Namibia’s HESS instrumental in detecting first GRB afterglow

21 November 2019 | Science

After a decade-long search, for the first time scientists have detected a gamma-ray burst (GRB) in very-high-energy gamma light.
This discovery was made in July last by the High Energy Stereoscopic System (HESS) collaboration using the huge 28-m telescope of the HESS array in Namibia. Surprisingly, this GRB – an extremely energetic flash following a cosmological cataclysm – was found to emit very-high-energy gamma rays long after the initial explosion.
Extremely energetic cosmic explosions generate GRBs, typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe. The burst is followed by a longer lasting afterglow, mostly in the optical and X-ray spectral regions whose intensity decreases rapidly.
The high energy gamma-ray emission is mostly composed of photons several hundred-thousands to millions of times more energetic than visible light, that can only be observed by satellite-based instruments.
Whilst these space-borne observatories have detected a few photons with even higher energies, the question if very-high-energy (VHE) gamma radiation (at least 100 billion times more energetic than visible light and only detectable with ground-based telescopes) is emitted, has remained unanswered until now.
On 20 July 2018, the Fermi Gamma-Ray Burst Monitor and a few seconds later the Swift Burst Alert Telescope notified the world of a gamma-ray burst, GRB 180720B. Immediately after the alert, several observatories turned to look at this position in the sky.
For HESS, this location became visible only 10 hours later. Nevertheless, the HESS team decided to search for a VHE afterglow of the burst. After having looked for a very-high-energy signature of these events for more than a decade, the efforts by the collaboration finally bore fruit.
A signature has now been detected with the large HESS telescope that is especially suited for such observations. The data collected during two hours from 10 to 12 hours after the gamma-ray burst showed a new point-like gamma-ray source at the position of the burst.
While the detection of GRBs at these VHEs had long been anticipated, the discovery many hours after the initial event, deep in the afterglow phase, came as a real surprise. The discovery of the first GRB to be detected at such very-high-photon energies is reported in a publication by the HESS collaboration in the journal 'Nature' yesterday (20 November 2019).

Long lasting
GRB 180720B was very strong and lasted about 50 seconds – a relatively long duration indicating the death of a massive star. In this process, its core collapses to a rapidly rotating black hole. The surrounding gas forms an accretion disk around the black hole, with gas jets ejected perpendicularly to the disk plane creating the gamma-ray flashes.
Elementary particles are accelerated in these jets to velocities nearly as high as the speed of light and interact with the surrounding matter and radiation, leading to the copious production of gamma rays.
The VHE gamma radiation which has now been detected not only demonstrates the presence of extremely accelerated particles in GRBs, but also shows that these particles still exist or are created a long time after the explosion.
Most probably, the shock wave of the explosion acts here as the cosmic accelerator. Before this HESS observation, it had been assumed that such bursts are likely observable only within the first seconds and minutes at these extreme energies, and not many hours after the explosion.
At the time of the HESS measurements the X-ray afterglow had already decayed considerably. Remarkably, the intensities and spectral shapes are similar in the X-ray and gamma-ray regions. There are several theoretical mechanisms for the generation of very-high-energy gamma light by particles accelerated to very-high-energies.
The HESS results constrain the emission to two potential mechanisms. In both cases, however, the observations raise new questions. “Although energetically one of these mechanisms is preferred, both the shape of the HESS spectrum, and the energy range of the emission at such late times presents a challenge to both emission scenarios,” says HESS scientist Andrew Taylor.
This breakthrough discovery provides new insights into the nature of gamma-ray bursts. As highlighted by Edna Ruiz Velasco, PhD. student at MPIK in Heidelberg and one of the corresponding authors of the publication: “This detection has already revolutionised the way we search for GRBs with Cherenkov Telescopes. Thanks to this GRB and the lessons learnt, our recently improved observational strategy has already paid off. We can expect a future with a great number of GRBs detections at very-high energies and with this, a deeper understanding of these phenomena.”

Mind of its own
As for the transient nature of GRBs, the HESS telescopes have an automated procedure to observe them as soon as an alert by a satellite comes in. “The GRB alert system has a mind of its own,” says Jimmy Shapopi, an MSc student in the Department of Physics at the University of Namibia (UNAM), who has been on observing night shifts at the HESS telescopes.
“It takes control from any observer at the telescope array and points the telescopes to the position of the GRB alert with lightning speed. This is impressive for an array of five telescopes with collective mass of 1000 tons.”
The HESS team consists of over 200 scientists from Germany, France, the United Kingdom, Namibia, South Africa, Ireland, Armenia, Poland, Australia, Austria, the Netherlands, Japan, and Sweden, supported by their respective funding agencies and institutions.
UNAM is a member of the HESS collaboration. Currently, there are six staff members, two PhD and three MSc students conducting their research in the context of HESS and gamma-ray astronomy in the Department of Physics at UNAM. In the near future, these will be joined by two postdoctoral fellows, supported by a grant from the UK Global Challenges Research Fund.

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