NASA’s Fermi Spots a Weird Pulse of High-Energy Radiation Racing Toward Earth

When the core of massive star collapses, it can form a black hole. Some of the surrounding matter escapes in the form of powerful jets that rush outward at almost the speed of light in opposite directions, as illustrated here. Normally jets from collapsing stars produce gamma rays for many seconds to minutes. Astronomers think the jets from GRB 200826A were shut down quickly, producing the shortest gamma-ray burst (magenta) from a collapsing star ever seen.
Credit: NASA’s Goddard Space Flight Center/Chris Smith (KBRwyle)

On August 26, 2020, NASA’s Fermi Gamma-ray Space Telescope detected a pulse of high-energy radiation that had been racing toward Earth for nearly half the present age of the universe. Lasting only about a second, it turned out to be one for the record books – the shortest gamma-ray burst (GRB) caused by the death of a massive star ever seen.

GRBs are the most powerful events in the universe, detectable across billions of light-years. Astronomers classify them as long or short based on whether the event lasts for more or less than two seconds. They observe long bursts in association with the demise of massive stars, while short bursts have been linked to a different scenario.

Astronomers combined data from NASA’s Fermi Gamma-ray Space Telescope, other space missions, and ground-based observatories to reveal the origin of GRB 200826A, a brief but powerful burst of radiation. It’s the shortest burst known to be powered by a collapsing star – and almost didn’t happen at all.
Credit: NASA’s Goddard Space Flight Center

“We already knew some GRBs from massive stars could register as short GRBs, but we thought this was due to instrumental limitations,” said Bin-bin Zhang at Nanjing University in China and the University of Nevada, Las Vegas. “This burst is special because it is definitely a short-duration GRB, but its other properties point to its origin from a collapsing star. Now we know dying stars can produce short bursts, too.”

Named GRB 200826A, after the date it occurred, the burst is the subject of two papers published in Nature Astronomy on Monday, July 26. The first, led by Zhang, explores the gamma-ray data. The second, led by Tomás Ahumada, a doctoral student at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, describes the GRB’s fading multiwavelength afterglow and the emerging light of the supernova explosion that followed.

“We think this event was effectively a fizzle, one that was close to not happening at all,” Ahumada said. “Even so, the burst emitted 14 million times the energy released by the entire Milky Way galaxy over the same amount of time, making it one of the most energetic short-duration GRBs ever seen.”

When a star much more massive than the Sun runs out of fuel, its core suddenly collapses and forms a black hole. As matter swirls toward the black hole, some of it escapes in the form of two powerful jets that rush outward at almost the speed of light in opposite directions. Astronomers only detect a GRB when one of these jets happens to point almost directly toward Earth.

Each jet drills through the star, producing a pulse of gamma rays – the highest-energy form of light – that can last up to minutes. Following the burst, the disrupted star then rapidly expands as a supernova.

Short GRBs, on the other hand, form when pairs of compact objects – such as neutron stars, which also form during stellar collapse – spiral inward over billions of years and collide. Fermi observations recently helped show that, in nearby galaxies, giant flares from isolated, supermagnetized neutron starsalso masquerade as short GRBs.

GRB 200826A was a sharp blast of high-energy emission lasting just 0.65 second. After traveling for eons through the expanding universe, the signal had stretched out to about one second long when it was detected by Fermi’s Gamma-ray Burst Monitor. The event also appeared in instruments aboard NASA’s Wind mission, which orbits a point between Earth and the Sun located about 930,000 miles (1.5 million kilometers) away, and Mars Odyssey, which has been orbiting the Red Planet since 2001. ESA’s (the European Space Agency’s) INTEGRAL satellite observed the blast as well.

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