Gamma-ray bursts (GRBs) are among the most explosive and energetic celestial events that have been observed by astronomers. Since its first detection back in the 1960s, scientists have been observing at least hundreds of these events every year. Yet still, discoveries behind these mega explosions in the distant galaxies never fail to fascinate us.
On January 14th, 2019, a GRB (officially named GRB 190114C) signal was initially detected by two observatories in Earth's orbit — the Neil Gehrels Swift Observatory and the Fermi Gamma-ray Space Telescope. Within 22 seconds, telescopes in multiple observatories around the globe started to point toward the origin of the GRB. Among them, was the MAGIC Collaboration, which operates two 17-meter (about 56 feet) diameter Cherenkov telescopes located in La Palma, Spain.
Thanks to the twin telescopes, scientists were able to discern some highly unusual characteristics of GRB 190114C. Its peak energy reached the tera-electronvolts (TeV) range, an extraordinary feat considering where the mega explosion originates - a distance of 4.5 billion light-years away.
In comparison, the gamma-ray emission from Crab Nebula (the strongest steady source in our galaxy, frequently used as a "standard candle" by TeV astrophysicists) did measure up to 100 TeV. However, the nebula is "nearby", only 6,500 light-years from Earth.
While the gamma rays from the GRB 190114C travel towards Earth, they may get absorbed. For this type of flux attenuation (which is stronger), the higher is the energy of the photon and the further is the source. It's due to its interaction with the low energy photons of the extra-galactic background light. The latter is a diffuse, abundant background of infrared, optical, and ultraviolet radiation emitted by all stars and galaxies during the existence of the Universe, which fills in the space. For example, the flux of gamma rays of 1 TeV from GRB 190114C is attenuated by about 300 times.
Therefore, we can only speculate how incredible intense the event of GRB 190114C could have benn if it would happen right in our galatic "backyard".
LabRoots reached out to the members of the MAGIC collaboration for comments on their work on GRB 190114C. Dr. Razmik Mirzoyan, Senior Astrophysicist at the Max-Planck-Institute for Physics (Munich, Germany), and Oscar Blanch, Senior Astrophysicist at the Institute for High Energy Physics (Barcelona, Spain) kindly responded to our request for email interviews.
"We announced our discovery in the night of 14/15th January 2019, in an ATel (the Astronomer's Telegram is an internet-based short notice for disseminating the latest astronomical discovery on the fly) and a GCN (Gamma-ray burst Coordinates Network) circular," said Mirzoyan, who's also the spokesperson of the MAGIC Collaboration.
"In May 2019, my colleagues from the H.E.S.S. collaboration announced a marginal signal from other GRB, where they have measured signal in the range up to 400 GeV," he added.
GRB 190114C stood out because its emission has the highest energy on the record among all gamma-ray bursts. However, it isn't the most energetic GRB. "The energy of the GRB adds up the energy of all photons of different wavelengths. Other GRB observed before would add up to more energy due to the number of photons of lower energy even if they did not emit gamma-ray photons at the energies seen in GRB 190114C," Blanch explained the difference between the terms "most energetics" and "highest energy" in GRB observations.
Through spectral and temporal analyses, MAGIC scientists revealed a new emission component in the afterglow of GRB 190114C. Its power is comparable to that of the synchrotron radiation, which is also known as the brightest light on Earth.
A synchrotron is a type particle accelerator whose magnetic field bends the particle beam and synchronizes the kinetic energy of the particles, producing extraordinary synchrotron light with enormous intensity.
Synchrotron radiation has been observed from astronomical objects such as the pulsar wind surrounding the Crab nebula, which could be as high as 25 giga-electronvolt (GeV).
Before GRB 190114C, the highest energy gamma ray measured from a GRB was a single photon event around 95 GeV. The advent of GRB 190114C changed the astromonical history: within a minute after its first detection, the GRB signal reached the range between 0.2 and 1 TeV, almost 10 folds higher than in intensity than the previous titleholder.
Reports on the detection of gravitational waves (GWs, disturbances in the curvature of spacetime) are also making headlines in space news these days. Scientists have confirmed that some of the past GRBs were caused by the merger of neutron stars, which can also cause ripples in the space-time fabric. However, for GRB 190114C there's zero implication on gravitational waves.
"Concerning gravitational waves, the link between gravitational waves and GRBs was stablished for short GRBs. There are two different kinds of GRB: short and long. They are believed to be originated by a different process. The GRB 190114c was a long GRB and thought to be originated by the collapse of massive stars, which is not one of the phenomena for which we are currently able to detect gravitational waves," said Blanch, who's also the Outreach Coordinator for the MAGIC group.
Besides, even if there was a GW signal that came out of GRB 190114C, we still wouldn't have sensed it, solely for the reason of distance.
"Today's GW arrays have a reach of only about 120 parsecs (391 light-years). On the other hand, GRB 190114C belongs to the long burst type (likely driven by very massive stars), and it happened at a distance of 4.5 Giga light-years," said Mirzoyan.
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The unique advantages of the twin MAGIC telescopes put them in a better position in detecting intense GRBs compared to other observatories.
"The MAGICs are pointing telescopes. They only see a small fraction of the sky. If we want to observe a particular position in the sky, for instance that of the GRB, we first have to move the telescopes so that they point there. It was a design to be light enough so that each can point to any direction in the sky in less than 25 seconds," said Blanch.
"Since the emission from GRB, even for the long ones, is fading very fast in time (every second counts), this is crucial to be able to catch the emission. It is the only of its type to have been designed like that. Other observatories see a big fraction of the sky and they are able to collect data from the position of the GRB just because is in their FoV (field of view). Those telescopes that are sensitive to very high energy gamma-rays need to collect are not very sensitive to short emissions like those of the GRBs. Hence the MAGIC telescopes are the best suited instruments for GRBs among those detecting very high energy gamma-rays."
In the featured presentation given at the 36th International Cosmic Ray Conference, Dr. Mirzoyan reflected on the challenges astrophysicists have been facing in unraveling one of the most mysterious phenomena in the universe:
"The frequent question that in the past year we asked ourselves is why it took so long, say 15 years, to measure a gamma-ray signal from a GRB at TeV?
The first answer is that these happen not so frequently at relatively low red-shifts (EBL absorption is strong). The second answer is that the probability is higher that these will happen at large zenith angles (i.e. at energies much higher than the threshold of a given ground-based instrument). One needs to correspondingly plan the reaction of instruments to alerts. The third reason is that one needs to observe a GRB at any, even at barely acceptable outdoor lighting conditions, once it is relevant; This further enhances the chance probability. And the last but not least answer is that one needed to show for the first time that in fact, it is possible to measure a (sub)-TeV signal from a GRB and this is just what MAGIC did.
The good thing is that the GRBs will become regular observational targets at (sub)-TeVs and the successful measurements will soon provide wealth of data allowing us to find clues to many puzzling questions about these monstrous explosions."
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