Space’s brightest explosion ever reveals possible hints of dark matter


aOn Sunday, October 9, Judith Rakusen was 35,000 feet in the air, on her way to a high-energy astrophysics conference, when the largest cosmic explosion in history occurred. “I landed and looked at my phone, and I had dozens of messages,” said Rakusen, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland. “It was really exceptional.”

The explosion was a long gamma-ray burst, a cosmic event where a dying massive star releases powerful jets of energy as it collapses into a black hole or neutron star. This particular explosion was so bright that it saturated the Fermi Gamma Ray Space Telescope, an orbiting NASA telescope designed in part to monitor such events. “There were too many photons per second for them to keep up,” said Andrew Levan, an astrophysicist at Radboud University in the Netherlands. The explosion appears to have swelled the volume of Earth’s atmosphere, the upper layer of Earth’s atmosphere, for several hours. “The fact that you can change the Earth’s ionosphere from an object halfway through the universe is incredible,” said Doug Welch, an astronomer at McMaster University in Canada.

Astronomers called it BOAT – “the brightest of all time” – and began pressing it for information about gamma-ray bursts and the universe in general. “Even 10 years from now there will be new understanding from this data set,” said Eric Burns, an astrophysicist at Louisiana State University. “It never occurred to me until now that this actually happened.”

Initial analysis suggests there are two reasons why the boat is so bright. First, they occurred about 2.4 billion light-years from Earth – fairly close to gamma-ray bursts (though outside our galaxy). It is also possible that the powerful plane of the boat is directed towards us. The two factors combined to make this type of event happen only once every few hundred years.

Perhaps the most important observation occurred in China. There, in Sichuan Province, the High Altitude Air Shower Observatory (LHAASO) tracks high-energy particles from space. In the history of gamma-ray burst astronomy, researchers have only seen a few hundred high-energy photons coming from these objects. LHAASO witnessed 5000 of this one event. “The gamma-ray burst essentially went off in the sky just above them,” said Silvia Chu, an astrophysicist at the German Electron Synchrotron (DESY) Center in Hamburg.

Among those discoveries is a suspected high-energy photon at 18 teraelectronvolts (TeV) – four times higher than anything seen from a gamma-ray burst before and more energetic than the highest energies achievable by the Large Hadron Collider. Such a high-energy photon would have been lost on its way to Earth, where it is absorbed by interactions with the universe’s background light.

How did you get here? One possibility is that after a gamma-ray burst, a high-energy photon was converted into an axion-like particle. Axions are putative lightweight particles that may explain dark matter; The axon-like particles are thought to be slightly heavier. High-energy photons can be converted into such particles by strong magnetic fields, such as those around an exploding star. Then the axle-like particle travels across a vast area unimpeded. When it reaches our galaxy, magnetic fields will convert it back into a photon, then make its way back to Earth.

In the week following the initial discovery, multiple teams of astrophysicists proposed this mechanism in papers uploaded to the scientific preprint website “It would be a very amazing discovery,” said Giorgio Galante, an astrophysicist at the National Institute of Astrophysics (INAF) in Italy, who co-authored one of the first such papers.

However, other researchers question whether the LHAASO discovery might be a case of misidentification. Perhaps the high-energy photon came from somewhere else, and its correct arrival time was just a coincidence. “I’m very skeptical,” said Milena Kronogorovic, an astrophysicist at the University of Maryland. “I currently tend to have it as a background event.” (To further complicate it, a Russian observatory has reported hitting a higher-energy 251 TeV photon from the explosion. But “the jury is still out” on that, said Rakusin, Fermi Telescope deputy project scientist. “I’m a little skeptical.”)

So far the LHAASO team has not released detailed findings of their observations. Burns, who is coordinating a global collaboration to study the boat, hopes they will. “I am very curious to see what they have,” he said. But he understands why a degree of caution might be warranted. “If I was sitting on the data that has a few percent chance of identifying evidence of dark matter, I would be very cautious right now,” Burns said. If the photon can be attached to the boat, “it is very likely evidence of new physics, and possibly dark matter,” Krnogorovich said. The LHAASO team did not respond to a request for comment.

Even without the LHAASO data, the massive amount of light seen from the event could enable scientists to answer some of the biggest questions about gamma-ray bursts, including key mysteries about the jet itself. “How does a plane launch? What happens in the plane as it propagates into space?” said Tyler Barsuttan, an astrophysicist at Goddard. “These are really big questions.”

Other astrophysicists hope to use BOAT to ascertain why only some stars produce gamma-ray bursts as they transition into a supernova. “This is one of the big mysteries,” said Yvette Sendez, an astronomer at the Harvard-Smithsonian Center for Astrophysics. “It must be a very massive star. A galaxy like ours probably produces gamma ray bursts every million years. Why do such a rare population have gamma ray bursts?”

Whether gamma-ray bursts lead to a black hole or a neutron star at the core of a collapsing star is also an open question. A preliminary analysis of BOAT indicates that the first occurred in this case. “There’s so much energy in the plane that it should basically be a black hole,” Burns said.

What is certain is that this is a cosmic event that will not be eclipsed for many, many lifetimes. “We’re all going to die before we get a chance to do it again,” Burns said.

Main image: The rings around the explosion, shown here in color data from NASA’s Swift Observatory, formed when X-rays scattered on hidden dust in our Milky Way. credit: NASA’s Swift Observatory; Processing: John Miller.

This article was originally published on Quanta abstraction Articles.

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