Gamma-Ray Bursts With Record Energy Detected

Gamma-ray bursts can be triggered by the explosion of a super massive star, collapsing into a black hole. From the vicinity of the black hole, powerful jets shoot in opposite directions into space, accelerating electrically charged particles, which in turn interact with magnetic fields and radiation to produce gamma rays. (Image: DESY, Science Communication Lab)
Gamma-ray bursts can be triggered by the explosion of a super massive star, collapsing into a black hole. From the vicinity of the black hole, powerful jets shoot in opposite directions into space, accelerating electrically charged particles, which in turn interact with magnetic fields and radiation to produce gamma rays. (Image: DESY, Science Communication Lab)

The strongest explosions in the universe produce even more energetic radiation than previously known: Using specialized telescopes, two international teams have registered the highest energy gamma rays ever measured from so-called gamma-ray bursts, reaching about 100 billion times as much energy as visible light.

The scientists of the H.E.S.S. and MAGIC telescopes present their observations in independent publications in the journal Nature. These are the first detections of gamma-ray bursts with ground-based gamma-ray telescopes. DESY plays a major role in both observatories, which are operated under the leadership of the Max Planck Society.

Gamma-ray bursts (GRB) are sudden, short bursts of gamma radiation happening about once a day somewhere in the visible universe. According to current knowledge, they originate from colliding neutron stars or from supernova explosions of giant suns collapsing into a black hole. David Berge, head of gamma-ray astronomy at DESY, explained:

 

The cosmic phenomenon was discovered by chance at the end of the 1960s by satellites used to monitor compliance with the nuclear test ban on Earth.

Since then, astronomers have been studying gamma-ray bursts with satellites, as Earth’s atmosphere very effectively absorbs gamma rays. Astronomers have developed specialized telescopes that can observe a faint blue glow called Cherenkov light that cosmic gamma rays induce in the atmosphere, but these instruments are only sensitive to gamma rays with very high energies.

Unfortunately, the brightness of gamma-ray bursts falls steeply with increasing energy. Cherenkov telescopes have identified many sources of cosmic gamma rays at very high energies, but no gamma-ray bursts. Satellites, on the other hand, have detectors that are much too small to be sensitive to the low brightness of gamma-ray bursts at very high energies. So it was effectively unknown if the monster explosions emit gamma rays also in the very-high-energy regime.

Scientists have tried for many years, to catch a gamma-ray burst with Cherenkov telescopes. Then suddenly, between summer 2018 and January 2019, two international teams of astronomers, both involving DESY scientists, detected gamma rays from two GRB events for the first time from the ground.

On 20 July 2018, the faint afterglow emission of GRB 180720B in the gamma-ray regime was observed with the 28-meter telescope of the High-Energy Stereoscopic System H.E.S.S. in Namibia. On 14 January 2019, a bright early emission from GRB 190114C was detected by the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes on La Palma, and they were immediately announced to the astronomical community.

Both observations were triggered by gamma-ray satellites of the U.S. space agency NASA that monitor the sky for gamma-ray bursts and send automatic alerts to other gamma-ray observatories upon detection. Cosimo Nigro, from the MAGIC group at DESY, who was in charge of the observation shift at that time, said:

MAGIC registered gamma-rays with energies between 200 and 1000 billion electron volts (0.2 to 1 teraelectronvolts). Elisa Bernardini, leader of the MAGIC group at DESY, said:

For comparison, visible light is in the range of about 1 to 3 electron volts. The rapid discovery allowed to quickly alert the entire observational community. As a result, more than 20 different telescopes had a deeper look at the target.

This allowed pinpointing the details of the physical mechanism responsible for the highest-energy emission, as described in the second paper led by the MAGIC collaboration. Follow-up observations placed GRB 190114C at a distance of more than 4 billion light-years. This means, its light traveled more than 4 billion years to reach us, or about a third of the current age of the universe.

GRB 180720B, at a distance of 6 billion light-years, even further away, could still be detected in gamma rays at energies between 100 and 440 billion electron volts long after the initial blast. Stefan Ohm, head of the H.E.S.S. group at DESY, said:

DESY theorist Andrew Taylor, who contributed to the H.E.S.S. analysis, added:

The detection of gamma-ray bursts at very high energies provides important new insights into the gigantic explosions. DESY researcher Konstancja Satalecka, one of the scientists coordinating GRB searches in the MAGIC collaboration, said:

To explain how the observed very-high-energy gamma rays are generated is challenging. Both groups assume a 2-stage process: First, fast electrically charged particles from the explosion cloud are deflected in the strong magnetic fields and emit so-called synchrotron radiation, which is of the same nature as the radiation that can be produced in synchrotrons or other particle accelerators on Earth, for example at DESY.

Cherenkov telescopes detect the bluish Cherenkov light generated by faster-than-light particles in Earth's atmosphere, produced by cosmic gamma rays. (Image: DESY, Science Communication Lab)

Cherenkov telescopes detect the bluish Cherenkov light generated by faster-than-light particles in Earth’s atmosphere produced by cosmic gamma rays. (Image: DESY, Science Communication Lab)

However, only under fairly extreme conditions would the synchrotron photons from the explosion be able to reach the very high energies observed. Instead, the scientists consider there is a second step, where the synchrotron photons collide with the fast particles that generated them, which boosts them to the very high gamma-ray energies recorded. The scientists call the latter step inverse Compton scattering. Berge concluded:

The scientists estimate that up to 10 such events per year can be observed with the planned Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory. The CTA will consist of more than 100 individual telescopes of three types that will be built at two locations in the northern and southern hemispheres.

DESY is responsible for the construction of the medium-sized telescopes and will host CTA’s Science Data Management Centre on its campus in Zeuthen. CTA observations are expected to start in 2023 at the earliest.

Provided by: Deutsches Elektronen-Synchrotron DESY [Note: Materials may be edited for content and length.]

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