November 21, 2019
Rikkyo University
Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU)
Extremely energetic cosmic explosions generate gamma-ray bursts (GRB), 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 prompt 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.
A signature has now been detected with the large H.E.S.S. 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 very-high-energies 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 H.E.S.S. collaboration et al., in the journal Nature on November 20.
H.E.S.S. is an international collaboration involving scientists from 13 countries, including Kavli IPMU Principal Investigator Tadayuki Takahashi and Rikkyo Univeristy Professor Yasunobu Uchiyama.
The very-high-energy 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 H.E.S.S. observation, it had been assumed that such bursts likely are observable only within the first seconds and minutes at these extreme energies, and not many hours after the explosion.
At the time of the H.E.S.S. measurements the X-ray afterglow had already decayed very 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 H.E.S.S results strongly 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 H.E.S.S. spectrum, and the energy range of the emission at such late times presents a challenge to both emission scenarios." - says H.E.S.S. 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 payed 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".
Paper details
Journal: Nature
Title: A new very-high-energy component deep in the Gamma-ray Burst afterglow
Authors: H.E.S.S. Collaboration
(corresponding authors: Edna Ruiz Velasco, Quentin Piel, Robert Daniel Parsons, Elisabetta Bissaldi, Clemens Hoischen, Andrew Taylor, Felix Aharonian, Dmitry Khangulyan)
Media contact
Motoko Kakubayashi
Press officer
Kavli Institute for the Physics and Mathematics of the Universe
University of Tokyo
TEL:+81-04-7136-5980
E-mail:press_at_ipmu.jp
* please change _at_ to @