VHE) gamma-ray sources. Difficulties with
separating primary photons from primary hadrons (mostly protons) in Cherenkov
experiments become larger at lower energies. I have calculated longitudinal and
lateral density distributions and their fluctuations at low energies basing on
Monte Carlo simulations (for vertical gamma cascades and protonic showers) to
check the influence of the detector parameters on the possible measurement.
Relative density fluctuations are significantly higher in proton than in photon
induced showers. Taking into account the limited detector field of view (FOV)
implies the changes of these calculated distributions for both types of primary
particles and causes an enlargement in relative fluctuations. Absorption due to
Rayleigh and Mie scattering has an impact on mean values but does not change
relative fluctuations. The total number of Cherenkov photons is more sensitive
to the observation height in gamma cascades than in proton showers at low
primary energies. The relative fluctuations of the density do not depend on the
reflector size in the investigated size range (from 240 m^2 up to 960 m^2).
This implies that a single telescope with a mirror area larger than that of the
MAGIC telescope cannot achieve better energy resolution than estimated and
presented in this paper. The correlations between longitudinal and lateral
distributions are much more pronounced for primary gamma-ray than for primary
proton showers.Comment: 21 pages, 11 figures, accepted for publication in Journal of Physics
Natural limit on the gamma/hadron separation for a stand alone air Cherenkov telescope
The gamma/hadron separation in the imaging air Cherenkov telescope technique
is based on differences between images of a hadronic shower and a gamma induced
electromagnetic cascade. One may expect for a large telescope that a detection
of hadronic events containing Cherenkov light from one gamma subcascade only is
possible. In fact, simulations show that for the MAGIC telescope their fraction
in the total protonic background is about 1.5% to 5.2% depending on the trigger
threshold. It has been found that such images have small sizes (mainly below
400 photoelectrons) which correspond to the low energy primary gamma's (below
100 GeV). It is shown that parameters describing shapes of images from one
subcascade have similar distributions to primary gamma events, so those
parameters are not efficient in all methods of gamma selection. Similar studies
based on MC simulations are presented also for the images from 2 gamma
subcascades which are products of the same pi^0 decay. The ratio of the number
of the expected background from false gamma and one pi^0 to the number of the
triggered high energy photons from the Crab direction has been estimated for
images with a small alpha parameter to show that the occurrence of this type of
protonic shower is the reason for the difficulties with true gamma selection at
low energies.Comment: 12 pages, 7 figures, published in Journal of Physics
Constraints on the steady and pulsed very high energy gamma-ray emission from observations of PSR B1951+32/CTB 80 with the MAGIC Telescope
We report on very high energy gamma-observations with the MAGIC Telescope of
the pulsar PSR B1951+32 and its associated nebula, CTB 80. Our data constrain
the cutoff energy of the pulsar to be less than 32 GeV, assuming the pulsed
gamma-ray emission to be exponentially cut off. The upper limit on the flux of
pulsed gamma-ray emission above 75 GeV is 4.3*10^-11 photons cm^-2 sec^-1, and
the upper limit on the flux of steady emission above 140 GeV is 1.5*10^-11
photons cm^-2 sec^-1. We discuss our results in the framework of recent model
predictions and other studies.Comment: 7 pages, 7 figures, replaced with published versio
Calibration and operation of SiPM-based cameras for gamma-ray astronomy in presence of high night-sky light
The SST-1M project for the Cherenkov Telescope Array
The SST-1M project, run by a Consortium of institutes from Czech Republic, Poland and Switzerland, has been proposed as a solution for implementing the small-size telescope array of the southern site of the Cherenkov Telescope Array. The technology is a pathfinder for efficient production of cost-effective imaging air Cherenkov telescopes. We report on the main system features and recent upgrades, the performances validation and the operation campaign carried out in 2018
Searching for VHE gamma-ray emission associated with IceCube neutrino alerts using FACT, H.E.S.S., MAGIC, and VERITAS
The realtime follow-up of neutrino events is a promising approach to searchfor astrophysical neutrino sources. It has so far provided compelling evidencefor a neutrino point source: the flaring gamma-ray blazar TXS 0506+056 observedin coincidence with the high-energy neutrino IceCube-170922A detected byIceCube. The detection of very-high-energy gamma rays (VHE, ) from this source helped establish the coincidence andconstrained the modeling of the blazar emission at the time of the IceCubeevent. The four major imaging atmospheric Cherenkov telescope arrays (IACTs) -FACT, H.E.S.S., MAGIC, and VERITAS - operate an active follow-up program oftarget-of-opportunity observations of neutrino alerts sent by IceCube. Thisprogram has two main components. One are the observations of known gamma-raysources around which a cluster of candidate neutrino events has been identifiedby IceCube (Gamma-ray Follow-Up, GFU). Second one is the follow-up of singlehigh-energy neutrino candidate events of potential astrophysical origin such asIceCube-170922A. GFU has been recently upgraded by IceCube in collaborationwith the IACT groups. We present here recent results from the IACT follow-upprograms of IceCube neutrino alerts and a description of the upgraded IceCubeGFU system.<br