Modelling gamma-ray absorption at cosmic distances

Abstract

Extragalactic background light (EBL) is the integrated light from all resolved and unresolved extragalactic sources since the recombination era. Its spectrum extends from infrared to ultraviolet (0.001−10eV). Due to the expansion of the Universe photons emitted from early structures, that are not observable yet individually with the current technology, are redshifted to longer wavelengths in the EBL regime. These photons, therefore, carry a wealth of information about the formation and evolution of the Universe. Studying these photons is crucial in understanding the different stages the Universe went through. In addition, low energy EBL photons play a significant role in the propagation of the very high energy γ-ray photons from extragalactic sources. Where they interact with the γ-ray producing electron-positron pair, thereby attenuating the γ-ray photons. This attenuation can be observed as a steepening in the index of the VHE spectrum of the extragalactic γ-ray sources such as Blazars. The main focus of this thesis is to study the EBL fluctuation in the Universe and its impact on the VHE γ-ray spectrum. The EBL fluctuations is investigated in three different ways. First, we study the spatial fluctuations in the Universe analytically (Kudoda & Faltenbacher, 2017). This was done through computing the fluctuations in the star formation rate (SFR) using the millennium simulation run 7 (MR7) galaxy catalogue. Then using the fluctuations in the SFR, we set upper and lower limits for the EBL following a forward EBL modelling approach. Our results show ±50% SFR fluctuation around the mean with a small (±1%) effect on the EBL. The fluctuations in the EBL, however, lead to ±10% variation in the γ-ray transmissivity in extreme cases, while its impact on the spectral index Γ of the VHE tail of the γ-ray spectrum is ±1%. Second, we investigate the fluctuations of the EBL in the γ-ray path from cosmological sources such as Blazars. In this part of the study we model the γ-ray path using a light cone. We develop a hybrid model of the EBL, where we combine the previous analytic model (Kudoda & Faltenbacher, 2017) with a numerical approach. By simulating the γ-ray path between z = 0 and z = 0.5 as cylinder with a radius of 50Mpc (Kudoda & Faltenbacher, 2018). In this approach we divide the EBL in two parts: local (within the 50Mpc) and homogeneous background. This was done to study and evaluate the impact of the presence of large scale structures along the γ-ray path. One order of magnitude fluctuation in the local EBL density is confirmed. However, in the total EBL this work is in agreement with our previous work (±1% fluctuation). In addition, we investigate the situation when the γ-ray encounter a galaxy in its path. We find the presence of a galaxy with stellar masses of M > 1011 M will have a significant effect on the γ-ray absorption, thus, the spectral index of the VHE sources. In the last part of this thesis we investigate the correlation between the power spectra of the EBL and the underlying matter (dark matter) distribution. This investigation was done by computing the power spectrum for the bolometric intensity of the EBL, and derive the bias between the two power spectra. If the EBL distribution is absolutely homogeneous, one expects the power spectrum to be flat. We confirm that the amplitude of the EBL fluctuations in general is small compared to the corresponding fluctuations in the matter distribution. However, on the scale of superclusters and voids (> 100 Mpc), it is more than two orders of magnitudes higher than small scales (< 10 Mpc). This is owing to the 1/r2 profile of the light distribution about point sources (i.e. galaxies), that flushes out the small fluctuations amplitudes.