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Investigation on Gamma-Electron Air Shower Separation for CTA

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Investigation on Gamma-Electron Air Shower Separation for CTA Taras Shevchenko National University of Kyiv The Faculty of Physics Astronomy and Space Physics Department Investigation on gamma-electron air shower separation for CTA Field of study: 0701 { physics Speciality: 8.04020601 { astronomy Specialisation: astrophysics Master's thesis the second year master student Iryna Lypova Supervisor: Dr. Gernot Maier leader of Helmholtz-University Young Investigator Group at DESY and Humboldt University (Berlin) Kyiv, 2013 Contents Introduction 2 1 Extensive air showers 3 1.1 Electromagnetic showers . 3 1.2 Hadronic showers . 7 1.3 Cherenkov radiation . 11 2 Cherenkov technique 13 2.1 Cherenkov telescopes . 13 2.2 Cherenkov Telescope Array . 18 2.3 Air shower reconstruction . 24 3 γ-electron separation with telescope arrays 31 3.1 Hybrid array . 31 3.2 The Cherenkov Telescope Array . 39 Summary 42 Reference 44 Appendix A 47 Appendix B 50 Introduction Very high energy (VHE) ground-based γ-ray astrophysics is a quite young science. The earth atmosphere absorbs gamma-rays and direct detection is possible only with satellite or balloon experiments. The flux of gamma rays falls rapidly with increasing energy and satellite detectors become not effective anymore due to the limited collection area. Another possibility for gamma- ray detection is usage of Imaging Atmospheric Cherenkov telescopes. The primary γ-ray creates a cascade of the secondary particles which move through the atmosphere. The charged component of the cascade which moves with velocities faster the light in the air, emits Cherenkov light which can be detected by the ground based optical detectors. The ground based gamma astronomy was pioneered by the 10 m single Cherenkov telescope WHIPPLE (1968) [1] in Arizona. It observed in the energy range between 300 GeV and 10 TeV. It was the first ground-based telescope which detected galactic (the Crab Nebula in 1989 [2]) and extragalactic (Mrk 421 in 1992 [3]) gamma ray sources. The stereo technique was pioneered by HEGRA array (1996) [4]. It consisted of 6 telescopes and operated on TeV energies. The stereoscopic view allowed a full geometrical reconstruction of the air shower that improved some experimental sensitivity compared to WHIPPLE. MAGIC [5], HESS [6], VERITAS [7] are the current generation of imaging atmospheric Cherenkov telescopes. They work in the stereoscopic regime with the energy threshold lower than 100 GeV. They achieve a sensitivity which allows to detect gamma ray sources weaker than 1% of the Crab Nebulae flux. But still, only a very small fraction of objects can be studied by the Cherenkov technique. The Cherenkov Telescope Array (CTA) [8] will be a new generation of ground based Cherenkov experiments. It will operate in a wide energy range from tens of GeV to hundreds of TeV. Improved temporal, angular and energy resolution will allow to measure faint and transient γ-ray sources. The background rate limits the sensitivity in middle energy range from 100 GeV to several TeV. A high efficiency of hadron background rejection allows to improve the sensitivity which will reach 10−3 of the Crab Nebulae flux at 1 TeV energy. Further improvement of the sensitivity by rejection of hadronic showers becomes complicated. The significant part of the leftover background consists of electron induced showers which can be hardly reduced by existing methods of separation. In this work the feasibility of γ-electron separation with array of the Cherenkov telescopes is studied. 2 1 Extensive air showers When a cosmic particle enters into the atmosphere it produces a cascade of secondary particles through interactions with nuclei. These cascades are called the Extensive Air Showers (EAS). There are two cascade sorts defined by the type of primary particle: hadronic (induced by protons or heavier nucleus) and electromagnetic showers (gamma, electron or positron primary). The secondary particles are high energetic and emit Cherenkov light mainly in UV part of the spectrum. The technique of Imaging Atmospheric Cherenkov Telescopes (IACT) is based on this effect. IACTs are used in the high-energy astronomy for detection of gamma-rays in energy range from tens of GeV to hundreds of TeV. 1.1 Electromagnetic showers Electromagnetic (EM) showers can be induced by gamma-rays, electrons or positrons. All three shower types are very similar. In the following paragraph the development of gamma-ray induced EM showers are explained. The primary gamma-ray creates electron-positron pair in the field of the atmospheric nucleus through pair production process with the characteristic length of 2 λpair ≈ 47 g/cm [9]. After pair creation the primary gamma disappears and each of the produced secondary particle gets roughly half of the primary energy. The electron and the positron continue to move through the atmosphere. They overcome ap- proximately 36.2 g/cm2 in the distance [9] till they emit a high energy photon through Bremsstrahlung process. These steps repeat continuously until the secondary electron energy drops below critical Ecrit = 82 MeV, when the Bremsstrahlung losses equal to the ionisation losses of the electron [10]. Often for simplification in the models of EM shower length of pair production and radiative length are equal (λpair = λbremss = X0). A simplified picture of the development of gamma induced showers is shown in fig. 1. With each step number of secondary particles (including n photons) doubles. After n = X=X0 steps the cascade consists of N = 2 secondary particles. Average energy per particle is roughly equal: n En ' Eprim=2 ; (1) where Eprim is energy of the primary particle, En is energy of the secondary particle after n steps of the shower development. As it was mentioned above, the shower develops until the energy of the secondary particles reach a critical value Ecrit. In this moment the shower achieves the maximum of development 2 hmax, from the top of the atmosphere, measured either in m or g/cm units [12]. The depth of the shower maximum can be calculated: 3 Figure 1: Schematic illustration of the electromagnetic air shower initiated by a primary gamma ray ([11]). ln (E =E ) X = X prim crit : (2) max 0 ln 2 The depth in the atmosphere is related with the height through the barometric formula. For a standard isothermal atmosphere it can be written in the simple form: X(h) = X(h = 0) exp (−h=H); (3) where X(h) is the depth in the atmosphere in relation from the height h, X(h = 0) is the vertical column density of the atmosphere at the sea level, H - scale height which is equal: RT H = ; (4) gµ where R [J/(K mol)] is the universal gas constant, T [K] is the absolute temperature, µ [g/mol] is the mean molecular weight, g [m/s2] is the gravitation acceleration. The location of the shower maximum depends on the primary energy. Showers with the higher energy have the maximum deeper in the atmosphere. The longitudinal development of the shower is demonstrated in fig. 2 [13]. Each curve represents a number of charged particles from the depth after first interaction for different primary energies. A maximum of each curve corresponds to the maximum of the shower. The rate of change of the shower maximum location versus the primary 4 Figure 2: Longitudinal development of the gamma-ray induced showers. The black curve illus- trates results obtained with CORSIKA [14] simulations. The red curve represents longitudinal distribution described by modified Greisen formula 5 [13]. energy is called elongation rate. Analytically longitudinal development modelled by the modified Greisen formula: a b a N (y; t) = p exp t 1 − ln s + 2 − p exp −t; (5) e y b − 1 y where Ne(y; t) is the number of charged particles as function of the distance to the point of the first interaction and primary energy, y = ln (Eprim=Ecrit) is energy in units of the critical energy, t = X=X0 is depth in the atmosphere in units of the radiation length, a and b are free parameters of the model, s is the shower age. The shower age is defined by the point of the first interaction, as following: b s = ; (6) 1 + c(b − 1)=t where c is depth of the shower maximum. The value of the shower age changes from 0 to 2 during shower development. In piont of the first interaction the shower age equals to "0". The value of "1" corresponds to the maximum of development. Electron (or positron) induced air showers are very similar to the gamma induced ones. Main difference between them is the process in the point of the first interaction. Gamma-ray creates 5 electron-positron pair and vanishes. Unlike gammas, the primary electron still can possess enough energy to emit a second Bremsstrahlung photon. It "stops" when its ionisation losses become dominated. Due to this electron may emit a few high energetic photons while passing through the atmosphere. This secondary photons create sub-showers. If energy of the primary electron is high enough and it does not deflect strongly in the Earth's magnetic field, then the sub-showers irradiate the same light pool on the ground. It means that such electron induced shower has a larger number of the secondary particles and accordingly a larger number of the Cherenkov photons from the high altitudes. The radiative length of the electron is smaller that the pair production length of the gamma (λpair > λbremss). This leads to another difference between gamma induced and electron induced showers. A point of the first interaction and, as result, a position of the maximum of development located higher in the atmosphere for the electron induced then for the gamma induced shower with the same primary energy. 6 1.2 Hadronic showers Another type of the showers is the hadronic showers.
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