Simulazioni Della Sensibilità Del Cherenkov Telescope Array Ai Gamma Ray Bursts

Università degli Studi di Napoli “Federico II” Scuola Politecnica e delle Scienze di Base Area Didattica di Scienze Matematiche Fisiche e Naturali Dipartimento di Fisica “Ettore Pancini” Laurea Magistrale in Fisica Simulazioni della sensibilità del Cherenkov Telescope Array ai Gamma Ray Bursts Simulations of the sensitivity of the Cherenkov Telescope Array to Gamma Ray Bursts Relatori: Candidato: Prof. Tristano Di Girolamo Antonio Circiello Dr.ssa Carla Aramo Matricola N94000468 A.A. 2019/2020 Simulations of the sensitivity of the Cherenkov Telescope Array to Gamma Ray Bursts Antonio Circiello Contents Abstract 1 1 GRBs 4 1.1 Properties . .5 1.1.1 Prompt Emission . .5 1.1.2 Classification beyond the Short-Long divide . .6 1.1.3 Afterglow . .7 1.2 Progenitors . .7 1.2.1 Short GRB progenitors . .8 1.2.2 Long GRB progenitors . .9 1.3 Dynamics . 10 1.3.1 Relativistic motion . 11 1.3.2 Shocks and acceleration . 13 1.3.3 Dissipation processes . 14 1.4 Beyond gamma rays . 16 1.4.1 Particle counterparts . 16 1.4.2 Gravitational wave counterpart . 19 2 GRB observations and simulations 20 2.1 The Cherenkov Telescope Array . 22 2.2 Description of the software utilities . 27 2.2.1 GammaLib . 28 2.2.2 ctools . 31 3 Simulations of the first two TeV GRBs 34 3.1 GRB 190114C . 35 3.1.1 CTA simulation . 36 3.1.2 Horizon of the observation . 39 3.2 GRB 180720B . 43 Conclusions 46 1 Abstract Gamma Ray Bursts (GRBs) are the most energetic explosive phenomena currently known in the universe. Since their serendipitous discovery in the late '60s, the great efforts put through their detection and analysis managed to un- veil many peculiar features of these remarkable events. The extreme energies involved in their evolution, the cosmological origin, and the production of all four messengers from the universe (photons, cosmic rays, neutrinos and gravitational waves) are only some of the properties that qualify GRBs as uniquely interesting among the broad selection of signals reaching the Earth. Still, we do not have a complete understanding of the processes involved in GRB production and evolution. In this work I focused on the electromagnetic signal from GRBs and its observation. Very High Energy (VHE) emission from GRBs has been detected only recently thanks to Imaging Air Cherenkov Telescopes (IACTs), able to detect the flashes of Cherenkov radiation from the Extensive Air Showers produced when an energetic gamma-ray photon collides with the atmosphere. In particular, I was interested in analysing the response to these signals with the Cherenkov Telescope Array (CTA), which represents the next generation of IACTs. CTA will be built in two arrays to observe both the Northern and Southern regions of the sky and will increase the energy range for IACT observations both in their lower and upper values, reaching a total range from 20 GeV to 300 TeV. With this goal, I used the ctools software package, distributed by the CTA collaboration, in order to simulate known GRB sources and test how CTA would carry out their observations. This package was developed to provide the scientific community with a common framework for the gamma-ray data analysis. Currently, all data taken from gamma-ray telescopes are stored using the Flexible Image Transport System (FITS) format, making them easy to share between different collaboration, while each instrument usually has its suite of software packages for the analysis. Thus, the ctools software package is implemented to be highly versatile and easily adaptable to one's analysis needs, whether the observation is carried out by a ground-based or a satellite telescope. In my master thesis I study two of the three VHE events (GRB 190114C and GRB 180720B) observed so far. GRB 190114C, though being not extremely energetic overall, emitted single high energy photons over 200 GeV during its early afterglow emission, as observed by the MAGIC telescopes. After simulating the CTA response to the same event, I performed several simulations moving the source farther away, analysing how 2 CONTENTS the significance of the observation is affected. GRB 180720B, on the other hand was a high energetic event observed by the H.E.S.S. telescopes during its faint late afterglow emission, over 10 hr after its trigger. In order to reach the lowest energy and detect such a faint signal, only the larger telescope in the H.E.S.S. array was operated. In this case, the aim of the CTA simulation was to test its efficiency at the lowest energies, without using any peculiar detection technique or pre-manipulation of data. When modelling the source to run the simulations, a few crucial steps are needed. First, I had to find the best way to reconstruct the evolution of the spectrum during the observation. This was a subtle process, as I wanted to in- troduce the minimum amount of assumptions to not bias the simulations. Sec- ond, the Extragalactic Background Light (EBL) absorption effect on gamma-ray propagation must be considered. Indeed, this is critical when simulating an observation carried out by a ground-based telescope, sensitive to energies at which this cosmological absorption effect can be strong, depending on the redshift of the source, as opposed to most satellite observations, which detect lower energies and are almost unhampered by EBL absorption effects. The EBL model I choose is by Dominguez et al., however other models could be preferred when considering high redshift observations. Third, I consider ideal observation conditions. Further development could include the effects of different conditions for night sky background and moon light. Nonetheless, the results drawn from the simulations showed how CTA will give a huge boost to our ability in detecting VHE signals from GRBs, both in the observable horizon and in the sensitivity to less energetic photons with IACTs. 3 Chapter 1 GRBs Gamma Ray Bursts (GRBs) are short, intense, non-repeating flashes of ≈ 100 keV to ≈ MeV photons. In spite of having a wide range of spectral and temporal properties, GRBs are usually classified by the duration-intensity of their spectrum in a short-hard or a long-soft category, the separation being at 2 seconds. GRBs were first detected by chance in the late 1960s by the Vela mili- tary satellites, which were monitoring the Test Ban Treaty between the United States and the Soviet Union. Over time, the interest about these signals raised as the improvements in observational techniques and procedures showed their cosmological origin, and therefore their incredibly high energy release. Thanks to the launch of the Compton Gamma Ray Observatory (CGRO), equipped with the Burst And Transient Source Explorer (BATSE), in 1991, it became clear that the GRB sky distribution is isotropic, suggesting a possible cosmological origin. The turning point in the study of GRBs came with the launch of the Beppo- SAX satellite, which in 1997 performed the first detection of a GRB afterglow emission, which follows the prompt emission. Information gained studying the afterglows were able to answer many of the questions we had on GRBs. First of all, the hypothesis of a cosmological origin, when combined with the short variability timescale and the large fluence (flux integrated over duration) of these signals, made us figure a plasma-like internal structure for GRBs, expect- ing them to have a typical thermal spectrum. The measured spectra, however, displayed a highly non-thermal shape, in the form of a power law. This ap- parent inconsistency was known as the 'compactness problem'. One can avoid it by considering the GRB plasma to expand with a high bulk Lorentz factor (Γ & 100), but this would in order lead to another problem, namely the 'baryon loading problem', as the plasma should be loaded with a very small baryon mass to achieve such an ultra-relativistic expansion. Furthermore, while first models for GRBs pictured an isotropic emission, we had no way of performing accurate measurements of their energy or localization. The observation of afterglows allowed us to analyse GRBs via optical spec- troscopy, getting some precious information about these phenomena. First of 4 CHAPTER 1. GRBS all, we were able to perform redshift measurements, directly confirming their cosmological origin. We also got observational evidence on the emission energy and geometry, noticing how GRBs are emitted as jets, with typical opening angles of ∼ 3 − 10◦ [1], rather than as isotropic outflows. At present, our knowledge about these signals is far from being complete. Sev- eral models were proposed in the last 50 years to explain their internal dynamics, while the search for their progenitors still goes on. 1.1 Properties GRBs are indeed uniquely interesting among the broad selection of signals that reach the Earth. To display a brief review of their properties, it is useful to analyse separately the prompt emission, which is the main GRB emission, and its afterglow, which is, instead, the delayed secondary emission from the interaction with the interstellar medium (ISM). 1.1.1 Prompt Emission The GRB itself, namely the γ-rays and any lower-energy emission that occurs simultaneously with them, is usually referred to as the prompt emission, to distinguish it from all the secondary emissions that come from the same phe- nomenon. Other than γ-rays, lower frequency emissions may also occur as an optical flash. The prompt emission is operationally defined as the time period when the γ-ray detector sees a signal above background. To properly characterize the prompt emission, one has to describe its spectral and temporal behaviours. The spectrum is highly non-thermal, with a peak energy usually located at a few hundred keV.

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