Hebrew U. professor decodes mechanism for gamma-ray bursts from space

Gamma-ray bursts are extremely energetic explosions that have been observed in distant galaxies. They are the brightest explosions in the universe.

August 3, 2019 19:47
2 minute read.
Gamma-ray bursts

Gamma-ray bursts. (photo credit: CREATIVE COMMONS)

A researcher from the Hebrew University of Jerusalem has been able to identify the “Rosetta Stone of gamma-rays bursts.”

Prof. Tsvi Piran, working with a group of other researchers, decoded the mechanism for gamma-ray bursts following an eruption observed last January. Now, in a paper published in Astrophysical Journal Letter, he describes how a stream of particles moving toward us at close to light speed emits the gamma radiation.
Gamma-ray bursts are extremely energetic explosions that have been observed in distant galaxies. They are the brightest explosions in the universe.

According to a release by the Hebrew University, since gamma-rays are blocked by the atmosphere, the bursts were discovered accidentally in the late 1960s by the Vela satellites: defense satellites sent to monitor man-made nuclear explosions in space.

Since their discovery, several dedicated satellites have been launched into space to explore the origins of the bursts. In the late 1990s, it was realized that bursts lasting more than a few seconds arise during the death and collapse of massive stars, while in the first decade of the 21st century it was discovered that bursts lasting less than a few seconds arise in neutron-star mergers.

Still, many mysteries involving these bursts remain.

Particularly puzzling was the question of how the high energy radiation is produced.

Last January, a gamma-ray detector on board NASA’s Neil Gehrls Swift satellite detected GRB190114C, a burst that took place 4.5 billion years ago in a distant galaxy. Following a trigger from Swift, The MAGIC telescopes – the Major Atmospheric Gamma Imaging Cherenkov detectors at the Roque de los Muchachos observatory in La Palma, Spain – focused on the burst’s location and detected extremely high energy photons originating from it at TeV (trillion electronvolt) levels.

The ultra-high energy TeV photons, which were observed about 50 seconds after the prompt emission in the so called “afterglow” phase, were at least 10 times more energetic than the highest energy photons detected previously from any burst.

The preliminary data of the MAGIC observations were published, but Piran and a colleague – Prof. Evgeny Derishev from the Russian Academy of Science’s Institute of Applied Physics in Nizhny Novgorod – took these data a step further, combining them with observations of lower energy (X-ray) photons carried out by the Neil Gehrles Swift. Their work demonstrated the details of the emission mechanism.

The results: The observed radiation must have originated in a jet moving toward us at 0.9999 of the speed of light, Piran and Derishev write.

“The high energy radiation observed by MAGIC was emitted by electrons accelerated to TeV energies within the jet,” according to a release. 

The emission process leads to the so-called inverse Compton mechanism, in which ultra-high energy electrons collide with low-energy photons and boosts their energy. The same relativistic electrons are also producing the low energy “seed” photons via synchrotron radiation, they found.

“MAGIC has found the Rosetta Stone of gamma-ray bursts,” Piran said. “This unique detection enables us for the first time to discriminate between different emission models and discover what are the exact conditions in the explosion. 

“We can also understand now why such radiation wasn’t observed in the past,” he added. 

Piran said the current detection suggests that many other such events will be detected in the future and will continue to shed light on this cosmic mystery.

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