Research themes

Cosmic rays are charged particles, deflected by magnetic fields as they travel – consequently, their arrival direction often does not correspond to their origin. However, cosmic ray interactions at their source location produce neutral gamma rays that are not deflected; by studying these gamma rays, we can learn a lot about the origins of cosmic rays.

Gamma-ray Astronomy and High Energy Astrophysics

Gamma-ray astronomy uses the observation of gamma-rays to study natural energetic particle accelerators in our universe. What kinds of systems accelerate particles and how? What energies do they reach? Which particle types are accelerated where? 

Imaging Atmospheric Cherenkov Telescopes (IACTs) observe very-high-energy (~1011-1014 eV) gamma-rays indirectly, by detecting Cherenkov radiation from extensive air showers – cascades of energetic particles initiated by a gamma-ray interaction in the atmosphere.

Images of gamma-ray extensive air showers are elliptical. The orientation provides information about the gamma-ray direction; the image brightness corresponds to the gamma-ray energy. Using multiple telescopes to make stereoscopic observations improves the experimental performance. 

Cherenkov Telescopes: H.E.S.S. and CTA

The High Energy Stereoscopic System (H.E.S.S.) is an array of five IACTs located in the Khomas Highlands of Namibia, currently the only IACT facility in the Southern hemisphere. Since 2013 I have been active within the H.E.S.S. collaboration, particularly in the analysis of pulsar wind nebulae and of calibration using muons.

The Cherenkov Telescope Array (CTA) is a next generation facility for the detection of the most energetic gamma-rays from space, signatures of astrophysical particle acceleration. 

CTA will comprise more than 100 telescopes of three different sizes; large, medium and small; with mirror diameters of 23m, 12m and 4m respectively. Since 2014, I have been active within the CTA consortium, particularly in software development for calibration and data analysis.

Particle Detector Arrays: SWGO

Particle detector arrays such as HAWC, LHAASO or the future Southern Wide-field Gamma-ray Observatory (SWGO) also observe extensive air showers – but by measuring the particles in situ. They are hence located typically at higher altitudes than IACTs and can operate continuously day and night, making them ideal survey instruments.

Pulsar Wind Nebulae

Pulsar Wind Nebulae are a common source class of astrophysical very high energy (VHE) gamma-rays.

Copious amounts of leptons (electrons and positrons) are produced in the immediate environment of the pulsar. These leptons interact with the surrounding medium to produce gamma-rays, detectable at Earth. 

The leptons lose energy as they travel away from the source, leading to signature energy-dependent morphology, such as seen in the pulsar wind nebulae HESS J1825-137. This can be used to derive the particle transport mechanism and speed. 

The pulsar wind nebula HESS J1825-137 reduces in size with increasing energy. See the full study for more details (HESS Collaboration, Astronomy & Astrophysics 621 A116 (2019) – Highlight paper)

Muons with Cherenkov Telescopes

Muons produce easily identifiable ring-shaped images in Cherenkov telescopes when hitting the mirror dish. However, as the distance of the muon from the mirror dish increases, less of the ring is seen, making muons harder to identify. 

Muons form a constant brightness light source and are images of full rings used to calibrate the light collection efficiency of Imaging Atmospheric Cherenkov Telescopes. 

Improving the identification of partial muon rings enables measurements of the muon production rate and distribution within extensive air showers, currently uncertain quantities. 

Novae

The smaller cousins of supernovae, nova outbursts occur in binary stellar systems, typically comprised of a compact white dwarf and a massive companion star.
Matter is accreted from the massive star (red giant or main sequence) onto the white dwarf. Due to gravity, the base of the accreted material heats up – when the temperature is high enough to ignite thermonuclear burning, a nova outburst occurs.

Whereas in supernovae, the progenitor star is destroyed, in novae the star survives – which means the process can happen again. There are ~10 known recurrent novae, those where more than one outburst has been observed from the same binary system. RS Ophiuchi is a recurrent nova – undergoing its two most recent outbursts in 2006 and 2021. The outburst starting on 8th August 2021 was the first nova event to be detected at energies beyond a few tens of GeV – up to ~ 1 TeV.
Many facilities observed the nova at the same time across the electromagnetic spectrum, including the H.E.S.S. telescopes, the MAGIC telescopes and the Fermi-LAT satellite covering the gamma-ray regime.

Results from H.E.S.S. observations of RS Oph were published online in the journal Science on 10th March 2022. https://www.science.org/doi/10.1126/science.abn0567

Artist’s impression of the RS Ophiuchi Nova outburst. The fast shockwaves form an hourglass shape as they expand, in which gamma-rays are produced. This gamma-ray emission is then detected by the H.E.S.S. telescopes. Credit: DESY/H.E.S.S. Science Communication Lab