"Gamma-ray bursts are flashes of light composed of x-rays and gamma rays observed in the sky, emitted by distant extragalactic sources", explained Edna Ruiz Velasco from the MPIK in Heidelberg, Germany, one of the co-authors of the article. "These are the most violent explosions in the universe, and those lasting more than a few seconds are probably associated with the collapse of a massive star into a black hole". Emissions from gamma-ray bursts are divided into two separate phases: an initial : chaotic phase lasting a few tens of seconds, followed by a long afterglow phase which fades slowly.
Exceptional observation of one of the closest gamma-ray bursts ever recorded
On 29 August 2019, the Fermi and Swift satellites detected a gamma-ray burst in the Eridanus constellation. The event, named GRB 190829A, after the date it appeared, was approximately one billion light years away, making it one of closest gamma-ray bursts ever observed (the usual distance is around twenty billion light years). The team picked up the afterglow of the explosion as soon as it became visible to the H.E.S.S. (High Energy Stereoscopic System) telescopes in Namibia.
"The signal was strong enough to be detected immediately, which meant we could inform the international community quickly", pointed out Fabian Schüssler from CEA-Irfu who is coordinating the team responsible for analysing this gamma-ray burst.
The relative closeness of this gamma-ray burst meant that the spectrum of the afterglow, i.e. the distribution of energy from the photons in the radiation, could be measured in detail. The energy spectrum of GRB 190829A was discovered to be up to 3.3 tera–electron volts, which is around a trillion times the energy of the photons in visible light. This is the highest energy ever recorded for a gamma-ray burst.
As GRB 190829A occurred in our cosmic 'back yard', its very high-energy photons were not absorbed by collisions with the background light on their way towards earth, as happens over longer distances.
An observation which calls into question the theory on gamma-ray bursts in the universe
"The H.E.S.S. was able to follow the afterglow for three days after the initial explosion" explains Fabian Schüssler.
The result was surprising: the observations revealed curious similarities between the emissions of X-rays and very high-energy gamma rays in the explosion's afterglow. Contrary to established theories which assume that the two components of the emission must be produced by separate mechanisms, observations of the afterglow of GRB 190829A show that the X-rays and gamma rays decreased in a synchronised way.
In addition, the gamma-ray energy spectrum clearly corresponds to an extrapolation of the X-ray spectrum. These results show that the X rays and the very high-energy gamma rays in this afterglow were produced by the same mechanism and at the same time.
@DESY - Science Communication Lab
"The significance of this possibility underlines the need for further studies on the afterglow emission of very high-energy gamma-ray bursts", indicated Mathieu de Naurois, deputy director of the H.E.S.S. observatory and researcher at LLR, France.
GRB 190829A is only the fourth gamma-ray burst ever to be detected from the ground. Previously detected explosions have occurred much further away in the cosmos and their afterglows could only be observed for a few hours and only at energies of 1 tera–electron volt or less.
@DESY - Science Communication Lab
Prospects for detecting gamma-ray bursts using next-generation instruments look promising: scientists are expecting that regular detections in the very high-energy band will become common with improved instruments, which will help to fully understand these gargantuan cosmic explosions.