The Astrophysical Search For Axion Like Particle Signatures In γ-ray Propagation

Abstract

The work undertaken during this Doctoral period involved studying interactions of Very High Energy (VHE) γ-rays with Astrophysical magnetic fields and the Extra-galactic Background Light (EBL). The primary goal was to search for signatures of ultralight Axion Like Particles (ALPs) via their interactions with VHE γ-rays. These particles are also a promising Dark Matter candidate.

Our first project attempted to find spectral “wiggles” induced by photon-ALP oscillations in the presence of Astrophysical magnetic fields. Our candidate VHE γ-ray source was the Messier 87 Active Galactic Nucleus, located at z ∼ 0.0042 at the center of the Virgo cluster. We chose to implement a Gaussian turbulence model for the Virgo cluster magnetic field as a radially varying function dependent on the electron density profile. Electron density measurements and calculations of Faraday Rotation measures from M87 and M84 were applied to constrain the model parameters, giving us a central magnetic field value of B0 = 34.2 µG. We modeled spectral wiggles as outcomes of various pseudo-randomized magnetic field realizations along the line of sight. We performed Likelihood ratio tests and other statistical analyses to verify any preferences for our models, fitting the pseudo-random models against data from H.E.S.S. observational campaigns. We chose observations from 2005, 2010 and 2018 when the source was reported to have been observed in flaring/high states. Our results were statistically inconclusive, showing at best only a preference of < 2σ for the ALPs case. Strictly speculatively, this may hint at a possible signal region within our parameter space. Reports of the magnetic field model and constraints, along with the ALP search were presented and published as proceedings at the Heidelberg International Symposium 2022 and at ICRC 2023 respectively.

Beyond our ALP searches, we were also able to make contributions toward the fields of EBL research and γ-ray Astronomy. We made a first-ever detection of intrinsic curvature in the VHE γ-ray spectrum of the AGN hosted by Messier 87 during flares. We detected such a spectral curvature in with a confidence of ∼ 4σ. In a combined analysis with colleagues from the H.E.S.S. Collaboration, we were able to utilize the same data set to place upper limits on the normalization term of three EBL models which are widely used in the field of γ-ray Astrophysics. The discovery of curvature, along with the EBL model constraints were published as a peer reviewed collaboration paper in the Astronomy & Astrophysics Journal in 2024.

Another project during the research period searched for ALP induced upturns in the VHE spectra of γ-ray sources. We searched for, and attempted to relate spectral upturns to ALP induced transparency of the Universe to VHE γ-rays. We model the upturns as outcomes of pseudo randomized realizations of the intergalactic and galactic magnetic fields along the line of sight. We fit these models against our data set and performed Likelihood ratio tests. We see at best one limiting case where the ALPs model can be ruled out at ∼ 4σ. We constructed an elaborate and robust dataset consisting of observations of our 13 candidate blazars, all of which have been observed in flaring states during H.E.S.S. observational campaigns. The data set was analyzed and temporally segmented where any source exhibited significant variability in its emission pattern with respect to its observed flux and spectral hardness. The data set has potential application for various types of Astrophysics research and will be made publicly available after publication of our results. This effort is being undertaken by colleagues.