Gamma-ray bursts with a high radiant power

Scientists are observing two cosmic bursts with an incomparably high energy output

Gamma-ray bursts are the most intense explosions in the universe. Now, astronomers are reporting in Nature magazine that they have observed two such bursts. The two MAGIC telescopes on La Palma have recorded the strongest burst of this kind to date with GRB 190114x. And the 28-meter telescope from the H.E.S.S. array in Namibia - the largest telescope of its kind in the world - reports the observation of the GRB 180720B burst, which is also very high-energy. With the measurements, the astrophysicists succeeded in proving that such cosmic flashes at the highest energy range do actually glow, something that had only been assumed for a long time.

Gamma-ray bursts are fleeting, lasting from just a few seconds to a maximum of a few minutes. They are probably caused by supernovae - the bursting of massive stars - and the melting of neutron stars or black holes. The gamma-ray bursts of the direct outbreak itself usually produce several thousand to a million times more high-energy than visible light and can only be observed from satellites.

Space observatories have however already been able to prove the existence of individual photons that have even more energy. To date, however, up to what energy level the bursts emit radiation and whether this also includes the very high-energy gamma rays, has remained a mystery.

In the case of the two gamma-ray bursts, GRB 190114c and GRB 180720B, the MAGIC and H.E.S.S. telescopes have also not registered the outbreak itself, but its afterglow. From their observations, scientists conclude that in this afterglow, different physical mechanisms are at work than have been assumed until now.

With GRB190114c, which occurred in January 2019, the MAGIC research network proved that such cosmic bursts glow for several minutes after the outbreak in the maximum energy range. For around 30 seconds, its afterglow was more than 100 times stronger than the Crab Nebula, the brightest known gamma source in our Milky Way. After that, the signal diminished relatively quickly. After just 30 minutes, MAGIC could no longer measure any emissions.

Initially, the energy monster was sighted by the Swift-BAT and Fermi satellites. After 22 seconds, the two scouts of the US space agency NASA alerted different telescopes, including the MAGIC Duo.

“Targeting gamma bursts from Earth is a difficult task”, says Razmik Mirzoyan, a scientist at the Max Planck Institute for Physics and spokesman for the MAGIC research network. “These bursts can light up somewhere in the sky at any time, and disappear again quickly. That’s why the MAGIC telescopes feature a fully automated system in order to process satellite signals”.

In fact the telescopes can quickly be brought into position with a powerful motor. “Despite their weight of 64 tonnes each, the telescopes can swivel towards new heavenly targets in an extremely short time. With the current gamma-ray burst, it was only 27 seconds after the first alert was issued”, says Mirzoyan.

Just a short time later, two dozen other instruments in total then began following events in the sky, including satellites such as NuSTAR, XMM-Newton, Hubble space telescope and several ground-based observatories. As a result, the researchers now have a detailed picture of this gamma outbreak. Using optical telescopes, they were able to estimate the distance of GRB190114c. The burst occurred in a galaxy situated around five billion light years away.

The results of the large H.E.S.S. telescope also prove that gamma-ray bursts glow in the highest energy range. In July 2018, GRB 180720B already entered the field of vision of the H.E.S.S. observatory stationed in Namibia about ten hours after the outbreak. While analysing the data, which was recorded in the two hours that followed, a new, punctiform gamma ray source appeared at the site of the explosion; 18 days later, it had disappeared.

GRB 180720B was very strong, and lasted around 50 seconds - a relatively long time, which indicates the death of a massive star. During the process, its core area collapses to form a rapidly rotating black hole. In the accretion disc surrounding this massive monster, the surrounding gas heats up extremely strongly. Gas jets emitted perpendicular to the disc plane ultimately produce gamma-ray bursts.

The maximum energy gamma-ray burst now discovered with H.E.S.S. not only proves the existence of extremely accelerated particles, but also shows that these particles still exist, or are even produced in the first place, many hours after the explosion. Here, the shockwave emitted by the explosion very likely acts as the cosmic accelerator. Prior to the H.E.S.S. measurement of GRB 180720B, it was assumed that such outbreaks can probably only be observed during the first seconds and minutes.

However, half a century after the first observation of a gamma-ray burst, it remains unclear what physical processes play out during the process. The fact that these bursts emit high-energy light particles, photons, had been predicted by several models; however, until now, it had not been possible to prove that this was the case.

However, one problem is the high energy in the afterglow of GRB190114c. The MAGIC telescopes registered a gamma radiation measuring one teraelectron volt (TeV) - the highest-energy photons ever measured from a gamma-ray burst. Another astonishing fact is that in the afterglow of GRB 180720B, the radiation intensity and the spectral form correlate in both the X-ray and in the maximum energy gamma range for hours on end. All this means that theoreticians are scrambling to find an explanation.

Gamma-ray bursts radiate in different energy ranges. According to the scientists, the lower-energy emissions are due to the synchrotron radiation. This occurs when charged particles move very quickly in a magnetic field. However, for the high-energy record radiation observed, this mechanism is highly implausible. Therefore, there must be another motor.

One possibility for such a mechanism would be the inverse Compton effect. In this process, high-energy electrons transfer their energy to photons. In this way, the light particles actually reach levels in the TeV range. However, with this explanation approach, it could be expected that the intensity of the gamma rays decreases more quickly than the decline that was measured.

Whatever the case, the two observations described certainly provide deeper insights in to the nature of gamma-ray outbreaks. “For over a decade, Cherenkov telescopes such as H.E.S.S. and MAGIC have searched for maximum energy gamma radiation from such bursts and have continuously improved observation strategies”, says Jim Hinton, Director at the Max Planck Institute for Nuclear Physics.

The fact that now, two bursts have been detected at the same time with maximum energy levels, and that the outbreaks also glow with very high energies for several hours and even days afterwards, opens up entirely new perspectives for the follow-up CTA (Cherenkov Telescope Array) instrument. “With this instrument, we will be able to study these extreme events in much greater detail”, explains Hinton.


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