November 30, 2022

The Strange Afterglow of a Gamma-Ray Burst – Unusual Features Challenge Models

Flash in area: An artists view of a gamma-ray burst. Credit: DESY, Science Communication Lab
Utilizing the H.E.S.S. observatory, researchers at GRB 190829A observe unusual features that challenge designs.
Researchers from the H.E.S.S. Collaboration was successful to obtain the intrinsic spectrum of the very-high-energy gamma-ray afterglow emission of a reasonably neighboring gamma-ray burst. Surprisingly, the gamma-ray spectrum looks like that of the much lower-energy X-rays, while the fading emission from both bands was observed to march in parallel over three nights. These amazing findings challenge the existing emission circumstances.
Gamma-ray bursts (GRBs) are brilliant X-ray and gamma-ray flashes observed in the sky, emitted by distant extragalactic sources. It was this afterglow emission that initially validated the extragalactic origin of GRBs. The X-ray afterglow radiation is produced by accelerated electrons losing and connecting energy within the blast wave magnetic field.

GRB afterglows are considered to be an exceptional cosmic laboratory for studying the acceleration of particles in the cosmos, due to the evident simplicity of the underlying physics. This remains in contrast with the prompt phase which is exceptionally complicated. Numerous elements of the afterglow emission are popular in X-rays, but the very-high-energy (VHE, >> 100 GeV) emission– 6 orders of magnitude more energetic than X-rays– has been a missing out on piece of the multi-wavelength puzzle.
In the VHE program, making a detection is especially challenging considering that the remote Universe is not completely transparent to VHE gamma rays due to their absorption in the background light that penetrates the Universe. In current years, significant steps have been made toward the understanding of GRBs at VHEs with two detections, the very first observed 10 hours after the afterglow onset and the second within the very first hour of the afterglow.
Fading burst: The H.E.S.S. sky maps of GRB190829A reveal the fading afterglow emission over the three observa-tion nights (panels A, B, C). Credit: © H.E.S.S. Collaboration
Now, the international group of scientists operating the H.E.S.S. (High Energy Stereoscopic System) range of atmospheric Cherenkov telescopes, reported the detection of a 3rd GRB– at a redshift of just z = 0.0785, a mere 1 billion light-years away. “A gamma-ray burst happening in our cosmic backyard like this one, is an extremely rare thing, and a wonderful opportunity to understand what is going on at the highest energies”– discussed Jim Hinton, director at MPIK.
On 29 August 2019, the Fermi Gamma-Ray Burst Monitor and the Swift Burst Alert Telescope discovered and localized GRB 190829A. Consequently, ground-based observatories consisting of H.E.S.S. relied on look at this position to keep track of the development of this burst over a very vast array of wavelengths. Observations with H.E.S.S. began 4 hours after the burst, when the source ended up being noticeable to its telescopes. Edna Ruiz Velasco, a PhD trainee from MPIK, was among the lead private investigators in the work: “We have actually been able to cover the GRB afterglow from 4 to 56 hours after the preliminary surge and determine its emission extremely accurately.” Dmitry Khangulyan, a H.E.S.S. member from Rikkyo University in Japan, and an MPIK alumnus, added: “This new outcome offers 2 unique and new observational insights about GRB afterglows.”
The precise determination of the spectrum over more than an order of magnitude in energy, from 0.18 to 3.3 TeV, and covering an extended temporal series of a number of days, was enabled by a combination of great instrumental sensitivity and the fortuitous proximity of the GRB. Such accurate measurements over a broad energy variety have permitted the intrinsic VHE spectrum to be dependably penetrated for the first time. As Carlo Romoli, a post-doctoral scientist at MPIK noted, “these new outcomes have exposed curious resemblances in between the X-ray and VHE gamma-ray emission.”
Such a strong connection, nevertheless, is unanticipated in basic GRB theory, which forecasts a different origin for the VHE part. “The far-reaching ramification of this possibility highlights the requirement for further studies of VHE GRB afterglow emission,” pointed out Felix Aharonian, external clinical member of MPIK and at DIAS in Ireland.
The abundance of GRB afterglow detections over the last few years indicates that routine detections in the VHE band will end up being rather common. A high bar has now been set by this H.E.S.S. result, which has actually highlighted the clinical significance of the detection at VHE of local GRBs, especially at late times in the afterglow.
For more on this research study, read Best View Yet of Exceptional Cosmic Explosion– Challenges Established Theory of Gamma-Ray Bursts in the Universe.
Recommendation: “Revealing x-ray and gamma ray spectral and temporal similarities in the GRB 190829A afterglow” by H.E.S.S. collaboration, 3 June 2021, Science.DOI: 10.1126/ science.abe8560.

It was this afterglow emission that initially verified the extragalactic origin of GRBs. In recent years, significant actions have actually been made toward the understanding of GRBs at VHEs with two detections, the very first observed 10 hours after the afterglow start and the second within the very first hour of the afterglow. “The significant implication of this possibility highlights the requirement for more studies of VHE GRB afterglow emission,” discussed Felix Aharonian, external clinical member of MPIK and at DIAS in Ireland.
The abundance of GRB afterglow detections over the last couple of years indicates that regular detections in the VHE band will become rather typical. A high bar has actually now been set by this H.E.S.S. outcome, which has highlighted the clinical significance of the detection at VHE of regional GRBs, particularly at late times in the afterglow.

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