Search for signal from self-annihilation of Dark Matter particles from the globular cluster M15 through MAGIC observations

Abstract

In this PhD thesis I investigate on the presence of dark matter inside a peculiar globular cluster, M15 (also known NGC 7078), analysing the data collected by the MAGIC telescopes along 2015 and 2016 for a total of 170 hours of integrated observation. M15 is one of the oldest object of the galaxy, located at 10.4 kpc from Earth and shows a very compact core, consequence of an initial phase of core collapse. It is likely that during this early phase the dark matter budget of M15 was concentrated inside the core. First Whipple telescope and later the High Energy Stereoscopic System (H.E.S.S.) have studied M15 in the VHE γ-ray range but the instruments did not find any signal attributable to annihilating dark matter particles. The MAGIC telescopes observed M15 in search for a VHE γ-ray excess due to pulsars. Data were collected for 173 hours and no significant excess was found. This same data set was then used in this work for a dark matter search in correlation with the new measurements on the density profile and mass to luminosity ratio in the inner parts of M15. The expected signal is given by the astrophysical factor, J-factor, and the spectrum of particles emitted from annihilation. The spectrum depends by the annihilation model and I have chosen 100% branching ratio into different SM particle pairs. J-factor describes the dark matter distribution inside the source and I derived its value from optical measurements of the star velocities. An important contribution of this thesis was to provide a method for computing the uncertainty of the J-factor, based on Monte Carlo techniques. No significant excess γ-ray signal from annihilating particles was found inthe MAGIC data set. I computed the upper limits on the cross-section as a function of several particles masses and for different channels of annihilation. Assuming that the DM represents only 1% of the the total missing matter inside M15, the upper limits are computed. The results have the same order of magnitude of other upper limits whichwere computed during the observation of dwarf galaxies by MAGIC Telescopes and are few lower than limit of H.E.S.S..