An international team of astronomers has detected a pair of gamma-ray bursts with energies more powerful than anything ever seen before. GRBs are the strongest explosions known in the cosmos, but these latest observations suggests we’ve significantly underestimated their true potential.
Three new papers published last week in Nature describe two new gamma-ray bursts – GRB 190114C and GRB 180720B – both of which yielded the highest-energy photons ever recorded for GRB events. The unprecedented observations are casting new light – quite literally – onto these mysterious cosmic events and the mechanics behind them.
Gamma-ray bursts are thought to be triggered when gigantic stars collapse into black holes, causing a supernova. The resulting explosion produces a powerful, concentrated jet that shoots material into space at 99.99 per cent the speed of light. The rapidly accelerating particles within the jet produce gamma rays through complex interactions with magnetic fields and radiation. The ensuing gamma rays continue to travel through interstellar space, some of which eventually reach Earth. When they come into contact with our atmosphere, gamma rays trigger a particle cascade that in turn generates a phenomenon known as Cherenkov light, which can be detected by specially equipped telescopes.
The afterglow of GRB 190114C and its home galaxy, as imaged by the Hubble Space Telescope on 11 February and 12 March 2019. (Image: NASA, ESA, and V. Acciari et al. 2019)
Astronomers have been studying GRBs for more than 50 years, but there is still much to learn, including needed insights into how gamma rays come into existence and the physics involved when materials are jettisoned from black holes at such extreme velocities, said Andrew Levan, an astronomy professor at the University of Warwick and a co-author of one of the new studies. The newly detected GRBs, with their unprecedented energies, are definitely helping.
“These new observations push the energy range over which we can observe the gamma rays and reveal a new component of the emission we’ve not seen before,” Levan wrote in an email to Gizmodo. “They are a fantastic demonstration of the technology that lets telescopes detect this kind of light. Most importantly, they offer a new way of understanding physics in the most extreme conditions in nature.”
Indeed, these observations wouldn’t have been possible without some very impressive technology. The GRB energies described in the new papers were measured by observing their effects on our atmosphere. When gamma rays plough into our skies, they shove large batches of particles, producing a kind of air shower. Moving at relativistic speeds, these showers generate a detectable bluish glow, called Cherenkov light, which can be detected by, appropriately enough, Cherenkov telescopes.
Conceptual image showing specially equipped telescopes detecting Cherenkov light, which is generated by incoming gamma-rays and detectable as blue light. (Image: DESY, Science Communication Lab)
In this case, those telescopes were the High-Energy Stereoscopic System (HESS) in Namibia and the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) in the Canary Islands, both of which are operated by the Max Planck Society. Satellites have previously been used to observe Cherenkov light, but their instruments are not sensitive enough to detect super high-energy events because they produce low light.
The first of these high-energy events, GRB 180720B, happened on 20 July 2018, and is described in a paper led by astronomers from the Max Planck Institute, Deutsches Elektronen-Synchotron (DESY), the International Centre for Radio Astronomy Research (ICRAR), and several other institutions. The second event, GRB 190114C, occurred on 14 January 2019, and is described in two new papers (here and here), both led by Razmik Mirzoyan from the Max Planck Institute for Physics. Over 300 scientists from around the world were involved in the research.
“What is remarkable about these particular bursts isn’t how much energy they emit in total, but the energy that we see from the individual bits of light,” explained Levan. “We can think of light as made up of little particles called photons, and each of these photons carries an energy. We usually measure that energy in a unit called an electron volt, which is the energy a single electron would get moving across 1 volt.”
The photons around us, which we see with our eyes, typically hold around 1 electron volt of energy, but the photons from GRB 190114C, as measured by MAGIC, carried upwards of 1 teraelectron volt (TeV), which is a trillion times more energy than we see with our eyes, explained Levan. By perspective, a record-setting GRB from 2013 was measured at 94 billion electron volts, or 0.094 TeV.
“It is a bit like having 10 cents to your name when the person standing next to you is Bill Gates,” he said. “Unsurprisingly, if a photon has so much energy, it can do different things – a bit like you can lead a very different life with $100 billion than 10 cents. So this very energetic light really does give a new window on the Universe.”
Data collected by MAGIC showed that energies from GRB 190114C were between 200 billion and 1,000 billion electron volts, or 0.2 to 1 teraelectron volt. This is now the strongest GRB event ever detected. Observations from supporting observatories placed the distance of this GRB to around 4 billion light-years from Earth. The earlier event, GRB 180720B, as measured by HESS, was slightly weaker, registering energies between 100 billion and 440 billion electron volts, or 0.1 to 0.44 TeV, and is estimated to be 6 billion light-years away.
Conceptual image showing the MAGIC observatory scanning the skies for gamma-rays. (Image: Superbossa.com and C. Righi)
“What surprises me most about these observations is how we have finally seen such high-energy emission after more than a decade of effort,” said Levan. In addition to these two events, another big GRB was recorded this past summer, the details of which have yet to be released, he said. “This implies that rather than being very rare, this kind of emission might be quite common in gamma-ray bursts. In that case, it is actually surprising we’ve had to wait so long for the conditions to be just right to find this exceptionally energetic light,” Levan told Gizmodo.
The new papers, in addition to characterising the new GRBs, also posited explanations for the high-energy photons, which are presumed to be produced by a two-pronged process known as inverse Compton scattering. At first, the rapidly accelerating particles bounce around in the strong magnetic field within the explosion itself, resulting in synchrotron radiation (yep, the same kind of radiation produced in synchrotrons and other particle accelerators on Earth, but that’s where the comparison ends). Then, in a second stage, the synchrotron photons smash into the fast particles that created them, boosting their energies to the extreme rates recorded in Earth’s atmosphere.
Diagram showing how GRBs are formed from a black hole. (Image: NASA’s Goddard Space Flight Center)
GRBs are recorded by satellites on an almost daily basis, but they’re actually quite rare from a cosmological perspective – and thank goodness for that. To put the power of these things in perspective, a “typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime,” explained astronomer Gemma Anderson, a study co-author from the Curtin University node of the International Centre for Radio Astronomy Research, in a press release. If a gamma-ray burst went off anywhere near our vicinity and it was focused directly at Earth, it could potentially trigger a mass extinction.
As Levan explained to Gizmodo, such an event may have actually happened in Earth’s ancient past.
“There is one mass extinction event that we can see in the geological record – the Ordovician extinction – which matches with what we’d expect from a gamma-ray burst,” Levan said. “If an event went off close enough to the Earth to affect us now, we’d have some somewhat paradoxical effects.”
First, the ozone layer would be destroyed by the gamma-rays, allowing copious amounts of UV light to reach the surface, said Levan. Visible light, by contrast, would likely be blocked due to the destruction of key molecules in the atmosphere and the presence of nitrous oxides, which would block sunlight, triggering an ice age. This double-whammy of atmospheric effects would be... bad.
“This is consistent with what was seen 440 million years ago during the Ordovician extinction, though it isn’t the only possible explanation,” said Levan. “However, to affect us, a gamma-ray burst would have to be close enough, with its jet pointing straight at us. Observations suggest that gamma-ray bursts are actually very rare in the Milky Way.”
To which he added: “We don’t really expect to be affected significantly more often than every billion years or so – there is no reason to lose sleep over the possibility.”
Once every billion years or so, eh? I like those odds.
Featured image: DESY, Science Communication Lab