Critical Evaluation of the Methods Used in Extragalactic Background Light Studies with IACTs and Application to Observations with MAGIC and LST-1

by Roger Grau Haro (IFAE)

Wednesday Jul 30, 2025, 11:00 AM → 1:00 PM Europe/Madrid

IFAE Seminar Room ((Hybrid))

Thesis supervisor: Abelardo Moralejo Olaizola

Thesis committee: Jonathan Biteau, Monica Seglar Arroyo, Pol Bordas Coma

Abstract: Even in the deepest voids of space, as far as possible from any source of light, the universe is not truly dark. In the wavelength range from the infrared to the ultraviolet bands, the electromagnetic radiation that fills the universe is known as the Extragalactic Background Light (EBL). The EBL is, in terms of energy density, the second most intense photon field in the universe, surpassed only by the cosmic microwave background (CMB). The bulk of the EBL is believed to be the accumulated light produced through the history of the universe by stars, and by dust that absorbs starlight and re-emits it at longer wavelengths. Its measurement provides valuable information about star formation history and galaxy evolution, but direct measurements are difficult due to bright foregrounds from local sources. An alternative indirect method to probe the EBL uses the γγ interaction between Very-High-Energy (VHE, > 100 GeV) photons and EBL photons, which leaves an observable imprint in the gamma-ray spectra of sources at cosmological distances. Measurements through this method have also provided an independent estimate of the Hubble constant. Furthermore, purported discrepancies between gamma-ray-based EBL constraints and expectations from models and other types of
measurements have occasionally been presented as evidence for physics beyond the standard model: from axion-like particles to dark matter and violation of Lorentz invariance. For these reasons, it is essential to understand the EBL and the details of the measurements and the analyses from which these results are derived.
This thesis focuses on examining the robustness of EBL constraints derived from gamma-ray data, with a particular focus on the assumptions made in previous studies about the intrinsic source spectra, the uncertainties of the observations, and the validity of the analysis tools employed. Using Monte Carlo simulations and archival MAGIC data, I evaluate the two central elements of this work. The first one is to test the validity of the statistical tools iused, such as Wilks’ theorem, in the analysis of data obtained with Imaging Atmospheric Cherenkov Telescopes (IACTs), focusing on the effect of potential energy-dependent systematic errors, which seem to have been ignored in the literature so far. The second one is to study the influence of spectral assumptions (concerning the intrinsic VHE spectra) on the derived EBL constraints.
Alternative approaches, with less demanding assumptions on the source spectra, are proposed to increase the robustness of the methods. The results show that the Extragalactic Background Light (EBL) constraints are very dependent on the chosen spectral assumptions, and that many of the results in the literature may have underestimated the uncertainties, especially on the lower constraints. This work presents proposals to improve the robustness of the EBL constraints and to ensure they have the desired statistical coverage. I expect these developments will be particularly relevant for the analysis of the observations with next-generation facilities such as the Cherenkov Telescope Array Observatory (CTAO).
In parallel, I have contributed to the development and testing of the Barcelona Raman LIDAR pathfinder (pBRL), an atmospheric monitoring tool being developed for the CTAO. Accurate atmospheric characterization is crucial to reduce systematic uncertainties in IACT data, a key goal for next-generation instruments in which statistical uncertainties will be strongly reduced compared to current facilities.
In conclusion, this thesis aims to advance the methodological foundation for EBL studies, paving the way for more accurate and reliable measurements in the era of next-generation gamma-ray observatories. By critically reassessing long-standing assumptions and addressing systematic uncertainties, it intends to contribute to a deeper and more robust understanding of the cosmic light that traces the history of structure formation in the universe.

Zoom link: https://us02web.zoom.us/j/89787514064?pwd=SkRaOElqanZRNFZXM2d2SE9PN1d0Zz09