A new Era in Gamma-Ray Astronomy: discovery of very high energy radiation from explosions connected to newly born stellar-mass black holes

On Jan. 14 2019 for the first time very high energy photons have been detected from a Gamma-Ray Burst (GRB), that is a violent explosion triggered by the formation of compact objects such as black holes. This radiation has been recorded by the MAGIC telescopes, located on La Palma in the Canary Islands. The radiation lasted half hour and reached energies up to a trillion times the energy of the visible light. This discovery opened a new investigation window on the mysterious sources known as GRB. An international team of 300 researchers led by Dr. Lara Nava, INAF researcher at the Observatories of Brera-Milan and Trieste, compared the data at different frequencies (radio waves, visible lights, X-rays and gamma-rays) with theoretical predictions and identified the physical process at the origin of the observed radiation. Fundamental was also the contribution of several researchers from INFN and Università di Trieste and Udine, and affiliated to the Institute for Fundamental Physics of the Universe (IFPU, Trieste). 

GRBs originate both from the collapse of massive stars and from the merger of neutron stars in distant galaxies, see Figure 1, the latter one being the same phenomenon originating gravitational waves. These explosions are picked up regularly almost once a day by satellites orbiting the Earth. After an initial, very bright flash of gamma-rays, with typical duration of several seconds, an afterglow radiation is detected, i.e. a long lasting emission observed at many different frequencies, from radio to gamma-rays, that fades with time and eventually disappears. This afterglow radiation is commonly observed both from Earth with ground telescopes and from the space, with satellites. Its origin is identified with synchrotron emission, a type of radiation that very energetic relativistic electrons produce when they are in presence of intense magnetic fields.

The GRB called 190114C has been detected in January 2019 by several space telescopes, including AGILE, Fermi and Swift, that in only 22 seconds calculated and communicated the position of the GRB to astronomers all around the world. The alert has been received also by the two MAGIC telescopes, that are 17-meter diameter Cherenkov telescopes (see Figure 2) located at La Palma (Canary Island, Spain). The two MAGIC telescopes, designed to promptly respond to alerts reporting the detection of a GRB, have been able to move and point the GRB in only 30 seconds, that is about 50 seconds after the beginning of the explosion.

Unbelievable as it may sound, all this procedure is quite standard and occurs several times per year. The peculiarity of this event was clear only after MAGIC pointed the GRB: while in similar occasions no signal was picked up by MAGIC, this time the telescopes have been illuminated by a huge amount of radiation, unequivocally produced by photons with very high energies, never detected before from this kind of astrophysical sources.

MAGIC promptly communicated the discovery to the whole astrophysics community. Given the relevance and uniqueness of the event, all major radio and optical telescopes on Earth and also satellites such as the Hubble Space Telescope and XMM-Newton pointed the GRB and started the data taking. Later on, about 300 researchers from all around the world shared their data, with the aim of putting together all available clues to understand the origin of the very high energy radiation detected for the first time by MAGIC. The study, led by the INAF researcher Dr. Lara Nava, from the Observatory of Brera-Milano and Trieste and affiliated to the INFN and IFPU of Trieste, has been published on the prestigious journal Nature on November 20th, 2019 . The study shows that the best candidate to explain the very high energy emission is the process called Inverse Compton: before leaving the region where they have been produced and travelling towards the Earth, photons produced by the synchrotron process collide with an electron, from which they receive a boost in energy, reaching then the huge energies with which MAGIC detected them. The two processes, the synchrotron and Inverse Compton, are then related to each other and influence each other. However, so far researcher have observed and accessed only one of them, missing therefore half of the information.

The new emission component identified by MAGIC, see Figure 3, brings along a wealth of additional information, as a new piece of the puzzle. The amount of radiation produced by the Inverse Compton process, in comparison with the amount produced as synchrotron light allows indeed the determination of: the energy of electrons responsible for the overall emission, the strength of the magnetic field, and the particle and photon density in the region where the radiation was produced. All this information has important repercussions on the study of how matter is emitted during the formation of stellar-mass black holes and on the environments where these phenomena take place.

A new generation of Cherenkov telescopes is already under construction in the two Observatories of La Palma and Paranal (Chile). The improved performances of this new array of telescopes, called the Cherenkov Telescope Array, as compared to current facilities will allow the observation of very high energy radiation for a much higher number of GRBs, hopefully up to several GRB per year . This will guarantee a rich collection of data covering the properties of GRB at very high energies, that will contribute to unveil the mystery that still surrounds these cosmic sources.


More info:

1)  Press release Trieste/Udine , 2)  The news on MediaINAF , 3)  The INAF- INFN-ASI joint press release , 4)  The NASA press release , 5)  The MAGIC press release  



Figura 1. Artist’s impression of a Gamma-Ray Burst: the collapse of the core of a massive start (in the center) results in the formation of a black hole and in a violent ejection of matter and radiation, ejected in the form of two opposite jets. This explosive phenomenon is detected on Earth as temporary emission of radiation covering the full electromagnetic spectrum, and is known as GRB. Credit: NASA.






Figura 2. A picture of the two MAGIC telescopes, located at the Observatory of Roque de los Muchachos at La Palma (Canary Islands), at an altitude of 2200 meters. Credit: Chiara Righi





Figura 3. Electromagnetic spectrum of GRB 190114C: putting together the data registered by different instruments that simultaneously observed GRB 190114C and comparing those data with theoretical models (blue curve), researchers from the MAGIC collaboration understood that the radiation they received from this source at energies above 10 11 electronVolt (eV, yellow circles) is radiation produced by the Inverse Compton mechanism. At lower energies (below 10 9 eV) the model (blue curve) used to explain the data (green and red circles and crosses) is the synchrotron model