Measurement of the gamma-ray opacity of the Universe with the MAGIC telescopes

Abstract

The extragalactic background light (EBL) is the collection of all the light produced in the history of the Universe. The wavelength of this background light goes from the UV-optical band to the near- and far-infrared. The EBL interacts with the gamma rays coming from active galactic nuclei (AGN) producing electron-positron pairs. This causes that the differential energy spectra observed at Earth from the AGNs, in the energy range of gamma rays, are modified by the interaction with the EBL. This works is focused in the observations and the data analysis from two AGNs: Markarian 421 and 1ES 1011+496. These two sources were observed in extraordinary flaring states by the MAGIC telescopes, providing good quality expectra at very high energies. From these observations, the effect of the EBL on the observed spectra could be measured. The measurements were performed using a likelihood maximization method, where the intrinsic spectrum of the source is modeled using a smooth convex function, modified by the effect of the EBL (from a model template) whose intensity is regulated using an opacity normalization factor. Then a scan is performed over the normalization factor to find the combination of parameters from the model for the intrinsic spectrum and the EBL that give the maximum likelihood to the observed spectrum. At the end, the maximum likelihood is compared with the likelihood of the null-EBL hypothesis with a likelihood ratio test. With this method it was found that with the data from 1ES 1011+496, the EBL model used in this work (by Dominguez et al. 2011) escaled by an opacity normalization factor of 1.07 (-0.13+0.09)stat (-0.10.+0.7)syst was prefered over the null-EBL hypothesis with a significance of 4.6 standard deviations. In the case of the data from Markarian 421, it was found that the EBL template scaled by the opacity normalization factor of 0.90 (-0.17+0.11)stat (-0.00+0.29)syst was prefered over the null-EBL hypothesis with a significance of 5.8 standard deviations. Additional analyses were performed of data from 1ES 0229+200 and 1ES 0647+250. No EBL measurements could be extracted from the observations of these two sources. Also in this work is presented a brief study of possible anomalies in the propagation of the gamma rays in the intergalactic medium caused by the theoretical axion-like particle.

Date of Award	24 Apr 2015
Original language	English
Supervisor	Enrique Fernandez Sanchez (Tutor) & Abelardo Moralejo Olaizola (Director)