On the night of January 14, 2019, astronomer Razmik Mirzoyan got a call at his home in Germany. The observers on shift at the Major Atmospheric Gamma Imaging Cherenkov Telescope (MAGIC) in the Canary Islands were on the other line. Alerted by two space telescopes—the Neil Gehrels Swift Observatory and Fermi Gamma-ray Space Telescope—the two MAGIC telescopes were pointed in the direction of emissions from an immensely powerful cosmic outburst that were arriving at Earth. Within the first 20 minutes of observation, the telescopes detected a strong and increasing signal that seemed to be from a gamma ray burst, the most energetic type of explosion known to occur in the universe.
Mirzoyan told the observers to keep measuring.
That night Mirzoyan, who is a researcher at the Max Planck Institute for Physics in Munich, dashed off a short note on the Astronomer's Telegram, hoping other telescope operators would turn their machines toward the signal. He described how the MAGIC telescopes saw the highest energy emissions ever measured from a gamma ray burst (GRB), with photon energies of up to 1,000 billion electronvolts, or 1 teraelectronvolt (TeV). These were also the first observations of a gamma ray burst (GRB) by MAGIC or any other ground-based telescope.
Without any sleep, Mirzoyan headed to Arizona the next day to celebrate the inauguration of a next-generation gamma ray telescope at Whipple Observatory. By the time he arrived, word had spread about the detection. Everyone in the room was eager to shake Mirzoyan's hand and congratulate the MAGIC team, says Jamie Holder, an astronomer from the University of Delaware who was there. "Almost every conversation I had that week centered around the discovery," he says. "What have they seen? What does it mean? Can we see it, too?"
A few months later, another group of scientists went through their archived observations and found that they, too, detected GRB emissions from the ground. In July 2018, the High Energy Stereoscopic System (HESS) array of telescopes in Namibia detected the faint afterglow emission of another GRB 10 hours after the initial explosion. Even after nearly half a day, the afterglow still had photons with energies of 100 to 440 gigaelectronvolts. Both teams published their results in separate papers the journal Nature today.
"These ground-based telescopes have been operating for more than a decade, and GRBs have been one of their main targets, and this is the first time they actually detected them," says astrophysicist Bing Zhang of University of Nevada, Las Vegas, who was not involved in the research but wrote an editorial about the new papers for Nature.
Gamma rays are the highest-energy form of radiation, with wavelengths that can be smaller than the nucleus of an atom. (Radio waves, for comparison, have wavelengths ranging between about a millimeter to hundreds of kilometers.) Gamma ray bursts are phenomena that occur in distant galaxies, and astronomers believe the violent outbursts can happen when a massive star dies and collapses in on itself, resulting in a supernova. In one second, a GRB can release as much energy as the sun will produce in its lifetime. The light arrives at Earth as a prompt "flash" of gamma rays. This flash is associated with the highly energetic jets of plasma that form as the core of a dying star becomes a black hole or a neutron star, Holder says, and the afterglow that follows comes from the shock waves as this jet plows into in the surrounding region.
Compared to space-based telescopes, which have been observing GRBs for years, ground-based telescopes have much larger surfaces for detection, but they have the disadvantage of being beneath Earth's atmosphere, which absorbs gamma radiation. Until now, detecting a GRB from Earth's surface has proven elusive.
"Now we know that it is possible to observe GRBs from the ground, to high energies, long after the burst occurred," says Holder. "This will allow us to tune our search strategies to discover more bursts, and to study them as a population."
Both of the GRBs that were observed are believed to be the result of supernovas. The burst seen by MAGIC, called GRB 190114C, came from about 4.5 billion light-years away, and the one seen by HESS, named GRB 180720B, came from 6 billion light-years away.
The observations show that GRBs produce even more energetic emissions than previously known. Konstancja Satalecka, a scientist at the German Electron Synchrotron (DESY) who was part of the MAGIC collaboration, said in a statement that researchers were missing about half of the energy budget of GRBs until now. "Our measurements show that the energy released in very-high-energy gamma-rays is comparable to the amount radiated at all lower energies taken together," she said. "That is remarkable!”
Now scientists also know that GRBs are able to accelerate particles within the explosion ejecta. After ruling out other theoretical explanations, both teams of scientists have suggested that the very-high-energy gamma ray photons had been scattered by electrons while traveling through space, boosting their energy in a process known as inverse Compton scattering.
"These results are very exciting," Dan Hooper, head of the Theoretical Astrophysics Group at the Fermi National Accelerator Laboratory, says in an email. "Astrophysicists have long expected gamma-ray bursts to emit photons in this energy range (the teraelectronvolt range), but until now this had never been observed." Hooper was also surprised by how high-energy emissions were able to persist in the long afterglow of GRB 180720B. "Considering that the initial burst is measured in tens of seconds, a 10-hour afterglow at such high energies is a remarkable feature."
The findings from MAGIC and HESS have scientists even more excited for the next generation of gamma ray telescopes. The new telescope that Mirzoyan was celebrating in Arizona is a prototype for the Cherenkov Telescope Array (CTA) Observatory, which will consist of 118 telescopes being built in Chile and the Canary Islands. Once in operation, these telescopes will be able to detect gamma rays in the range of 20 GeV to 300 TeV, with about ten times better sensitivity than other current observatories.
Edna Ruiz-Velasco, a researcher at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, who is part of the HESS team, says these new observatories will be able to detect GRBs several days after the initial burst, covering longer timescales of the total emissions. Better detections might also help scientists investigate the possible connection between gamma ray bursts and gravitational waves, or the ripples in spacetime that scientists have only recently observed directly.
After decades of waiting, Mirzoyan says he thinks that observations of GRBs from the ground will become much more routine. Already, the HESS team posted another notice on the Astronomer's Telegram that they spotted another burst in August. With so much more data pouring in, astronomers may soon unravel the mysteries of the most immense explosions in the universe.
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Gamma-ray bursts are the strongest and brightest explosions in the universe, thought to be generated during the formation of black holes. Though they last mere seconds, gamma-ray bursts produce as much energy as the sun will emit during its entire 10-billion-year existence.
The enigmatic phenomena were first seen in 1967 by a U.S. Air Force satellite called Vela. The probe was designed to keep watch for secret Soviet nuclear testing, but it ended up spotting dazzling gamma-rays — the most powerful electromagnetic radiation — coming from beyond the solar system, according to NASA. When such an event happened, it would briefly become the brightest gamma-ray object in the observable universe.
It wasn't until 1991 that astronomers launched the Compton Gamma Ray Observatory with the Burst and Transient Source Experiment (BATSE), which discovered roughly one new gamma-ray burst per day. BATSE found that gamma-ray bursts were distributed evenly across the sky, meaning they were occurring everywhere in the cosmos, according to the Swinburne University of Technology in Australia. BATSE also showed that there were two types of gamma-ray bursts with distinct signatures: those that lasted 2 to 30 seconds, and those that flashed for less than 2 seconds.
Since then, researchers have learned a great deal more about gamma-ray bursts by developing a network of rapid-response satellites and ground-based observatories that all converge on a gamma-ray burst as soon as it's detected. This network has provided data showing that gamma-ray bursts are located in galaxies billions of light-years away and that, after the initial gamma-ray flare, the source of the burst produces an afterglow in less-energetic wavelengths.
Where do gamma-ray bursts come from?
The longer-lived versions of gamma-ray bursts have been found to be associated with ultrapowerful supernovas called hypernovas, which occur when stars between five and 10 times the mass of our sun end their lives and implode into black holes, according to NASA. Hypernovas are 100 times brighter than typical supernovas and are thought to be generated by stars that are spinning particularly fast or have an especially strong magnetic field, imparting extra energy to their combustions.
But the short-lived gamma-ray bursts, which make up 30% of such events, remained a mystery until 2005, mainly because they are too quick and fleeting for follow-up observations. After being launched in 2004, NASA's Neil Gehrels Swift Observatory (previously called the Swift Gamma-Ray Burst Explorer) was finally able to record enough data to see the afterglow of short-lived gamma-ray bursts and figure out that they were likely caused when two ultradense stellar corpses known as neutron stars collided and formed a black hole, or when a black hole ate a neutron star.
Such outbursts are so strong that they produce ripples in the fabric of space-time called gravitational waves. Now that researchers have fired up the Laser Interferometer Gravitational-Wave Observatory (LIGO), which can detect gravitational waves from these collisions, they are expected to be able to gather even more information about the processes underlying short-lived gamma-ray bursts.
Still bursting with mystery
There are still many unknowns about gamma-ray bursts. Recent observations have shown that the photons emitted from gamma-ray bursts all oscillate in the same direction, but for some reason, the direction changes over time. "What this could be, we really don't know," Merlin Kole, a scientist at the University of Geneva in Switzerland and one of the lead researchers on the study, said in a statement after this 2019 discovery.
Gamma-ray bursts also seem to focus their energy in a narrow beam, rather than emitting it equally in every direction, meaning that our satellites are missing many of them. Astronomers estimate that, although satellites spot about one gamma-ray burst per day, roughly 500 are occurring within the same time period.
So far, gamma-ray bursts have only been detected in distant galaxies. However, it is possible for one to occur in our Milky Way galaxy. The Ordovician extinction — one of five big extinction events in our planet's history — happened around 450 million years ago and might have been caused by an ice age triggered by a gamma-ray burst. If a new gamma-ray burst were to happen near Earth, it would strip our planet's protective ozone layer away and expose all life to deadly ultraviolet radiation. So, although scientists might appreciate the opportunity to witness a gamma-ray burst up close one day, they're also OK with not observing one in our home galaxy.
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