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Report in the Current Edition of the Journal Nature

Research Team Proves Outbreak of Cosmic Stellar Explosion

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A giant telescope. © Dominik Elsässer​/​TU Dort­mund
On the Canary Island of La Palma, the Dort­mund physicists work with the MAGIC telescopes.

For the first time, scientists in the MAGIC collaboration have verified ultra-high energy rays from a gamma ray burst using ground-based telescopes. In doing so, they proved physical theories. Researchers from TU Dort­mund University also played a seminal role in the discovery. They report on their insights in the current edition of the respected journal Nature.

Gamma ray bursts (GRBs) are perplexingly short but very bright events that suddenly appear in the sky roughly once a day. It is assumed that some of them are the result of massive stars exploding at the end of their lifespan – and thus, so to speak, the birthing cry of black holes or neutron stars. They are characterized by an initial, very bright flash, known as prompt emission, which lasts from a fraction of a second to hundreds of seconds. This is followed by the so-called afterglow, a weaker, but rather longer, light emission over a broad range of wavelengths that fades over time. The MAGIC telescopes have now captured evidence of the highest energy photons ever to issue from the explosion of massive stars – that is, of that which constitutes electromagnetic radiation.

This breakthrough delivers crucial new knowledge for our understanding of what are still the puzzling physical processes that are responsible for GRBs. On January 14, 2019, a GRB was discovered by two satellite observatories: NASA’s Neil Gehrels Swift Ob­ser­va­tory and the Fermi Gamma-ray Space Telescope.

A bright blue flash of light across a night sky full of stars. © TU Dort­mund
A gamma ray burst looks like a bright flash and often only lasts a few seconds.

In accordance with the date of discovery, the event was named GRB 190114C. Within 22 seconds, the coordinates of the gamma ray burst in the sky were sent to astronomers around the world in the form of an electronic alarm signal, including to the MAGIC collaboration that operates two gamma ray telescopes with a diameter of 17 meters on La Palma, Spain. Because GRBs appear at unpredictable places in the sky and then quickly fade, observing them with such large telescopes as MAGIC demands a sophisticated strategy.

Despite weighing 64 tons, the telescope can be rotated quickly

“An automatic system processes the GRB warnings from satellite instruments in real time and quickly rotates the MAGIC telescopes to the point in the sky where the GRB occurs,” says Professor Wolfgang Rhode of TU Dort­mund University. The telescopes were specifically designed to be very light and thus quick to rotate in the pursuit of GRBs: despite weighting 64 tons, they can rotate 180 degrees in just 25 seconds. That is why, in the case of GRB 190114C, MAGIC could trigger observation just 50 seconds after the GRB started.

Data analysis of the first ten seconds shows that the emission of photons in the afterglow reaches energies that are trillions of times larger than the visible light. During this period, GRB 190114C was by far the brightest object in this energy range in the entire sky. As anticipated with GRB afterglow, the emission faded. MAGIC registered the last photons from the object half an hour later.

The Dort­mund research group particularly specializes in fast, efficient analysis and in setting up the simulations required to evaluate the data. This has paid off: after carefully examining the data, they were able to share the MAGIC findings with the worldwide research community only hours after the event. As a result, more than two dozen observatories or instruments were able to undertake a comprehensive campaign of follow-up observations of GRB 190114C. The latter delivered a complete image of this GTB from the radio range to gamma radiation. Optical observations, in particular, enabled them to measure the distance to GRB 190114C. It was around five billion light years.

Photons with the highest energy from a newly-discovered emission process

Although the high-energy emission in the GRB’s afterglow had been predicted in some theoretical studies, pursuing it proved very difficult and required years of constantly improving strategies as well as the efficiency of the MAGIC telescopes. But the scientific reward for these patient efforts is considerable: “Our measurements suggest that the high-energy gamma radiation from the afterglow possibly stems from a different process than the emission at low energies,” explains Dr. Dominik Elsässer who is also involved in MAGIC at TU Dort­mund University. “We suspect that energy-rich electrons transfer their energy to photons by so-called inverse Compton scattering, thus generating the brilliance captured by MAGIC. But to test this suspicion we need observations that go beyond the electromagnetic spectral range.”

So, 50 years after they were discovered, there are still lots of GRB puzzles to be solved. This is especially true of the question as to whether some of them also produce energy-rich neutrinos. These are those spectral elementary particles the scientists around Wolfgang Rhode are trying to track down at the South Pole using the IceCube Detector. “The MAGIC results have encouraged us to keep refining our methods and extending our experiments. Thanks to the continuation of funding from the state of NRW and the federal government, which is so important for such in­ter­na­tio­nal research projects, we hope to pave the way for a much deeper understanding of these fascinating cosmic explosions,” Rhode concludes.

 

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