Observations challenge the established theory of gamma-ray bursts in the universe
GERMAN ELECTRON SYNCHROTRON DESY
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PICTURE: ARTIST’S IMPRESSION OF A RELATIVIST JET OF A GAMMA-RAY BURST (GRB) SPREADING OUT OF A COLLAPSED STAR AND MEANS VERY HIGH ENERGY PHOTONS. Show more CREDIT: DESY, SCIENCE COMMUNICATION LAB
Scientists have gained the best view of the brightest explosions in the universe to date: A specialized observatory in Namibia has recorded the most energetic radiation and the longest gamma-ray afterglow of a so-called gamma-ray burst (GRB). The observations with the High Energy Stereoscopic System (HESS) question the established notion of how gamma rays are generated in these colossal star explosions, the birth screams of black holes, as the international team reports in the journal Science.
“Gamma-ray bursts are bright X-ray and gamma-ray bursts that are observed in the sky and are emitted from distant extragalactic sources,” explains DESY scientist Sylvia Zhu, one of the authors of the study. “They are the largest explosions in the universe and are related to the collapse of a rapidly rotating massive star into a black hole. A fraction of the released gravitational energy feeds the generation of an ultra-relativistic pressure wave. Your emission is divided into two different phases: an initial chaotic prompting phase that lasts for several tens of seconds, followed by a long lasting, gently fading afterglow phase. “
On August 29, 2019, the Fermi and Swift satellites detected a gamma-ray burst in the constellation Eridanus. The event, cataloged as GRB 190829A after its date, turned out to be one of the closest gamma-ray bursts observed to date, about a billion light years away. For comparison: the typical gamma-ray burst is about 20 billion light years away. “We were really sitting in the front row when this gamma-ray flash happened,” explains co-author Andrew Taylor from DESY. The team caught the afterglow of the explosion as soon as it became visible to the HESS telescopes. “We were able to observe the afterglow for several days and with unprecedented gamma-ray energies,” says Taylor.
The comparatively short distance to this gamma-ray flash allowed detailed measurements of the afterglow spectrum, ie the distribution of the “colors” or photon energies of the radiation, in the very high energy range. “We were able to determine the spectrum of GRB 190829A up to an energy of 3.3 tera electron volts, which is 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 in Heidelberg. “That is the extraordinary thing about this gamma-ray flash – it happened in our cosmic backyard, where the very high-energy photons on their way to earth were not absorbed in collisions with the background light, as is the case over great distances in the cosmos.”
The team was able to track the afterglow for up to three days after the first explosion. The result was surprising: “Our observations showed strange similarities between the X-ray and the high-energy gamma radiation from the afterglow of the burst,” reports Zhu. Established theories assume that the two emission components must be generated by separate mechanisms: The X-ray component comes from ultrafast electrons that are deflected in the strong magnetic fields around the burst. This “synchrotron” process is very similar to how particle accelerators on earth generate bright X-rays for scientific research.
However, according to existing theories, it seemed very unlikely that even the most powerful explosions in the universe could accelerate electrons enough to directly generate the very high-energy gamma rays observed. This is due to a “burn-up limit”, which is determined by the balance between acceleration and cooling of particles within an accelerator. In order to generate high-energy gamma rays, electrons with energies are required that are far above the burn-up limit. Instead, current theories assume that in a gamma-ray burst, fast electrons collide with synchrotron photons, raising them to gamma-ray energies in a process called synchrotron self-Compton.
But the afterglow observations from GRB 190829A now show that both components, x-rays and gamma-rays, fade out in sync. The gamma-ray spectrum also clearly agreed with an extrapolation of the X-ray spectrum. Taken together, these results strongly suggest that X-rays and very high energy gamma rays were generated by the same mechanism in this afterglow. “It is rather unexpected to observe such remarkably similar spectral and temporal properties in the energy bands of X-rays and the very high-energy gamma radiation if the emission in these two energy ranges had different origins,” says co-author Dmitry Khangulyan of Rikkyo University in Tokyo . This poses a challenge to the synchrotron-self-Compton origin of the very high-energy gamma-ray emission.
The far-reaching implication of this possibility underscores the need for further studies of the very high-energy GRB afterglow emission. GRB 190829A is only the fourth gamma-ray burst detected from the ground. However, the explosions discovered earlier occurred much further away in the cosmos and their afterglow could only be observed for a few hours and not up to energies above 1 tera-electron volt (TeV). “With a view to the future, the prospects for the detection of gamma-ray flashes by next-generation instruments such as the Cherenkov Telescope Array, which is currently being built in the Chilean Andes and on the Canary Island of La Palma, are very promising,” says HESS spokesman Stefan Wagner from of the State Observatory Heidelberg. “The general frequency of gamma-ray bursts leads us to expect that regular detections in the very high energy band will become quite frequent, which helps us to fully understand their physics.”
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More than 230 scientists from 41 institutes in 15 countries (Namibia, South Africa, Germany, France, Great Britain, Ireland, Italy, Austria, the Netherlands, Poland, Sweden, Armenia, Japan, China and Australia) form the international HESS collaboration, has to contributed to this investigation. HESS is a system of five imaging atmospheric Cherenkov telescopes that investigate 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 rays. HESS is located in Namibia, near the Gamsberg, an area known for its excellent optical quality. Four HESS telescopes went into operation in 2002/2003, the significantly larger fifth telescope – HESS II – has been in operation since July 2012 and extends the energy coverage towards lower energies and further improves sensitivity. In the years 2015-2016, the cameras of the first four HESS telescopes with state-of-the-art electronics and in particular the NECTAr readout chip, which was developed for the next big experiment in this field, the Cherenkov Telescope Array (CTA), were completely refurbished for the The Data Science Management Center is hosted by DESY at the Zeuthen location.
DESY is one of the world’s leading particle accelerator centers and researches the structure and function of matter – from the interaction of tiny elementary particles to the behavior of novel nanomaterials and vital biomolecules to the great secrets of the universe. The particle accelerators and detectors that DESY develops and builds at its locations in Hamburg and Zeuthen are unique research tools. They generate the strongest X-rays in the world, accelerate particles to absorb energies and open new windows to the universe. DESY is a member of the Helmholtz Association, Germany’s largest scientific association, and is funded by the Federal Ministry of Education and Research (BMBF) (90 percent) and the federal states of Hamburg and Brandenburg (10 percent).
Reference:
Uncover temporal and spectral similarities of X-rays and gamma rays in GRB 190829A afterglow; HESS cooperation; Science, 2021; DOI: 10.1126 / science.abe8560
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