The brightest space explosion in history reveals possible hints of dark matter

Oon Sunday, October 9 Judith Racusin he was 35,000 feet in the air, on his way to a high-energy astrophysics conference, when the biggest cosmic explosion in history took place. “I landed, looked at my phone and got dozens of messages,” said Racusin, an astrophysicist at NASA’s Goddard Space Flight Center in Maryland. “It was really exceptional.”

The explosion was a long burst of gamma rays, 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 burst was so bright that it oversaturated the Fermi Gamma-ray Space Telescope, an orbiting NASA telescope designed in part to observe such events. “There were so many photons per second that they couldn’t keep up,” he said Andrew Levan, an astrophysicist at Radboud University in the Netherlands. The burst even appears to have caused the Earth’s ionosphere, the upper layer of the Earth’s atmosphere, to swell in size for several hours. “The fact that you can change the Earth’s ionosphere from an object halfway across the universe is pretty incredible,” he said. Doug Welchastronomer at McMaster University in Canada.

Astronomers cheekily called it BARCEL—“the brightest of all time”—and began to squeeze it for information about gamma-ray bursts and the cosmos in general. “Even 10 years from now there will be a new understanding of this data set,” he said Eric Burns, an astrophysicist at Louisiana State University. “It still hasn’t hit me that this actually happened.”

Initial analysis suggests that there are two reasons why the BOAT was so brilliant. First, it occurred about 2.4 billion light-years from Earth, quite close to gamma-ray bursts (though far outside our galaxy). It is also likely that BARCEL’s powerful jet was aimed at us. The two factors combine to make this the kind of event that only happens once every few hundred years.

Perhaps the most consequential observation happened 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 seen only a few hundred high-energy photons from such objects. LHASO I’ve seen 5,000 of this single event. “The gamma-ray burst basically shot up into the sky directly above them,” he said Silvia Zhuastrophysicist at the German Electron Synchrotron (DESY) in Hamburg.

Among these detections was a suspected high-energy photon at 18 teraelectron volts (TeV), four times higher than anything seen before in a gamma-ray burst and more energetic than the highest energies possible by Large Hadron Collider. A high-energy photon should have been lost on the way to Earth, absorbed by interactions with the background light of the universe.

So how did it get here? one possibility is that, after the gamma-ray burst, a high-energy photon became an axion-like particle. shares are hypothesized light particles that can explain dark matter; Axion-like particles are thought to be slightly heavier. High energy photons could be converted into these particles by strong magnetic fields, such as those surrounding an exploding star. The axion-like particle would then travel through the vastness of space unimpeded. When it reached our galaxy, the magnetic fields would turn it back into a photon, which would then reach Earth.

In the week following the initial detection, several teams of astrophysicists suggested this mechanism in articles uploaded to the scientific preprint site arxiv.org. “It would be a very incredible discovery,” said Giorgio Galanti, an astrophysicist at the National Institute of Astrophysics (INAF) in Italy, who co-authored one of the first of these papers.

However, other researchers wonder if the detection of LHAASO could be a case of mistaken identity. Perhaps the high-energy photon came from somewhere else and its precise arrival time was a coincidence. “I’m very skeptical,” he said Milena Crnogorcevic, an astrophysicist at the University of Maryland. “I’m currently leaning towards it being a background event.” (To further complicate matters, a Russian observatory reported hit by an even higher-energy 251 TeV photon from the burst. But “the jury is still out” on that, said Racusin, assistant project scientist for the Fermi telescope. “I’m a little skeptical.”)

So far, the LHAASO team has not published detailed results of their observations. Burns, who is coordinating a global collaboration to study BOAT, hopes they will. “I’m very curious to see what they have,” he said. But he understands why some degree of caution may be warranted. “If I were sitting on data that had even a small percentage chance of being definitive evidence of dark matter, I would be extraordinarily cautious at this point,” Burns said. If the photon can be matched to the BOAT, “it would most likely be evidence of new physics and potentially dark matter,” Crnogorčević said. The LHAASO team did not respond to a request for comment.

Even without the LHAASO data, the vast amount of light seen from the event could allow scientists to answer some of the biggest questions about gamma-ray bursts, including major puzzles about the jet itself. “How is the jet launched? What is happening to the jet as it propagates through space?” said Tyler Parson, Goddard astrophysicist. “These are really big questions.”

Other astrophysicists hope to use BOAT to find out why only some stars produce gamma-ray bursts as they go supernova. “This is one of the great mysteries,” he said Yvette Cendes, astronomer at the Harvard-Smithsonian Center for Astrophysics. “It must be a very massive star. A galaxy like ours will produce a gamma-ray burst perhaps every million years. Why is such a rare population producing gamma-ray bursts?”

Whether gamma-ray bursts give rise to a black hole or a neutron star in the core of the collapsed star is also an open question. A preliminary analysis by BARCA suggests that the former happened in this case. “There’s so much energy in the jet that it basically has to be a black hole,” Burns said.

What is certain is that this is a cosmic incident that will not be eclipsed for many, many lifetimes. “We’ll all be dead long before we get a chance to do it again,” Burns said.

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

This article was originally published to the How many Abstractions blog

• Jonathan O’Callaghan

  Posted on October 27, 2022

  Jonathan O’Callaghan