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New discovery on gamma rays

22 November, 2019

The decisive contribution of researchers from the Universities of Trieste and Udine, the Trieste section of the INFN, the Trieste Astronomical Observatory of the INAF.

On January 14, 2019, for the first time ever, high-energy photons emitted by a flash of gamma rays, or GRB, were revealed. These photons, which have reached energies of the order of teraelettronvolt, that is thousands of billions of times higher than those of visible light, were captured by the MAGIC telescopes on the Canary Islands. The Italian scientific contribution was fundamental for the discovery, also in this case. In the international team, researchers from the Universities of Trieste and Udine, researchers from the Trieste section of the INFN, the Trieste Astronomical Observatory and involved in the GRB project of the Institute for Fundamental Physics of the Universe (IFPU) played a leading role.

A new light on the phenomenon of GRBs

Gamma-ray burst (GRB) are short but powerful cosmic explosions that suddenly appear in the sky, about once a day. The flashes of gamma rays are characterized by a very bright initial flash in gamma rays that has a typical duration ranging from fractions of a second to a few hundred seconds. This initial flash is followed by the so-called afterglow, an emission of light observable at all wavelengths that is dimming over time. The study of afterglow was done at all wavelengths using instrumentation from both ground and space. Today we know that GRBs are the result of the explosion of very massive stars or the fusion of neutron stars in distant galaxies. Although we have identified the origin of these phenomena still much is the mystery that surrounds the phenomenon itself and the physics that characterizes it.

In particular, observations with high-energy gamma-ray telescopes are fundamental to solve these still mysterious aspects because they allow us to observe directly the heart of the phenomenon. Exactly what happened with the observations of the GRB190114C made by MAGIC telescopes in the Canary Islands that for the first time revealed the emission of photons from this source to the energy of the teraelettronvolt.

"The event observed by various space telescopes, including AGILE and Fermi, was called GRB 190114C and its coordinates, which identified its position in the sky, were distributed via the internet to astronomers around the world in 22 seconds from the detection of the explosion," says Prof. Francesco Longo, of the University and INFN of Trieste and a member of the collaborations AGILE, Fermi and MAGIC.

To receive the alert there were two MAGIC telescopes, Cherenkov light telescopes, each with a mirror diameter of 17 m, located on the island of La Palma, in the Canary Islands (Spain). MAGIC telescopes are designed to respond quickly to GRB alerts and also have a dedicated follow-up strategy. In the case of the GRB 190114C, MAGIC was able to start the observation about 30 seconds after the arrival of the GRB alert from the satellites, i.e. about 50 seconds after the detection of the phenomenon.

After pointing in the direction of the GRB 190114C, the MAGIC telescopes captured for the first time the highest energy photons ever measured for this type of celestial event. An unprecedented result, which provides new information essential for understanding the physical processes taking place in GRBs. "The photons revealed by MAGIC must in fact originate from a different process than the radiation commonly observed in the afterglow of GRBs, distinct from the physical process responsible for the emission of GRBs at the lowest energies" - says Davide Miceli, PhD student at the University of Udine, in the group of astroparticle physics coordinated by Prof. Barbara De Lotto.

Although the emission up to the energies of TeV in GRBs had been provided for in some theoretical studies, it had remained unnoticed until now despite the numerous researches carried out in recent decades with various instruments that work on these energies, including MAGIC.

The high-energy photons were observed by MAGIC until half an hour after the explosion of the GRB so, thanks to both the intensity of the signal received and the procedure for analyzing data in real time available at the observatory, it was possible to communicate within a few hours of observation to the international astronomical community the discovery of the first unequivocal detection of high-energy photons from a GRB. This communication highlighted the importance of this astronomical event and gave rise to an extensive campaign of follow-up observations at all wavelengths of the GRB 190114C by more than two dozen observers or instruments from the radio band to TeV energies.

All the observations made offer a very complete multifrequency overview for this event and provide unequivocal evidence that the high energy emission observed by MAGIC originates from a further, distinct emission process in the afterglow so far never observed: "From our analysis, the favorite candidate for explaining the very high energy emission is the so-called reverse Compton process in which the photons gain a considerable amount of energy in the collisions with even higher energy electrons that have been accelerated by the collision with the material surrounding the GRB" says Dr. Lara Nava of the INAF of Brera and associated with the INFN and the INAF of Trieste, which continues: "Vice versa, the low-energy photons observed in afterglows originate from the so-called synchrotron process in which the observed photons are generated by the interaction between electrons and magnetic fields".

MAGIC has opened a new window to study GRBs. These results indicate that we are certainly able to detect many more GRBs to TeV energies with both MAGIC and the new generation Cherenkov instruments.

The direct news for the MAGIC experiment:

https://magic.mpp.mpg.de/index.php?id=252

 

More information:

Francesco Longo: francesco.longo@ts.infn.it

Department of Physics - University of Trieste

Davide Miceli: davide.miceli@ts.infn.it

Lara Nava: lara.nava@inaf.it