The origin of cosmic rays

Since the first detection of cosmic rays early in the 20th century there have been many efforts to try to find out how and where they are produced. An intrinsic problem in that process is that cosmic rays consist of charged particles and are therefore deflected in interstellar and intergalactic magnetic fields, which makes it impossible to track down their origin.

Compilation of measurements of the energy spectrum of charged cosmic rays. The observations can be described by a power-law with spectral breaks at 4 PeV, referred to as the knee, a second knee at 400 PeV and the ankle at 1 EeV. Click for a larger image. (Hillas 2006, preprint arXiv:astro-ph/0607109 v2).

It is possible, though, to measure the energy spectrum and the composition of cosmic rays, and it has been done so in many experiments. Above a few GeV, the spectrum can be described by a powerlaw shape with changes of the spectral index at 4 PeV (the knee), at 400 PeV (the second knee), and at 1 EeV (the ankle). More than 99 % of all cosmic rays are hadrons, mainly protons and helium nuclei.

To examine the question of the origin of cosmic rays, two principal ways have been suggested. Firstly, neutrinos are expected to be produced and accelerated by decay of high energy mesons in the vicinity of cosmic ray accelerators. As neutrinos are not charged and do in fact interact very little at all, they are perfectly suited as messengers. However, detection of cosmic neutrinos is challenging due to their small interaction cross section, and all efforts in that direction haven't led to much sucess so far. Also, if anything will be detected, the signals would be weak and a detailed study (of spectra, for example) may not be possible.

The second possible way to explore the origin of cosmic ray is the search for high energy photons (i.e. gamma rays) that are produced in the same processes. Both direct (satellite based) and indirect (ground-based) methods have yielded in many interesting results during the last years. While satellite based experiments are excellent in detecting relatively low energy gamma rays, their area is too small to collect enough photons at higher energies (as those are much rarer). Photons with energies above 100 GeV can be detected with current Cherenkov telescopes (see link section). Current telescopes such as H.E.S.S have effective sensitive areas of about 105 m2 which enables them to collect enough statistics up to about 10 TeV. The planned Cherenkov Telescope Array (CTA) will both lower the threshold below 100 GeV and, by its larger instrumented area, raise the upper energy limit far above the capability of current systems. Nevertheless, it will probably not be affordable to instrument areas far above one square kilometer with Cherenkov Telescopes.

HiSCORE is being designed in order to access energies beyond the range of CTA. The use of simple, inexpensive detector stations, which are placed several hundred meters apart, makes it possible to instrument a huge area. The energy threshold will be around a few 10 TeV and it will be perfectly suited for energies from 100 TeV up to at least 1 EeV (see simulation section). With it, it will be possible to extend the energy range of gamma ray spectrums of many known sources to higher energies which will help to adress many interesting questions.

Currently detected gamma ray sources can seldom be identified for sure as accelerators of cosmic rays, as gamma rays may be produced by other processes as well. In the leptonic scenario it is assumed that the gammas are accelerated by high energy electrons via the inverse Compton effect. This effect, however, becomes inefficient at certain, higher, energies (Klein-Nishina regime). Examing the higher part of energy spectra of gamma ray sources should therefore settle the ambiguity between the leptonic and the hadronic scenario. If a source can is found to emit gamma rays at energies in the multi-TeV range, it would be a strong indication of it being an accelerator of high energy cosmic rays.

Galactic photon background

If spectra of sources are assumed to be known, the distortion of the spectrum by cosmic absorption can be used to infer information about the galactic photon fields.

Large structures

The starburst area in Cygnus OB2 is dominated by young, bright, hot stars and has been identified as a source of cosmic gamma rays by MILAGRO. This is an infrared image of the area taken by the Infrared Astronomical Satellite (Picture by Jürgen Knödlseder / Centre d'Etude Spatiale des Rayonnements)

As HiSCORE offers a wide field of view it is perfectly suited to monitor extended gamma ray emitting structures such as molecular gas clouds, dense regions or large scale structures such as star forming regions or the galactic plane. Gamma-ray signals are expected from the interaction of the molecular gas with charged cosmic rays trapped in our Galaxy (meson decay). Spectral analysis of the faint diffuse gamma-ray signal correlated with the gas density can be used to derive the cosmic ray energy density at the clouds and thus map cosmic ray accelerator properties and propagation throughout the galaxy. Here again, observations at the highest energies with a wide field of view are required. Recently, MILAGRO was able to detect largely extended gamma-ray emission from the Cygnus region. The successor experiment HAWC will improve the sensitivity and lead to further progress here. However both experiments are optimized for a low energy threshold and it will be difficult to achieve the necessary sensitivities in the cut-off regime of cosmic accelerators.

Particle Physics

Proton-proton cross section at high energies

As HiSCORE offers the possibility to reconstruct the altitude of the shower maximum and therefore the height of the first interaction of a primary proton, the interaction cross section between protons can be measured. The energy range of HiSCORE offers to investigate center-of-mass energies between 1 TeV and 100 TeV, therefore extending greatly the range of the measurements at the LHC.

Dark Matter Search

The spectrums of gamma ray sources are distorted by the absorption of gamma rays in the interstellar photon fields and the CMB. A strong absorption feature is expected around 100 TeV. If the spectrum of the source is assumend to be known, examination of the absorption can bring information about the interstellar photon fields. On the other hand, photons might be converted into axions or hidden photons along the way, particles which might travel long distances without being affected by normal absorption processes and the convert back into normal photons. If the absorption is less than expected, this might be an indication for the presence of hidden photons or axions. Also, the decay of heavy supersymmetric particles might lead to a detectable signal in HiSCORE. Therefore, HiSCORE can also be used for testing theories about Dark Matter.