Extraordinary observation challenges theory of the strongest explosions in the universe
The brightest explosions in the universe are possibly stronger particle accelerators than expected: this shows an extraordinarily detailed observation of such a cosmic flash of gamma rays. With the special telescopes of the HESS observatory in Namibia, an international research team has registered the most energetic radiation from a gamma flash to date and followed the longest afterglow in the field of gamma radiation. The evaluation of the data suggests that X-rays and gamma rays of these huge stellar explosions have the same cause and are not, as previously assumed, caused by separate processes, as the team reports in the journal “Science”.
“Gamma-ray bursts are bright bursts of X-ray and gamma radiation in the sky that come from sources outside our own galaxy,” explains DESY researcher 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.” Part of the gravitational energy released in the process drives an extremely fast, ultra-relativistic shock wave. In it, subatomic particles are accelerated like electrons, which in turn can generate gamma radiation. Gamma ray bursts (GRB) split into two phases: a short and chaotic burst phase that lasts a few dozen seconds, and a long, slowly fading afterglow.
On August 29, 2019, the two satellites “Fermi” and “Swift” of the US space agency NASA registered a gamma-ray flash in the southern constellation Eridanus. The event was cataloged by date as GRB 190829A. At a distance of around a billion light years, it turned out to be one of the closest gamma-ray bursts observed to date. For comparison: the typical gamma-ray burst is around 20 billion light years away. “We really saw this gamma-ray flash from the front row,” says DESY researcher Andrew Taylor, co-author of the study. The team registered the afterglow as soon as it came into the field of view of the HESS telescope. “We were able to watch the afterglow for several days and at unprecedented energies,” reports Taylor.
The comparatively short distance of the gamma-ray flash enabled detailed measurements of the high-energy spectrum of its afterglow, ie the “color” or energy distribution of the X-ray and gamma photons. “We were able to measure the spectrum of GRB 190829A up to an energy of 3.3 tera electron volts, which is around a trillion times more energetic than visible light,” says co-author Edna Ruiz-Velasco from the Max Planck Institute for Nuclear Physics in Heidelberg. “This is what makes this gamma-ray burst so extraordinary – it happened in our direct cosmic neighborhood, so that its very high-energy photons were not absorbed by collisions with background light, as happens over long distances in the cosmos.
HESS followed the afterglow of the gamma-ray burst until the third day after the original explosion. “Our observations reveal a striking similarity between the X-ray component and the very high-energy gamma radiation in the afterglow,” reports Zhu. This is surprising, because the generally accepted theory assumes that these two radiation components have to be produced by different mechanisms: The X-ray radiation therefore comes from strongly accelerated electrons that are deflected by the strong magnetic fields in the vicinity of the explosion. Using this “synchrotron process”, particle accelerators on earth also produce intensive X-rays for scientific investigations.
For the production of very high-energy gamma radiation, however, according to current theories, the synchrotron process is initially out of the question. The fault is a so-called burn-off limit, which is determined by the ratio of acceleration and cooling of the particles in an accelerator. The production of gamma radiation requires electrons with energies well above the burn-off limit, which even the strongest explosions in space cannot actually produce. Instead, the theory assumes that the fast electrons collide with the already high-energy synchrotron photons and raise them to gamma energies in the process. This complicated process is called Synchrotron Self Compton (SSC).
However, the observations of the afterglow of GRB 190829A show that both components – that is, X-rays and gamma rays – have faded synchronously. In addition, the gamma-ray spectrum goes well with an extension of the X-ray spectrum. Taken together, these properties are a strong indication that both radiation components have been generated by the same process. “We would not expect to observe such remarkably similar spectral and temporal properties of X-ray radiation and the very high-energy gamma radiation if these radiation components had separate origins,” explains co-author Dmitry Khangulyan from Rikkyo University in Tokyo. This calls into question the SSC process as the origin of gamma radiation.
Whether the theory of gamma-ray bursts needs to be changed can only be clarified by further observations of the very high-energy component of their afterglow. However, GRB 190829A is only the fourth gamma-ray flash that can be detected at these high energies. However, the previously discovered gamma-ray bursts came from a much greater distance, and their afterglow could only be observed for a few hours and not at energies above one tera-electron volt (TeV). “The 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, have very promising prospects of tracking such gamma-ray bursts regularly,” says HESS spokesman Stefan Wagner from the State Observatory in Heidelberg.
More than 230 researchers from the HESS cooperation from 41 institutes from 15 countries (Namibia, South Africa, Armenia, Germany, France, Italy, Great Britain, Ireland, Austria, the Netherlands, Poland, Sweden, Japan, China and Australia) contributed to this work ) contributed. HESS is a system of five so-called imaging Cherenkov telescopes for the investigation of cosmic gamma rays. The name stands for High-Energy Stereoscopic System (stereoscopic system for observing high-energy radiation) and also pays tribute to the discoverer of cosmic rays, Victor Franz Hess, who received the Nobel Prize in Physics in 1936 for his work. HESS is located in Namibia in the Gamsberg region, which is known for its excellent observation conditions. Four of the five HESS telescopes went into operation in 2003/2003, the fifth, much larger telescope has been in operation since July 2012. It has not only increased the sensitivity of the facility significantly, but also expanded the observable energy range. In 2015/2016, the cameras of the first four HESS telescopes were renewed and brought up to the state of the art, using the NECTAr chip, which was developed for the next-generation observatory, the Cherenkov Telescope Array CTA. In 2019, the camera in the large fifth telescope was equipped with a CTA prototype camera. using the NECTAr chip, which was developed for the next-generation observatory, the Cherenkov Telescope Array CTA. In 2019, the camera in the large fifth telescope was equipped with a CTA prototype camera. using the NECTAr chip, which was developed for the next-generation observatory, the Cherenkov Telescope Array CTA. In 2019, the camera in the large fifth telescope was equipped with a CTA prototype camera.
Featured image: Artist’s impression of the relativistic matter jet of a gamma-ray burst (GRB) that shoots out of the collapsing star and generates high-energy gamma radiation. Image: DESY, Science Communication Lab
Original publication: Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow; HESS collaboration; “Science”, 2021; DOI: 10.1126/science.abe8560
Provided by DESY