from the Be-scared-of-GRBs dept.
Scientists have gained the best view yet of the brightest explosions in the universe: A specialized 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 (H.E.S.S.) 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 international team reports in the journal Science.
"Gamma-ray bursts are bright X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources," explains DESY scientist Sylvia Zhu, 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."
On August 29, 2019, the satellites Fermi and Swift detected a gamma-ray burst in the constellation of Eridanus. The event, cataloged 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 20 billion lightyears away. "We were really sitting in the front row when this gamma-ray burst happened," explains co-author Andrew Taylor from DESY. The team caught the explosion's afterglow immediately when it became visible to the H.E.S.S. telescopes. "We could observe the afterglow for several days and to unprecedented gamma-ray energies," reports Taylor.
The comparatively short distance to this gamma-ray burst allowed detailed measurements of the afterglow's spectrum, which is the distribution of "colors" 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 in Heidelberg. "This is what's 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."
H.E.S.S. Collaboration, H. Abdalla, F. Aharonian, et al. Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow [$], Science (DOI: 10.1126/science.abe8560)