Astronomers Detect High-Energy Gamma Rays from Afterglow of Long Gamma-Ray Burst

Long gamma-ray bursts are bright flashes of extragalactic gamma rays produced during the collapse of a massive star. A gamma-ray burst itself lasts only a few seconds but is followed by an afterglow that can persist for hours or days. Astronomers using the High Energy Stereoscopic System (H.E.S.S.), an array of five gamma-ray telescopes located in Namibia, have now observed the afterglow of GRB 190829A, a nearby long gamma-ray burst.

An artist’s impression of a relativistic jet of a gamma-ray burst, breaking out of a collapsing star, and emitting very-high-energy photons. Image credit: Science Communication Lab, DESY.

An artist’s impression of a relativistic jet of a gamma-ray burst, breaking out of a collapsing star, and emitting very-high-energy photons. Image credit: Science Communication Lab, DESY.

“Gamma-ray bursts (GRBs) are the biggest explosions in the Universe and associated with the collapse of a rapidly rotating massive star to a black hole,” said Dr. Sylvia Zhu, an astrophysicist at the Deutsches Elektronen-Synchrotron (DESY).

“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.”

Using the H.E.S.S. system, Dr. Zhu and her colleagues from the H.E.S.S. Collaboration observed the afterglow of GRB 190829A on three consecutive nights.

They detected emission at energies of several tera-electronvolts from 4.3 hours to 55.9 hours after the gamma-ray burst began, and determined the intrinsic high-energy spectrum.

These emissions had similar spectral properties and decay profiles to the X-ray emission coming from GRB 190829A.

“Our observations revealed curious similarities between the X-ray and very-high energy gamma-ray emission of the burst’s afterglow,” Dr. Zhu said.

The astrophysicists argue that this spectral behavior doesn’t match current emission models for gamma-ray burst afterglows, but speculate that a faster jet or different energy distributions of emitted particles could explain the discrepancy.

“It is rather unexpected to observe such remarkably similar spectral and temporal characteristics in the X-ray and very-high energy gamma-ray energy bands, if the emission in the two energy ranges had different origins,” said Dr. Dmitry Khangulyan, an astrophysicist at Rikkyo University.

“This poses a challenge for the synchrotron self-Compton origin of the very-high energy gamma-ray emission.”

“Looking to the future, the prospects for the detection of gamma-ray bursts by next-generation instruments like the Cherenkov Telescope Array that is currently being built in the Chilean Andes and on the Canary Island of La Palma look promising,” said H.E.S.S. spokesperson Dr. Stefan Wagner, from Landessternwarte Heidelberg.

“The general abundance of gamma-ray bursts leads us to expect that regular detections in the very-high energy band will become rather common, helping us to fully understand their physics.”

The study was published in the June 4, 2021 edition of the journal Science.

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H. Abdalla et al. (H.E.S.S. Collaboration). 2021. Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow. Science 372 (6546): 1081-1085; doi: 10.1126/science.abe8560

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