On April 15, 2020, a powerful gamma-ray burst (GRB) reached Earth. GRBs are among the most powerful release of energy in the universe, providing astronomers with insight into many different extreme processes. GRB 200415A was just as extreme as one would expect, but what makes it even more special is its location. It was produced in a galaxy very close to the Milky Way.
The discovery is reported in Nature Astronomy, and it is presented at the Virtual 237th meeting of the American Astronomical Society. GRB 200415A is what is known as a short GRB – anything shorter than two seconds enters this classification. Neutron star collisions are known to produce such a phenomenon. But in this case, it a single neutron star with an extreme magnetic field: a magnetar. These are a type of extreme stellar remnant which are prone to flare-ups, and these flares can be energetic enough to release gamma rays. The GRB in question comes from a magnetar in galaxy NGC 253, which is located 11.4 million light-years away in the direction of the southern constellation Sculptor.
"Magnetars are up to a thousand times more magnetic than ordinary neutron stars. Most emit X-rays every now and then. But so far, we know of only a handful of magnetars that produced giant flares. The brightest we could detect was in 2004. Then GRB 200415A arrived in 2020," lead author Prof Soebur Razzaque from the University of Johannesburg, said in a statement.
The first emission lasted 140 milliseconds, and it was followed by a second more powerful and (relatively) longer GRB 20 seconds afterward. The two GRBs after a giant flare was a prediction made by Razzaque back in 2005. This is the first time that it was actually observed.
"It's only in the last 20 years or so, that we have all the instruments in place to detect these GRB events in many different ways - in gravitational waves, radio waves, visible light, X rays and gamma rays," Razzaque added. "GRB 200415A was the first time ever that both the first and second explosions of a giant flare were detected.".
Short GRBs make up 30 percent of the total gamma-ray burst menagerie seen in the Universe. The remaining 70 percent are significantly longer and they are produced when giant stars go supernova.
There are only 31 known magnetars in the Milky Way, and these objects remain extremely mysterious. Work such as this tells us just how powerful they can be. “This was the most accurately localized magnetar outside of our galaxy so far, and we’ve really pinned it down now, not just to a galaxy, but a part of a galaxy where we expect star formation is going on, and stars are exploding. That is where the supernovas should be and the magnetars, too,” co-author Kevin Hurley at the University of California, Berkeley, said in a statement. “The April 15 event is a game changer.”
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