545 research outputs found
Using Interstellar Clouds to Search for Galactic PeVatrons: Gamma-ray Signatures from Supernova Remnants
Interstellar clouds can act as target material for hadronic cosmic rays;
gamma rays subsequently produced through inelastic proton-proton collisions and
spatially associated with such clouds can provide a key indicator of efficient
particle acceleration. However, even in the case that particle acceleration
proceeds up to PeV energies, the system of accelerator and nearby target
material must fulfil a specific set of conditions in order to produce a
detectable gamma-ray flux. In this study, we rigorously characterise the
necessary properties of both cloud and accelerator. By using available
Supernova Remnant (SNR) and interstellar cloud catalogues, we produce a ranked
shortlist of the most promising target systems, those for which a detectable
gamma-ray flux is predicted, in the case that particles are accelerated to PeV
energies in a nearby SNR. We discuss detection prospects for future facilities
including CTA, LHAASO and SWGO; and compare our predictions with known
gamma-ray sources. The four interstellar clouds with the brightest predicted
fluxes >100 TeV identified by this model are located at (l,b) = (330.05, 0.13),
(15.82, -0.46), (271.09, -1.26), and (21.97, -0.29). These clouds are
consistently bright under a range of model scenarios, including variation in
the diffusion coefficient and particle spectrum. On average, a detectable
gamma-ray flux is more likely for more massive clouds; systems with lower
separation distance between the SNR and cloud; and for slightly older SNRs.Comment: Accepted for publication in MNRAS. 30 pages, 16 figures, 7 table
Arc-minute-scale studies of the interstellar gas towards HESSJ1804216: Still an unidentified TeV -ray source
The Galactic TeV -ray source HESSJ1804216 is currently an
unidentified source. In an attempt to unveil its origin, we present here the
most detailed study of interstellar gas using data from the Mopra Southern
Galactic Plane CO Survey, 7 and 12mm wavelength Mopra surveys and Southern
Galactic Plane Survey of HI. Several components of atomic and molecular gas are
found to overlap HESSJ1804216 at various velocities along the line of
sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both
correlation and anti-correlation between the gas and TeV -ray emission
have been identified in various gas tracers, enabling several origin scenarios
for the TeV -ray emission from HESSJ1804216. For a hadronic
scenario, SNRG8.70.1 and the progenitor SNR of PSRJ18032137
require cosmic ray (CR) enhancement factors of times the
solar neighbour CR flux value to produce the TeV -ray emission.
Assuming an isotropic diffusion model, CRs from both these SNRs require a slow
diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM
gas. The morphology of gas located at 3.8kpc (the dispersion measure
distance to PSRJ18032137) tends to anti-correlate with features of the
TeV emission from HESSJ1804216, making the leptonic scenario possible.
Both pure hadronic and pure leptonic scenarios thus remain plausible.Comment: 29 pages, 23 figures, 5 tables, accepted for publication in PAS
FACT - How stable are the silicon photon detectors?
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Since the properties of G-APDs depend on auxiliary
parameters like temperature, a feedback system adapting the applied voltage
accordingly is mandatory.
In this presentation, the feedback system, developed and in operation for
FACT, is presented. Using the extraction of a single photon-equivalent (pe)
spectrum as a reference, it can be proven that the sensors can be operated with
very high precision. The extraction of the single-pe, its spectrum up to
10\,pe, its properties and their precision, as well as their long-term behavior
during operation are discussed. As a by product a single pulse template is
obtained. It is shown that with the presented method, an additional external
calibration device can be omitted. The presented method is essential for the
application of G-APDs in future projects in Cherenkov astronomy and is supposed
to result in a more stable and precise operation than possible with
photo-multiplier tubes
FACT - Threshold prediction for higher duty cycle and improved scheduling
The First G-APD Cherenkov telescope (FACT) is the first telescope using
silicon photon detectors (G-APD aka. SiPM). The use of Silicon devices promise
a higher photon detection efficiency, more robustness and higher precision than
photo-multiplier tubes. Being operated during different light-conditions, the
threshold settings of a Cherenkov telescope have to be adapted to feature the
lowest possible threshold but also an efficient suppression of triggers from
night-sky background photons. Usually this threshold is set either by
experience or a mini-ratescan. Since the measured current through the sensors
is directly correlated with the noise level, the current can be used to set the
best threshold at any time. Due to the correlation between the physical
threshold and the final energy threshold, the current can also be used as a
measure for the energy threshold of any observation. This presentation
introduces a method which uses the properties of the moon and the source
position to predict the currents and the corresponding energy threshold for
every upcoming observation allowing to adapt the observation schedule
accordingly
Mirror Position Determination for the Alignment of Cherenkov Telescopes
Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with
large apertures to map the faint Cherenkov light emitted in extensive air
showers onto their image sensors. Segmented reflectors fulfill these needs
using mass produced and light weight mirror facets. However, as the overall
image is the sum of the individual mirror facet images, alignment is important.
Here we present a method to determine the mirror facet positions on a segmented
reflector in a very direct way. Our method reconstructs the mirror facet
positions from photographs and a laser distance meter measurement which goes
from the center of the image sensor plane to the center of each mirror facet.
We use our method to both align the mirror facet positions and to feed the
measured positions into our IACT simulation. We demonstrate our implementation
on the 4 m First Geiger-mode Avalanche Cherenkov Telescope (FACT).Comment: 11 figures, small ray tracing performance simulation, and
implementation demonstratio
FACT - Long-term Monitoring of Bright TeV-Blazars
Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated
successfully on the Canary Island of La Palma. Apart from the proof of
principle for the use of G-APDs in Cherenkov telescopes, the major goal of the
project is the dedicated long-term monitoring of a small sample of bright TeV
blazars. The unique properties of G-APDs permit stable observations also during
strong moon light. Thus a superior sampling density is provided on time scales
at which the blazar variability amplitudes are expected to be largest, as
exemplified by the spectacular variations of Mrk 501 observed in June 2012.
While still in commissioning, FACT monitored bright blazars like Mrk 421 and
Mrk 501 during the past 1.5 years so far. Preliminary results including the Mrk
501 flare from June 2012 will be presented.Comment: 4 pages, 4 figures, presented at the 33rd ICRC (2013
FACT - Long-term stability and observations during strong Moon light
The First G-APD Cherenkov Telescope (FACT) is the first Cherenkov telescope
equipped with a camera made of silicon photon detectors (G-APD aka. SiPM).
Since October 2011, it is regularly taking data on the Canary Island of La
Palma. G-APDs are ideal detectors for Cherenkov telescopes as they are robust
and stable. Furthermore, the insensitivity of G-APDs towards strong ambient
light allows to conduct observations during bright Moon and twilight. This gain
in observation time is essential for the long-term monitoring of bright TeV
blazars. During the commissioning phase, hundreds of hours of data (including
data from the the Crab Nebula) were taken in order to understand the
performance and sensitivity of the instrument. The data cover a wide range of
observation conditions including different weather conditions, different zenith
angles and different light conditions (ranging from dark night to direct full
Moon). We use a new parmetrisation of the Moon light background to enhance our
scheduling and to monitor the atmosphere. With the data from 1.5 years, the
long-term stability and the performance of the camera during Moon light is
studied and compared to that achieved with photomultiplier tubes so far.Comment: 3 pages, 3 figures, FACT Contribution to the 33rd International
Cosmic Ray Conference (ICRC), Rio de Janeir
Data compression for the First G-APD Cherenkov Telescope
The First Geiger-mode Avalanche photodiode (G-APD) Cherenkov Telescope (FACT)
has been operating on the Canary island of La Palma since October 2011.
Operations were automated so that the system can be operated remotely. Manual
interaction is required only when the observation schedule is modified due to
weather conditions or in case of unexpected events such as a mechanical
failure. Automatic operations enabled high data taking efficiency, which
resulted in up to two terabytes of FITS files being recorded nightly and
transferred from La Palma to the FACT archive at ISDC in Switzerland. Since
long term storage of hundreds of terabytes of observations data is costly, data
compression is mandatory. This paper discusses the design choices that were
made to increase the compression ratio and speed of writing of the data with
respect to existing compression algorithms.
Following a more detailed motivation, the FACT compression algorithm along
with the associated I/O layer is discussed. Eventually, the performances of the
algorithm is compared to other approaches.Comment: 17 pages, accepted to Astronomy and Computing special issue on
astronomical file format
FACT - Monitoring Blazars at Very High Energies
The First G-APD Cherenkov Telescope (FACT) was built on the Canary Island of
La Palma in October 2011 as a proof of principle for silicon based photosensors
in Cherenkov Astronomy. The scientific goal of the project is to study the
variability of active galatic nuclei (AGN) at TeV energies. Observing a small
sample of TeV blazars whenever possible, an unbiased data sample is collected.
This allows to study the variability of the selected objects on timescales from
hours to years. Results from the first three years of monitoring will be
presented. To provide quick flare alerts to the community and trigger
multi-wavelength observations, a quick look analysis has been installed on-site
providing results publicly online within the same night. In summer 2014,
several flare alerts were issued. Results of the quick look analysis are
summarized.Comment: 2014 Fermi Symposium proceedings - eConf C14102.
Performance of a Small Array of Imaging Air Cherenkov Telescopes sited in Australia
As TeV gamma-ray astronomy progresses into the era of the Cherenkov Telescope
Array (CTA), there is a desire for the capacity to instantaneously follow up on
transient phenomena and continuously monitor gamma-ray flux at energies above
eV. To this end, a worldwide network of Imaging Air Cherenkov
Telescopes (IACTs) is required to provide triggers for CTA observations and
complementary continuous monitoring. An IACT array sited in Australia would
contribute significant coverage of the Southern Hemisphere sky. Here, we
investigate the suitability of a small IACT array and how different design
factors influence its performance. Monte Carlo simulations were produced based
on the Small-Sized Telescope (SST) and Medium-Sized Telescope (MST) designs
from CTA. Angular resolution improved with larger baseline distances up to 277m
between telescopes, and energy thresholds were lower at 1000m altitude than at
0m. The 300 GeV energy threshold of MSTs proved more suitable for
observing transients than the 1.2 TeV threshold of SSTs. An array of four
MSTs at 1000m was estimated to give a 5.7 detection of an RS
Ophiuchi-like nova eruption from a 4-hour observation. We conclude that an
array of four MST-class IACTs at an Australian site would ideally complement
the capabilities of CTA.Comment: 10 pages, 13 figures, 2 tables, accepted for publication in PAS
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