Limits to the energy resolution of a single air Cherenkov telescope at low energies

Szanecki

et al. 2018

Astroparticle Physics

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The upcoming Cherenkov Telescope Array (CTA) project is expected to provide unprecedented sensitivity in the low-energy ( 100 GeV) range for Cherenkov telescopes. Most of the remaining background in this energy range results from misidentified hadron showers. In order to fully exploit the potential of the telescope systems it is worthwhile to look for ways to further improve the available analysis methods for γ/hadron separation. We study the composition of the background for the planned CTA-North array by identifying events composed mostly of a single electromagnetic subcascade or double subcascade from a π 0 (or another neutral meson) decay. We apply the standard simulation chain and state-of-the-art analysis chain of CTA to evaluate the potential of the standard analysis to reject such events. Simulations show a dominant role of such single subcascade background for CTA up to energies ∼ 70 GeV. We show that a natural way of rejection of such events stems from a shifted location of the shower maximum, and that the standard stereo reconstruction method used by CTA already exploits most of expected separation.

Section: Rejection Of Ses-dominated Eventsmentioning

confidence: 99%

Nature of the low-energy, γ-like background for the Cherenkov Telescope Array

Sitarek

Szanecki

et al. 2018

Astroparticle Physics

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“…The former effect involves the change of the orientation of the individual shower images by the GF, which obviously spoils the direction reconstruction. The altitude effect, studied in detail in [27,32], involves the radial distribution of the photon density in the light pool. Namely, the increase of the Cherenkov light density at the higher-altitude sites (underlying the enhanced trigger efficiency, studied in the previous section) occurs mostly close to the shower axis and, therefore, showers with smaller impact parameters are more frequently recorded.…”

Section: Image Parametersmentioning

confidence: 99%

Influence of the geomagnetic field on the IACT detection technique for possible sites of CTA observatories

Szanecki

Bernlöhr

et al. 2013

Astroparticle Physics

We investigate the influence of the geomagnetic field (GF) on the Imaging Air Cherenkov Telescope technique for two northern (Tenerife and San Pedro Martir) and three southern (Salta, Leoncito and Namibia (the H.E.S.S.-site)) site candidates for Cherenkov Telescope Array (CTA) observatories. We use the CORSIKA and sim telarray programs for Monte Carlo simulations of gamma ray showers, hadronic background and the telescope response. We focus here on gamma ray measurements in the low energy, sub-100 GeV, range. Therefore, we only consider the performance of arrays of several large telescopes. Neglecting the GF effect, we find (in agreement with previous studies) that such arrays have lower energy thresholds, and larger collection areas below 30 GeV, when located at higher altitudes. We point out, however, that in the considered ranges of altitudes and magnetic field intensities, 1800-3600 m a.s.l. and 0-40 µT, respectively, the GF effect has a similar magnitude to this altitude effect. We provide the trigger-level performance parameters of the observatory affected by the GF effect, in particular the collection areas, detection rates and the energy thresholds for all five locations, which information may be useful in the selection of sites for CTA. We also find simple scaling of these parameters with the magnetic field strength, which can be used to assess the magnitude of the GF effect for other sites; in this work we use them to estimate the performance parameters for five sites: South Africa-Beaufort West, USA-Yavapai Ranch, Namibia-Lalapanzi, Chile-La Silla and India-Hanle. We roughly investigate the impact of the geophysical conditions on gamma/hadron separation procedures involving image shape and direction cuts. We note that the change of altitude has an opposite effect at the trigger and analysis levels, i.e. gains in triggering efficiency at higher altitudes are partially balanced by losses in the separation efficiency. In turn, a stronger GF spoils both the shape and the direction discrimination of gamma rays, thus its effects at the trigger and analysis levels add up resulting in a significant reduction of the observatory performance. Overall, our results indicate that the local GF strength at a site can be equally important as its altitude for the low-energy performance of CTA.

“…The impact of a cloud cover on the shower images depends on the impact parameter of the shower because the density of Cherenkov light is correlated with the height of production of Cherenkov photons in the γ -ray initiated shower [11]. The Hillas parameters called DIST, WIDTH and LENGTH describe the position and the shape of the image.…”

Section: Resultsmentioning

confidence: 99%

The impact of clouds on image parameters in IACT at very high energies

Bednarek

2015

EPJ Web of Conferences

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Abstract. The effective observation time with the Cherenkov telescopes arrays is limited to clear sky conditions due to considerable absorption of Cherenkov light by the possible presence of clouds. However below the cloud altitude the primary particles with high energies can still produce enough Cherenkov photons to allow the detection by the large telescopes. In this paper, using the standard CORSIKA code, we investigate the changes of image parameters due to the absorption of Cherenkov radiation by the cloud (for γ -ray and proton showers with various energies -from 2 TeV to 100 TeV and from 10 TeV to 200 TeV, respectively). We consider the clouds with different transmissions located at various altitudes above the ground level (between 8 km and 3 km). We show that, for both simulated primary particles at fixed energy, the WIDTH and the DIST distributions are shifted towards larger values in the presence of clouds in comparison to the clear sky simulations. This shift decreases with the cloud altitude. The LENGTH distributions are shifted towards smaller values for images of primary γ -rays, while for primary protons this shift is not observed. We conclude that the large Cherenkov telescopes with large camera FOV could be used for observation of γ -ray showers with high energies in the presence of clouds.