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

Abstract

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.

Monte Carlo simulations of alternative sky observation modes with the Cherenkov Telescope Array

“…The quality of the direction reconstruction is given by the angular resolution, defined by the containment radius, r f , of the area in the camera plane enclosing a fixed fraction f of the reconstructed directions (for details see Sect. 4.5 in [21]).…”

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

“…First, at low energies the geomagnetic field has more impact on γ-ray showers (making them appear more hadron-like) than hadron initiated showers thus the efficiency of primary particle selection is worse (see e.g. Bowden et al, 1992;Commichau et al, 2008;Szanecki et al, 2013). Second, larger fluctuations of the image parameters are expected due to the larger fluctuations of the Cherenkov light density at ground in the low energy region (Chitnis & Bhat, 1998;Sobczyńska, 2009).…”

Monte Carlo studies of medium-size telescope designs for the Cherenkov Telescope Array

“…The strength and orientation of the GF is thus an important consideration for the selection of candidate sites for an IACT observatory. Its influence can be as large or larger than the site elevation [45]. The magnitude of the induced deflection is proportional to the perpendicular component of the GF (B ⊥ ) and therefore the strength of the GF effect depends on both the magnitude of the GF vector as well as its orientation relative to the shower trajectory.…”