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Gamma-ray emission of type ia supernovae

  • Autores: Alina Hirschmann
  • Directores de la Tesis: Eduardo Bravo Guil (dir. tes.), Josep Isern Vilaboy (dir. tes.)
  • Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2009
  • Idioma: inglés
  • Tribunal Calificador de la Tesis: Margarita Hernanz Carbó (presid.), Domingo García Senz (secret.), Vincent Tatischeff (voc.), Rosario López Hermoso (voc.), Inmaculada Domínguez Aguilera (voc.)
  • Materias:
  • Resumen
    • Eventhough the research on Type Ia supernovae (SNeIa) has increased considerably with the help of new observatories and data, the mechanism that triggers these explosions is still an enigma, Gamma-rays and positrons are the product of the decay of radioactive elements generated in the explosion and therefore, play a crucial role in understanding the origin of Type Ia SN. They provide information on the internal structure of the ejecta and the effects of the burning front on it and are also the main energy source that powers the lightcurve. In addition, they can give information on the turbulence of the system, explosion asymmetries, Doppler broadening of emission lines, etc. In order to determine the appropriate mechanism of explosion which reproduces at best the observations, several theoretical models have been built taking into account that these events could lodge subsonic (deflagration), supersonic (detonation) or both (delayed detonation) flame regimes.

      This work focuses on understanding the transport of gamma-ray photons and positrons in the ejecta and how they can provide information on its internal structure. To study this, we have built a Monte Carlo transport code and computed gamma-ray emission simulations in 1D and 3D, using as input the set of mentioned explosion models.

      The Monte Carlo technique can help solve the problem of radiative transfer by studying the behavior of a representative number of photons and positrons created in SNeIa explosions. The basics of the Monte Carlo method concerns the use of random numbers with appropriate distributions to describe physical aspects.

      Simulations in 1D are used basically to discern between explosion models while 3D simulations provide information concerning asymmetries in the explosion, rotation and convective mixing features, and possible interaction of the ejecta with the secondary star of the system.

      As for the transport of gamma-ray photons, gamma-ray spectral and lightcurve simulations have shown to provide information on he ejecta through the analysis of the line shapes and widths. However, these tools alone are not determinant in the discernment of the explosion mechanism. Diagnostic tools such as the study of the different line ratios, determination of supernova age, determination of 56 Ni mass, energy of the explosion, fwhm, energy deposition in the ejecta and photoelectric cut-off energy have been used as complementary tools to help constrain the model through future explosion observations.

      In 3D, we have analyzed the flux as a function of the direction of observation, assuming that we are capable of viewing the system from different lines of sight. The spectra and lightcurves have shown that differences in the flux as a function of the line of sight can reach 10% at the most, which means that the explosion is not very asymmetric. In addition, we have included the binary companion (a main sequence star) in an input model and have carried out simulations for such scenario to see the effects on the gamma-ray emission. The results have shown that the flux is not very sensitive to the change in the line of sight even with the presence of the main sequence companion. A future study will focus on the type of effects the emission might show considering different binary companions.

      Complementary to the gamma-ray emission, we have also studied the positron transport in the ejecta since these particles play an important role in powering the lightcurve at late times and they can also be closely linked to the positron-electron annihilation 511 keV line seen by INTEGRAL in the Galactic bulge and disk.

      The study of the positron escape fraction has allowed us to determine a needed supernova rate in the Galaxy to explain the emission INTEGRAL has measured. The rate obtained from our simulations suggests that Type Ia supernovae could be the strongest candidates since they produce the necessary positrons to explain such emission.

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