Searching for an optical counterpart to Fast Radio Bursts

Hanmer, Kira Yasmin

Searching for an optical counterpart to Fast Radio Bursts

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Astronomy

Faculty

Faculty of Science

Abstract

Fast radio bursts (FRBs) are bright, short-lived bursts of radio emission of extragalactic origins. Despite being immensely luminous, the answer to what actually causes FRBs remains a mystery. Numerous models have been proposed, but it is difficult to narrow down the list because, up until now, FRBs have only ever been observed at radio wavelengths. Some models predict that, when a FRB is emitted, light at other wavelengths should be emitted too. Other models predict only radio emission. If we were to detect emission at other wavelengths associated with FRBs, we would be able to conclusively strike off at least some of the proposed models, getting us closer to solving the mystery around what they actually are. But observing FRBs simultaneously in radio and other wavelengths is extremely difficult: they only last for a few thousandths of a second, meaning that any hopes of observing a FRB and then following up with another telescope in another wavelength is not feasible, unless the FRB is one of the handful that actually burst repeatedly. Most FRBs, however, only happen once. The way to observe FRBs across multiple wavelengths, would be with a system consisting of a radio telescope, and a telescope in another wavelength that is dedicated to observing simultaneously with the radio telescope. Such systems are rare, but we have one here in South Africa: MeerLICHT is an optical 0.6 m telescope located in Sutherland, and it was commissioned specifically to observe simultaneously with the MeerKAT array near Carnarvon. This means that we have a unique opportunity to search for new FRBs using MeerKAT, and to see any associated optical emission with MeerLICHT, regardless of whether the FRBs found by MeerKAT are rare repeaters, or the much more common non-repeaters. In this thesis, I will present an overview of FRBs and their known properties to date, and the results of our search for an optical counterpart to these enigmatic bursts using MeerLICHT and MeerKAT. I present our results for FRB 20230808F, which was newly discovered by MeerKAT, and for which we obtained an optical image from MeerLICHT 3.4 seconds later. The FRB is as yet non-repeating. The previous shortest delay between radio and optical observations of a non-repeating FRB was approximately 2 hours, meaning that our delay of 3.4 seconds is a significant improvement. Although we detected no optical counterpart on this timescale, we are able to set upper limits on any optical emission associated with the FRB. Additionally, we are able to localise the FRB to a pair of galaxies at a redshift of 0.3472, and to determine its scattering timescale, dispersion measure, and polarisation fractions. Additionally, MeerKAT detected its first ever repeating FRB, FRB 20240619D. We managed to obtain strictly simultaneous optical and radio observations of this FRB, and I present the results of our analysis of the optical data obtained for FRB 20240619D.

Keywords

Fast Radio Bursts

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