Limits to the violation of Lorentz invariance using the emission of the CRAB pulsar at TeV energies, discovered with archival data from the MAGIC telescopes
Universitat Autònoma de Barcelona
Campus Bellaterra, 08193 Barcelona (Spain)
External url: http://bit.ly/PhDGarrido
Gamma-ray astronomy is devoted to study the most energetic emitters in the Universe, starting from 100 mega-electronVolts. The MAGIC telescopes, located at the Roque de los Muchachos observatory in the Canary island of La Palma (Spain), are able to detect very-high-energy gamma-rays with more than 50 giga-electronVolts and allow us to explore some of the most violent cosmic events. One of these sources are pulsars, neutron stars spinning up to hundreds of times every second. Its radiation is collimated in two axis, so we detect them as a double, periodic flash with the same frequency as their rotation. One of the most studied pulsars is the one known as the Crab, located at the center of the nebula with the same name. The first part of this thesis is centered on the analysis of two years of Crab pulsar data taken with the MAGIC telescopes. This was part of a wider effort to analyze seven years available Crab observations, the biggest analysis ever performed by any gamma-ray telescope. The global result of this work is the discovery of the emission of very-high-energy gamma-rays from the Crab pulsar, between 400 and 1700 GeV for the intermediate pulse, whereas the main peak could be detected up to energies of 500 giga-electronVolts. This is in direct contradiction with the theoretical models currently accepted to explain the emission of this source, which predict a spectral cut-off at energies of a few hundreds of giga-electronVolts. This discovery requires a revision of these models. On the other hand, the second topic of this thesis is to contribute to test one of the phenomena predicted by Quantum Gravity theories using the above mentioned discovery. This theory, still under construction, tries to fulfill the old dream of combining together Einstein gravitation with quantum field theory. It is believed that this theory will predict certain observable phenomena, like the violation at the highest energies of one of the best-established physical symmetries: the Lorentz symmetry. In this case, the speed of light would not be as constant as we thought, and so the photons velocity in vacuum could depend on their energy. This would make high energy photons emitted by objects in the Universe get advanced or delayed with respect to the low energy ones. And they would arrive to the Earth at different times. This difference could only be detected if the following conditions are fulfilled: the emission source is located at far away distances, it must emit light at the highest energies, and its flux has to vary suddenly and simultaneously in a big energy range. So far, such an effect has never been measured. In this thesis we make use of the Crab pulsar photons above 400 giga-electronVolts to do one such tests. The Crab pulses arrival time should be modified if Lorentz symmetry was broken but we have not found any significant correlation between arrival time and energy of these photons. The temporal coincidence of pulses at different energies allows us to establish a lower limit in the energy scales where such an effect would start to dominate at E(QG2) > 4x10^10 giga-electronVolts for a quadratic dependence, which is half-way to the current best limit for this term. This result has been obtained using for the first time the method of maximization of the likelihood for a periodic, background-dominated signal.