UNIVERSITADEGLISTUDIDISIENA` FACOLTADISCIENZEM.F.N` Tesi di Dottorato in Fisica Sperimentale PhD Thesis in Experimental Physics Very High Energy observation of GRBs with the MAGIC Telescope Candidato: Relatore: Nicola Galante Prof. Riccardo Paoletti XVIII CICLO To my family, who supported me in all hard challenges of this job. To Riccardo, who has been an example of devotion to science. To Antonio, who has been an example of submission to science. To Pisa, that strengthened my nature. And to uncle B, who has always been an example of life. “Software is not for everybody” Nicola Galante iv Contents Introduction 1 1 The MAGIC Telescope 7 1.1TheScientificCase........................ 10 1.1.1 ActiveGalacticNuclei.................. 11 1.1.2 PulsarsandSupernovaRemnants............ 11 1.1.3 Extragalactic Background Light . ........... 13 1.1.4 Gamma-RayBursts.................... 14 1.2IACTTechnique......................... 15 1.2.1 ElectromagneticShowers................. 15 1.2.2 HadronicandCosmicElectronShowers......... 21 1.2.3 ImageReconstruction................... 22 1.2.4 EnergyThreshold..................... 22 1.2.5 Sensitivity......................... 26 1.3TheTelescope........................... 28 1.3.1 TheMainStructure.................... 29 1.3.2 TheReflectingSurface.................. 30 1.3.3 TheDriveSystem..................... 31 1.3.4 TheCamera........................ 33 1.3.5 TheDAQ......................... 37 1.3.6 TheTrigger........................ 38 2 Gamma Ray Bursts 45 2.1GRBAngularDistribution.................... 47 2.2GRBDurationandTemporalProfile.............. 47 2.3Spectrum............................. 52 2.4Afterglow,HostGalaxiesandCosmologicalScattering..... 54 2.5EnergeticandBeaming...................... 58 v vi CONTENTS 2.6ConnectionwithSupernovae................... 61 2.7HEandVHEObservations.................... 64 3 The Theoretical Point 69 3.1TheCompactnessProblemandRelativisticMotion...... 69 3.2TheFireballModel........................ 71 3.3TheCannonballModel...................... 74 3.4HighEnergyTail......................... 75 4 The GRB Alert System 79 4.1TheGamma-RayBurstCoordinatesNetwork......... 79 4.2TheMAGICGRBAlertSystem................. 81 4.3TheGRBObservationStrategy................. 82 4.3.1 AlertSelectionCriteria.................. 83 4.3.2 ObservationduringMoonTime............. 84 4.3.3 TimingConsideration.................. 84 5GRBAnalysis 89 5.1DataReductionandCalibration................. 89 5.1.1 DataPreprocessing.................... 90 5.1.2 Calibration........................ 91 5.2 Hillas Parameters Calculation . ................ 93 5.3 γ-hadronSeparation....................... 93 5.4DataQualityCheckandSelectionofCuts........... 96 5.4.1 SelectionoftheOFFDataSample........... 96 5.4.2 Pre-selection of Events and γ-hadronSeparation....107 5.4.3 γ-hadronSelection....................108 5.5AlphaPlotsandLightCurves..................110 5.6UpperLimitsCalculation....................121 6 Results 131 6.1 GRB050421 . ...........................132 6.2 GRB050502a ...........................134 6.3 GRB050505 . ...........................134 6.4 GRB050509a ...........................135 6.5 GRB050528 . ...........................135 6.6 GRB050713a ...........................135 6.7 GRB050904 . ...........................137 CONTENTS vii 6.8 GRB060121 ............................138 6.9 GRB060203 ............................138 6.10 GRB060206 ............................139 6.11Discussion.............................139 Conclusion 141 A Test for GRB Isotropy 145 A.1DipoleandQuadrupoleMoments................145 A.2HypothesisTest..........................149 A.2.1PearsonTest.......................149 A.2.2 Kolmogorov Test .....................152 B Upper Limit in Presence of Background Noise 155 B.1 The Confidence Region for µ and b ...............156 B.2 The Confidence Interval for µ ..................157 B.3ASecondBackgroundSource..................159 Bibliography 173 viii CONTENTS Introduction Since a couple of decades the astronomical and astrophysical exploration has been proceeding through two major streams. The former in the study and observation of far and weak sources in the Universe, with the consequent need to develop new technologies to increase the instruments sensitivity in order to explore the Universe, the latter in the observation and study of the cosmic radiation, composed by charged cosmic particles, neutrinos and high energy photons, at different energies in order to investigate their sources and the related physical phenomena. Such stream of research is called astroparticle physics, being strictly connected to many issues, physics items and instru- mental technologies common to particle physics. The γ-astronomy is, therefore, a branch of astroparticle physics whose target is to study all those astrophysical sources responsible for the emission of High Energy (HE) radiation. Several kind of sources are responsible for the γ radiation emmission, both in the galactic enviroment and at extragalactic distances. γ radiation can be studied from few tens of keV up to several tens of TeV, covering an energy range of nine orders of magnitude. The related physical phenomena involved in the production of γ radiation can be very different and the experimental techniques to detect the γ particles can vary a lot. This is the main reason behind the efforts of the astronomical com- munity for the so-called Multi Wave Lenght (MWL) campains, observational campains on sources at different wave lenght (energy). The γ cosmic radiation is mainly detected by using two techniques. The space based technique, that consists in the direct detection of the primary γ by instruments installed onboard of a satellite. This technique has been used for the last three decades, and until the end of ’80s it has been the only possible technique suited for the detection of the γ cosmic radiation. Space 1 2 INTRODUCTION based instruments for γ radiation usually consists in particle detectors, while rejecting the charged ones by using anti-coincidence techniques. Space based instruments can observe the low energy γ radiation – from few tens of keV up to few GeV. Only recently new instruments capable to reach hundreds of GeV have been built. This kind of instruments typically have a large Field of View (FoV) and therefore can perform a sky-survey activity, becoming then well suited for transient phenomena search like Gamma-Ray Bursts or other Target of Op- portunity (ToO) like flaring blazars. On the other hand, the effective area available on such kind of detectors is very low, of the order of 1 m2,which makes the observation of sources beyond GeV energies very difficult or even impossible because of the steep decrease of the γ radiation flux. The second technique, used by ground based experiments, consists in the indirect observation of the primary γ thanks to the atmosphere which acts as a calorimeter. The primary γ, in fact, when interacting with the hadronic nuclei of the atmosphere, decays into an electron-positron pair that starts to develop a particle shower. All the charged particles of the shower are moving relativistically through the atmosphere, thus producing light by Cherenkov effect. Information about the primary γ photon and the direction of incidence can be retrieved by the analysis of the particle shower shape by using two techniques. The one used by Extensive Air Shower (EAS) arrays, which simply collects the charged particles of the shower reaching the ground, and reconstruct the shape and direction of the primary γ by looking at their arrival time and distribution at ground. The other technique, used by Imag- ing Air Cherenkov Telescopes (IACT) instruments, reconstructs the image of the shower from the Cherenkov light produced in the atmosphere by the shower itself. Ground based instruments are thus complementary to space based ones in γ astronomy since they cover a different energy range extending from hundreds of GeV up to several TeV. Low γ fluxes at high energies be- come observable thanks to the greater sensitivity available for ground based telescopes, of the order of 105-106 m2 i.e. five or six orders of magnitude bigger than that one available on a space based instruments. On the other hand, EAS arrays can perform a sky survey while IACT can only perform a follow-up observation because of their small FoV. Moreover local ground conditions strongly constrains the IACT duty cycle, affecting then the ob- servation schedule as well as the search for transient phenomena. A detailed description of the ground based IACT technique can be found in §1.2. INTRODUCTION 3 The Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC Telescope) exploits several new tecnologies for the observation of gamma cosmic radiation, to be applied in the context of IACT technique. A complete technical description of the MAGIC Telescope can be found in §1. The general design is oriented to reduce the energy threshold down to few tens of GeV and to reduce the slewing time to few tens of seconds, in order to observe the prompt emission of of transient phenomena like Gamma-Ray Bursts. With its large reflective surface, consisting in a 17m ∅ dish tasseled with square aluminum mirrors, the MAGIC telescope can achieve a 30 GeV en- ergy threshold at trigger level, the lowest energy threshold currently available on a IACT, covering the energy gap between the observations by satellites and previous generation Cherenkov detectors. Moreover the extremely light carbon fibre structure reduces the total weight of the telescope as well as its