Image of gamma-ray burst taken by HESS
Credit: HESS

Oxford astronomers in front-row view of exceptional cosmic explosion

Astronomy and astrophysics
Particle astrophysics & cosmology

An international team of scientists including astronomers from the University of Oxford have gained the best view yet of the brightest explosions in the universe. A specialised observatory in Namibia has recorded the most energetic radiation and longest gamma-ray afterglow of a so-called gamma-ray burst (GRB) to date. The observations with the High Energy Stereoscopic System (HESS) challenge the established idea of how gamma-rays are produced in these colossal stellar explosions which are the birth cries of black holes, as the team reports in the journal Science.

Darkest and clearest skies on Earth

‘The HESS observatory is sited under some of the darkest and clearest skies on Earth, in the Khomas highlands in central Namibia,’ says Professor Garret Cotter, leader of the HESS group at Oxford. ‘These conditions allow us to detect the brief flashes of light, lasting only a few billionths of a second, that occur when gamma-rays from space smash into the top of the Earth’s atmosphere.’

‘Gamma-ray bursts are bright X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources,’ explains Sylvia Zhu, scientist at DESY in Berlin and one of the authors of the paper. ‘They are the biggest explosions in the universe and associated with the collapse of a rapidly rotating massive star to a black hole. A fraction of the liberated gravitational energy feeds the production of an ultrarelativistic blast wave. Their emission is divided into two distinct phases: an initial chaotic prompt phase lasting tens of seconds, followed by a long-lasting, smoothly fading afterglow phase.’

Closest gamma-ray burst observed so far

On 29 August 2019 the satellites Fermi and Swift detected a gamma-ray burst in the constellation of Eridanus. The event, catalogued as GRB 190829A according to its date of occurrence, turned out to be one of the nearest gamma-ray bursts observed so far, with a distance of about one billion lightyears. For comparison: the typical gamma-ray burst is about 40 billion lightyears away. ‘We were really sitting in the front row when this gamma-ray burst happened,’ explains co-author Andrew Taylor from DESY. ‘Following the alert from the satellites, the HESS telescopes on the ground could then follow the course of explosion in fine detail of the course of the following three nights,’ says Professor Cotter.

The comparatively short distance to this gamma-ray burst allowed detailed measurements of the afterglow's spectrum, which is the distribution of “colours” or photon energies of the radiation, in the very-high energy range. ‘We could determine GRB 190829A's spectrum up to an energy of 3.3 tera-electronvolts, that's about a trillion times as energetic as the photons of visible light,’ explains co-author Edna Ruiz-Velasco from the Max Planck Institute for Nuclear Physics. ‘This is what is so exceptional about this gamma-ray burst – it happened in our cosmic backyard where the very-high-energy photons were not absorbed in collisions with background light on their way to Earth, as it happens over larger distances in the cosmos.’

Following the afterglow

The team could follow the afterglow up to three days after the initial explosion. The result came as a surprise: ‘Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst's afterglow,’ reports Zhu. Established theories assume that the two emission components must be produced by separate mechanisms: the X-ray component originates from ultra-fast electrons that are deflected in the strong magnetic fields of the burst's surroundings. This “synchrotron” process is quite similar to how particle accelerators on Earth produce bright X-rays for scientific investigations.

However, according to existing theories it seemed very unlikely that even the most powerful explosions in the universe could accelerate electrons enough to directly produce the observed very-high-energy gamma rays. This is due to a “burn-off limit”, which is determined by the balance of acceleration and cooling of particles within an accelerator. Producing very-high energy gamma-rays requires electrons with energies well beyond the burn-off limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons and thereby boost them to gamma-ray energies in a process dubbed synchrotron self-Compton.

Challenging established theory

But the observations of GRB 190829A's afterglow now show that both components, X-ray and gamma ray, faded in sync. Also, the gamma-ray spectrum clearly matched an extrapolation of the X-ray spectrum. Together, these results are a strong indication that X-rays and very-high-energy gamma rays in this afterglow were produced by the same mechanism. ‘It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high energy gamma-ray energy bands, if the emission in these two energy ranges had different origins,’ says co-author Dmitry Khangulyan from Rikkyo University in Tokyo. This poses a challenge for the synchrotron self-Compton origin of the very-high energy gamma-ray emission.

The far-reaching implication of this possibility highlights the need for further studies of very-high energy GRB afterglow emission. GRB 190829A is only the fourth gamma-ray burst detected from the ground. However, the earlier detected explosions occurred much farther away in the cosmos and their afterglow could only be observed for a few hours each and not to energies above 1 tera-electronvolts (TeV). ‘Looking to the future, the prospects for the detection of gamma-ray bursts by next-generation instruments like the Cherenkov Telescope Array that is currently being built in the Chilean Andes and on the Canary Island of La Palma look promising,’ says HESS spokesman Stefan Wagner from Landessternwarte Heidelberg. ‘The general abundance of gamma-ray bursts leads us to expect that regular detections in the very-high energy band will become rather common, helping us to fully understand their physics.’ Oxford is leading work on the high-energy cameras for the Cherenkov Telescope Array which, with the now-demonstrated ability for ground-based observing, will be of prime importance for future advances in our knowledge of gamma-ray busts.

Dark sky tourism

Astronomers at Oxford have also been following the events using telescopes at radio wavelengths. ‘The radio appearance is similar to that of previously-known GRBs. This leads us to the exciting possibility that all GRBs might be producing photons at the highest energies,’ says doctoral student Lauren Rhodes, who has been leading the follow-up with the MeerKAT radio observatory, situated in South Africa. ‘Astronomy across the wavebands in the southern African region has a highly significant impact on local science and technology,’ says Professor Cotter. ‘Oxford, in collaboration with our colleagues at the University of Namibia, has a strong record in capacity building in science education, outreach, and even the development of the tourist sector under the amazing night skies in Namibia.’
More than 230 scientists from 41 institutes in 15 countries (Namibia, South Africa, Germany, France, the UK, Ireland, Italy, Austria, the Netherlands, Poland, Sweden, Armenia, Japan, China and Australia), comprising the international HESS collaboration, contributed to this research. HESS is a system of five Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays. The name HESS stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Franz Hess, who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. Support for HESS operations in Oxford comes from the UK’s Science and Technology Facilities Council and from Exeter College.
Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow, HESS collaboration; Science, 2021; DOI: 10.1126/science.abe8560