TAIGA: A Large Multi-Component Detector for Gamma- and Cosmic Rays from a Few TeV to EeV Energy Regime
TAIGA includes several types of detectors for identifying isotropic cosmic rays with high confidence level and for correspondingly efficient selection of several orders of magnitude less intense gamma rays from celestial sources. Cosmic gamma-rays and hadrons bombard the air molecules and atoms in earth’s atmosphere and via chain of electromagnetic and nuclear reactions produce cascades of superluminal secondary particles. These emit Cherenkov light which is measured by the several detector components of TAIGA. While the operation principle of wide angle Cherenkov light integrating detectors of Tunka-133 (185 detector stations spread over ~3 km2) and HiSCORE (currently 30 detector stations cover an area of ~0.3 km2) is based on nanosecond fast-sampling of the light front by the largely spaced detector stations, the first Imaging Air Cherenkov Telescope (IACT) produces images of air showers. The surface and deployed in 2 m depth underground scintillation detectors of Tunka-Grande measure the number of muons, electrons and positrons. The radio emission from air showers is measured by Tunka-Rex (currently 46 stations).
The goal of TAIGA is to address several key problems in both high-energy gamma-ray astronomy and in cosmic ray physics. One of the most important problems of contemporary astrophysics and cosmic ray physics is to find the galactic sources where the elementary particles are accelerated till PeV energies, i.e. to find PeVatrons, emitting gamma-rays at the energy range of ~100 TeV. The really innovative feature of TAIGA, dedicated to the search for PeVatrons, is the planned coincident operation of the IACT(s) and HiSCORE. This combination shall allow us to arrive at a huge collection area of the order of several km2 at a very low cost. We plan to operate a single IACT with HiSCORE stations up to the shower impact distance of 600m. Currently it is not yet known how the IACTs operate in coincidence when their inter-telescope distance is well beyond 120m. There exist several Monte Carlo studies which, despite their non-completeness, predict that the telescopes could operate reasonably well till the inter-telescope distance of ~300m. At big heights the e± are still energetic and the Cherenkov light emission angle is larger or comparable to the multiple scattering angle. Most of this light will be impinging within a radius of < 150m from the shower core. When impact of a shower axis is beyond ~150m, most of the light is coming due to the multiple scattering of relatively low energy e± from low heights in a shower. Moreover, the measured light intensity is rapidly decreasing, so at a distance of ~300m there is about one order of magnitude less light than within ~150m.
Thus when the shower axis moves away from the telescope, the observed image has less and less photons and on top of that, it becomes progressively more truncated in height (i.e. the image parameter “length” becomes shorter and more “fuzzy”). This impacts the gamma-hadron separation power of the telescope.
For large impact distances 300-600m we plan to recover this loss of discrimination power of the IACT by using the shower axis impact point and direction information measured by the HiSCORE detector.
The second telescope of TAIGA will be set at ~300m away from the first one. It will allow us to scan the impact parameter range, at least along the connection line of the telescopes, up to which distance the telescopes can collect showers and reliably analyze them.
Thus the simple cell of the discussed instrument can be seen as a single IACT telescope surrounded by a number of HiSCORE stations, especially at the large impact distances beyond 300m. Because of the low-cost and simplicity of the HiSCORE station, such an array has a promise of being a very cost-effective one.
We want to prove that with the 600m operational range of a single IACT surrounded by HiSCORE, we can arrive at a detector, providing a collection area in excess of ~1 km2 at high energies. Five such cells, for example, can cover an area in excess of 5.5 km2, which is a very reasonable size for measuring PeVatrons at ~100 TeV energies in a reasonably short time.
Note that our approach is in strong contrast with the currently planned or being under design experiments like, for example, the Cherenkov Telescope Array (CTA), where 50-70 Small Size (~4m in diameter) IACT Telescopes (SST) are planned to be built in Chile for measuring the PeVatrons and the estimated cost of this array is ~45-50.-MEUR.
The total cost of TAIGA, assuming a similar performance as the CTA SST array, will be at least one order of magnitude less.
In our presentation we are going to report on the status of TAIGA and about the first hints of a gamma-signal from a PeVatron candidate.