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Register a new userMeasurement of the Iron Spectrum in Cosmic Rays with VERITAS
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Added by
Henrike Fleischhack Dr.
Publication date
2017
fields
Physics
and research's language is
English
Authors
Henrike Fleischhack
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The elemental energy spectra of cosmic rays play an important role in understanding their acceleration and propagation. Most current results are obtained either from direct measurements by balloon or satellite detectors, or from indirect measurements by air shower detector arrays on the Earths surface. Imaging Air Cherenkov Telescopes (IACTs), used primarily for gamma-ray astronomy, can also be used for cosmic-ray physics. They are able to measure Cherenkov light emitted both by heavy nuclei and by secondary particles produced in their air showers, and are thus sensitive to the charge and energy of cosmic ray particles with energies of tens to hundreds of TeV. A measurement of the energy spectrum of iron nuclei, based on 71 hours of data taken by the VERITAS array of IACTs between 2009 and 2012, will be presented. The energy and other properties of the primary particle are reconstructed using a template-based likelihood fit. The event selection makes use of direct Cherenkov light, which is emitted by the primary particle before starting the air shower. A multivariate method is used to estimate the remaining background. Using these methods, the iron spectrum was measured in the energy range from 20 TeV to 500 TeV.
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We present a new measurement of the energy spectrum of iron nuclei in cosmic rays from 20 to 500 TeV. The measurement makes use of a template-based analysis method, which, for the first time, is applied to the energy reconstruction of iron-induced air showers recorded by the VERITAS array of imaging atmospheric Cherenkov telescopes. The event selection makes use of the direct Cherenkov light which is emitted by charged particles before the first interaction, as well as other parameters related to the shape of the recorded air shower images. The measured spectrum is well described by a power law $frac{mathrm{d} F}{mathrm{d} E}=f_0cdot left(frac{E}{E_0}right)^{-gamma}$ over the full energy range, with $gamma = 2.82 pm 0.30 mathrm{(stat.)} ^{+0.24}_{-0.27} mathrm{(syst.)}$ and $f_0 = left( 4.82 pm 0.98 mathrm{(stat.)}^{+2.12}_{-2.70} mathrm{(syst.)} right)cdot 10^{-7}$m$^{-2}$s$^{-1}$sr$^{-1}$TeV$^{-1}$ at $E_0=50$TeV, with no indication of a cutoff or spectral break. The measured differential flux is compatible with previous results, with improved statistical uncertainty at the highest energies.
One of several working groups established for this workshop was charged with examining results and methods associated with the UHECR energy spectrum. We summarize the results of our discussions, which include a better understanding of the analysis choices made by groups and their motivation. We find that the energy spectra determined by the larger experiments are consistent in normalization and shape after energy scaling factors are applied. Those scaling factors are within systematic uncertainties in the energy scale, and we discuss future work aimed at reducing these systematics.
The Calorimetric Electron Telescope (CALET), in operation on the International Space Station since 2015, collected a large sample of cosmic-ray iron over a wide energy interval. In this Letter a measurement of the iron spectrum is presented in the range of kinetic energy per nucleon from 10 GeV$/n$ to 2.0 TeV$/n$ allowing the inclusion of iron in the list of elements studied with unprecedented precision by space-borne instruments. The measurement is based on observations carried out from January 2016 to May 2020. The CALET instrument can identify individual nuclear species via a measurement of their electric charge with a dynamic range extending far beyond iron (up to atomic number $Z$ = 40). The energy is measured by a homogeneous calorimeter with a total equivalent thickness of 1.2 proton interaction lengths preceded by a thin (3 radiation lengths) imaging section providing tracking and energy sampling. The analysis of the data and the detailed assessment of systematic uncertainties are described and results are compared with the findings of previous experiments. The observed differential spectrum is consistent within the errors with previous experiments. In the region from 50 GeV$/n$ to 2 TeV$/n$ our present data are compatible with a single power law with spectral index -2.60 $pm$ 0.03.
A self-consistent model of a one-dimensional cosmic-ray (CR) halo around the Galactic disk is formulated with the restriction to a minimum number of free parameters. It is demonstrated that the turbulent cascade of MHD waves does not necessarily play an essential role in the halo formation. Instead, an increase of the Alfven velocity with distance to the disk leads to an efficient generic mechanism of the turbulent redshift, enhancing CR scattering by the self-generated MHD waves. As a result, the calculated size of the CR halo at lower energies is determined by the halo sheath, an energy-dependent region around the disk beyond which the CR escape becomes purely advective. At sufficiently high energies, the halo size is set by the characteristic thickness of the ionized gas distribution. The calculated Galactic spectrum of protons shows a remarkable agreement with observations, reproducing the position of spectral break at ~ 0.6 TeV and the spectral shape up to ~ 10 TeV.
ALICE, a general purpose experiment designed to investigate nucleus-nucleus collisions at the CERN Large Hadron Collider (LHC), has also been used to detect atmospheric muons produced by cosmic-ray interactions in the atmosphere. In this contribution the analysis of the multiplicity distribution of the atmospheric muons detected by ALICE between 2010 and 2013 is presented, along with a comparison with Monte Carlo simulations. Special emphasis is given to the study of high-multiplicity events, i.e. those containing more than 100 reconstructed muons. Such high-multiplicity events demand primary cosmic rays with energy above $10^{16}$ eV. The frequency of these events can be successfully described by assuming a heavy mass composition of primary cosmic rays in this energy range, using the most recent interaction models to describe the development of the air shower resulting from the primary interaction.
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