Event reconstruction techniques for the wide-angle air Cherenkov detector HiSCORE

Abstract

Wide-angle, non-imaging air Cherenkov detectors provide a way to observe cosmic gamma-rays which is complementary to observations by imaging Cherenkov telescopes. Their particular strength lies in the multi-TeV to ultra high energy range (E γ > 30 TeV), where large effective areas, yet small light sensitive areas per detector station are needed. To exploit this potential to full extent, a large station spacing is required to achieve a large effective area at a reasonable effort.In such a detector, the low number of signals per event, the absence of imaging information, and the poor signal to noise ratio of Cherenkov light to night sky brightness pose considerable challenges for the event reconstruction, especially the gamma hadron separation. The event reconstruction presented in this paper has been developed for the wide-angle detector HiSCORE, but the concepts may be applied more generically. It is tested on simulated data in the 10 TeV to 5 PeV energy range using the air shower simulation CORSIKA and the HiSCORE detector simulation sim score. For the tests, a regular grid of 22 × 22 detector stations with a spacing of 150 m is assumed, covering an area of 10 km 2 . The angular resolution of individual events is found to be about 0.3• near the energy threshold, improving to below 0.1 • at higher energies. The relative energy resolution is 20% at the threshold and improves to 10% at higher energies. Several parameters for gamma hadron separation are described. With a combination of these parameters, 80% to 90% of the hadronic background can be suppressed, while about 60% of the gamma-ray events are retained. The point source sensitivity to gamma-ray sources is estimated, using conservative assumptions, to be about 8 × 10 −13 erg s −1 cm −2 at 100 TeV gamma-ray energy for a 10 km 2 array. With more optimistic assumptions, and a 100 km 2 array, a sensitivity of about 1 × 10 −13 erg s −1 cm −2 can be achieved (at 100 TeV). Even in the former case the detector is sensitive enough to measure the continuation of currently known gamma-ray spectra into the ultra high energy domain. Due to its large field of view of 0.6 sr it also offers a great potential for the discovery of new gamma-ray sources at the so far largely unexplored energies of 100 TeV and above.

The TAIGA timing array HiSCORE - first results

“…3). The value of the angular resolution for gamma rays is 0.4 • at threshold and below 0.2 • above 100 TeV, as predicted in [11]. The sensitivity (after 200 h for the timing array and 50 h for the TAIGA IACTs) of the current 0.25 km 2 TAIGA timing array (HiSCORE) and the planned hybrid stage, consisting of an extension of the timing array to 0.6 km 2 in combination with two IACTs.…”

“…Two independent time-calibration methods were sucessfully employed, reaching the required sub-ns accuracy [8,9]. The primary event parameters are reconstructed using signal timing and the amplitude [9][10][11]. Additionally, a small IACT (4.75 m dish diameter) is currently in commissioning and will be operated using a new hybrid approach, combining imaging and timing (see e.g.…”

“…In dotted red, the sensitivity is shown for 5 km 2 , including 10 IACTs. TAIGA curves are based on simulations and reconstruction methods presented in [7,10,11,13]. A parametrization of the Crab Nebula flux by [15] is shown as a solid grey line.…”

Simulation of the hybrid Tunka Advanced International Gamma-ray and Cosmic ray Astrophysics (TAIGA)

“…Also, from the same CORSIKA output, the HiSCORE detector response is simulated with sim score [7], a custom simulation software based on the IACT package, to determine the core position/resolution and shower direction/angular resolution of the pure array and to evaluate different array layouts and electronic systems.…”

“…In the low energy range (<10 TeV) however this method works only poorly, especially for gammas and protons [7].…”