215 research outputs found
Monte Carlo studies on the sensitivity of the HEGRA imaging atmospheric Cerenkov telescope system in observations of extended gamma-ray sources
In this paper, we present the results of Monte Carlo simulations of
atmospheric showers induced by diffuse gamma rays as detected by the
high-energy gamma ray astronomy (HEGRA) system of five imaging atmospheric
Cerenkov telescopes (IACTs). We have investigated the sensitivity of
observations on extended gamma ray emission over the entire field of view of
the instrument. We discuss a technique to search for extended gamma ray sources
within the field of view of the instrument. We give estimates for HEGRA
sensitivity of observations on extended TeV gamma ray sources.Comment: 21 pages, 7 figures, accepted for publication in "Journal of Physics
G: Nuclear and Particle Physics
Comparison of techniques to reconstruct VHE gamma-ray showers from multiple stereoscopic Cherenkov images
For air showers observed simultaneously by more than two imaging atmospheric
Cherenkov telescopes, the shower geometry is overconstrained by the images and
image information should be combined taking into account the quality of the
images. Different algorithms are discussed and tested experimentally using data
obtained from observations of Mkn 501 with the HEGRA IACT system. Most of these
algorithms provide an estimate of the accuracy of the reconstruction of shower
geometry on an event-by-event basis, allowing, e.g., to select higher-quality
subsamples for precision measurements.Comment: 14 Pages, 6 figures, Late
Improved energy resolution for VHE gamma-ray astronomy with systems of Cherenkov telescopes
We present analysis techniques to improve the energy resolution of
stereoscopic systems of imaging atmospheric Cherenkov telescopes, using the
HEGRA telescope system as an example. The techniques include (i) the
determination of the height of the shower maximum, which is then taken into
account in the energy determination, and (ii) the determination of the location
of the shower core with the additional constraint that the direction of the
gamma rays is known a priori. This constraint can be applied for gamma-ray
point sources, and results in a significant improvement in the localization of
the shower core, which translates into better energy resolution. Combining both
techniques, the HEGRA telescopes reach an energy resolution between 9% and 12%,
over the entire energy range from 1 TeV to almost 100 TeV. Options for further
improvements of the energy resolution are discussed.Comment: 13 Pages, 7 figures, Latex. Astroparticle Physics, in pres
Search for point sources and diffuse emission from the Galactic plane with the HEGRA-IACT-system
The HEGRA-IACT-system with a FoV of ~1.5 deg radius has been used for
surveying one quater of the Galactic disc in respect to point sources,
moderately extended sources and for diffuse emission in the energy range above
1 TeV. In total 140 h of good observation time were accumulated.
No new source has been discoverd. Limits on the level of 20% or lower of the
Crab flux on about 87 potential sources like SNR, Pulsars and EGRET sources are
derived. A limit on the diffuse emission is given on the level of dPhi/dE(E=1
TeV) = 6.1 10^-15 ph cm-2 s-1 sr-1 Mev-1 resulting in a lower limit of 2.5 on
the spectral index for the extrapolation of the meassured EGRET flux for the
diffuse emission.Comment: 4 pages, 5 figures, 1 table, Submitted to ICRC, Hambur
Statistical Classification Techniques for Photometric Supernova Typing
Future photometric supernova surveys will produce vastly more candidates than
can be followed up spectroscopically, highlighting the need for effective
classification methods based on lightcurves alone. Here we introduce boosting
and kernel density estimation techniques which have minimal astrophysical
input, and compare their performance on 20,000 simulated Dark Energy Survey
lightcurves. We demonstrate that these methods are comparable to the best
template fitting methods currently used, and in particular do not require the
redshift of the host galaxy or candidate. However both methods require a
training sample that is representative of the full population, so typical
spectroscopic supernova subsamples will lead to poor performance. To enable the
full potential of such blind methods, we recommend that representative training
samples should be used and so specific attention should be given to their
creation in the design phase of future photometric surveys.Comment: 19 pages, 41 figures. No changes. Additional material and summary
video available at
http://cosmoaims.wordpress.com/2010/09/30/boosting-for-supernova-classification
Statistical classification techniques for photometric supernova typing
Future photometric supernova surveys will produce vastly more candidates than can be followed up spectroscopically, highlighting the need for effective classification methods based on light curves alone. Here we introduce boosting and kernel density estimation techniques which have minimal astrophysical input, and compare their performance on 20 000 simulated Dark Energy Survey light curves. We demonstrate that these methods perform very well provided a representative sample of the full population is used for training. Interestingly, we find that they do not require the redshift of the host galaxy or candidate supernova. However, training on the types of spectroscopic subsamples currently produced by supernova surveys leads to poor performance due to the resulting bias in training, and we recommend that special attention be given to the creation of representative training samples. We show that given a typical non-representative training sample, S, one can expect to pull out a representative subsample of about 10 per cent of the size of S, which is large enough to outperform the methods trained on all of
High-Redshift Cosmography
We constrain the parameters describing the kinematical state of the universe
using a cosmographic approach, which is fundamental in that it requires a very
minimal set of assumptions (namely to specify a metric) and does not rely on
the dynamical equations for gravity. On the data side, we consider the most
recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows
to further extend the cosmographic fit up to , i.e. up to redshift for
which one could start to resolve the low z degeneracy among competing
cosmological models. In order to reliably control the cosmographic approach at
high redshifts, we adopt the expansion in the improved parameter .
This series has the great advantage to hold also for and hence it is
the appropriate tool for handling data including non-nearby distance
indicators. We find that Gamma Ray Bursts, probing higher redshifts than
Supernovae, have constraining power and do require (and statistically allow) a
cosmographic expansion at higher order than Supernovae alone. Exploiting the
set of data from Union and GRBs catalogues, we show (for the first time in a
purely cosmographic approach parametrized by deceleration , jerk ,
snap ) a definitively negative deceleration parameter up to the
3 confidence level. We present also forecasts for realistic data sets
that are likely to be obtained in the next few years.Comment: 16 pages, 6 figures, 3 tables. Improved version matching the
published one, additional comments and reference
NTT and NOT spectroscopy of SDSS-II supernovae
Context. The SDSS-II Supernova Survey, conducted between 2005 and 2007, was
designed to detect a large number of Type Ia supernovae (SNe Ia) around z~0.2,
the redshift "gap" between low-z and high-z SN searches. The survey has
provided multi-band photometric lightcurves for variable targets, and SN
candidates were scheduled for spectroscopic observations, primarily to provide
SN classification and accurate redshifts. We present SN spectra obtained in
2006 and 2007 using the NTT and the NOT. Aims. We provide an atlas of SN
spectra in the range z =0.03-0.32 that complements the well-sampled lightcurves
from SDSS-II in the forthcoming three-year SDSS SN cosmology analysis. The
sample can, for example, be used for spectral studies of SNe Ia, which are
critical for understanding potential systematic effects when SNe are used to
determine cosmological distances. Methods. The spectra were reduced in a
uniform manner, and special care was taken in estimating the uncertainties for
the different processing steps. Host-galaxy light was subtracted when possible
and the SN type fitted using the SuperNova IDentification code (SNID). We also
present comparisons between spectral and photometric dating using SALT
lightcurve fits to the photometry from SDSS-II, as well as the global
distribution of our sample in terms of the lightcurve parameters: stretch and
colour. Results. We report new spectroscopic data from 141 SNe Ia, mainly
between -9 and +15 days from lightcurve maximum, including a few cases of
multi-epoch observations. This homogeneous, host-galaxy subtracted, SN Ia
spectroscopic sample is among the largest such data sets and unique in its
redshift interval. The sample includes two potential SN 1991T-like SNe (SN
2006on and SN 2007ni) and one potential SN 2002cx-like SN (SN 2007ie). In
addition, the new compilation includes spectra from 23 confirmed Type II and 8
Type Ib/c SNe.Comment: Accepted for publication in A&
- …