Astronomers believe they have found a rare, massive flare erupting from a highly magnetic dead star, or magnetar, bright enough to illuminate the entire galaxy. If true, the discovery would represent the first sighting of gamma rays from a “recently deceased” neutron star that exploded outside the Milky Way.
The flare was first detected by the Integral Science Data Center in Geneva as a short burst of high-energy gamma rays lasting about a tenth of a second. Integral sent an alert to astronomers just 13 seconds after the flare, noting that these gamma rays appeared to be coming from the bright galaxy Messier 82 (M82), nicknamed the “Cigar Galaxy” for its elongated shape; The Cigar Galaxy is located approximately 12 million light-years from Earth.
But all this left astronomers with a mystery to solve. Was this the fairly common gamma-ray burst they saw from this galaxy, which is also home to intense star formation, or did it definitely represent the rare flash of a highly magnetic magnetar?
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“We immediately realized that this was a special warning,” Sandro Mereghetti, research leader and scientist at the National Institute for Astrophysics (INAF-IASF), said in a statement. “Gamma-ray bursts can come from far away and from anywhere in the sky, but this burst came from a bright galaxy nearby.”
Mereghetti and his colleagues quickly made follow-up observations of the burst’s source with the XMM-Newton space telescope to look for the gamma-ray flare. They reasoned that if this gamma-ray burst was a brief gamma-ray burst that resulted from a powerful event such as two neutron stars colliding and merging together, then there should also be an associated afterglow seen in X-rays and visible light. . This event would also cause space-time to “ring” with waves called “gravitational waves”.
“XMM-Newton’s observations showed only the hot gas and stars in the galaxy,” said team member and INAF researcher Michela Rigoselli. “If this explosion had been a short gamma-ray burst, we would have seen a weakening X-ray source coming from its location, but this afterglow was not present.”
Integral’s role in enabling researchers to move quickly to investigate this gamma-ray flash – we apologize in advance – integral To determine their true origins and trace them back to the magnetar explosion in M82.
“When unexpected observations like this are detected, Integral and XMM-Newton can be flexible in their programs, which is crucial for time-critical discoveries,” Integral project scientist Jan-Uwe Ness said in a statement. said. “In this case, if the observations had been made even just a day later, we would not have such strong evidence that this was indeed a magnetar and not a gamma-ray burst.”
A dead and glowing magnetar
Magnetars are a type of neutron stars noted for their incredibly strong magnetic fields. Like all neutron stars, magnetars are born when a star with a mass at least eight times that of the Sun consumes the fuel it needs for nuclear fusion in its core. This puts an end to the external force due to radiation pressure that keeps stars from collapsing under the influence of their own gravity for millions, and sometimes billions, of years.
As this protection ends, the core of this dying star collapses, while its outer layers, representing the majority of the star’s mass, are blown away in a supernova explosion. The result is a dead stellar core with a mass between one and two times the mass of the Sun, compressed into a width no larger than 12 miles (20 kilometers).
This rapid collapse causes neutron stars to form from the densest known matter in the universe; Just one tablespoon would weigh 1 billion tons when brought to Earth. This collapse has two more extreme consequences.
Just as an ice skater on Earth takes advantage of the conservation of angular momentum by pulling his arms to increase his spin rate, the rapid radial decay of a dying stellar core causes a newborn neutron star to spin at incredible speeds. Some young neutron stars have been found to spin as fast as 700 times per second.
Additionally, the collapse causes the field lines of the stellar core’s magnetic field to come closer together. The closer the field lines are, the stronger the magnetic field. This means that some neutron stars have the strongest magnetic fields in the entire universe. Both the spin rate and intense magnetism of neutron stars fade as these stellar remnants age.
“Some young neutron stars have extra-strong magnetic fields, more than 10,000 times greater than typical neutron stars. These are called magnetars. They emit energy in flares, and these flares sometimes reach gigantic sizes,” said European Space Agency research associate Ashley Chrimes. aforementioned.
Magnetar explosions, thought to be caused by “starquakes” on the surface of these highly magnetic young neutron stars disrupting their intense magnetic fields, are both massive and vanishingly rare.
In 50 years of observing the universe with gamma rays, humanity had previously caught only three flares. These were found in 1979, 1998 and 2004 and were all magnetars found in the Milky Way.
However, perhaps it is fortunate that glowing magnetars are rare. The sample seen in December 2004, caused by a magnetar 30,000 light-years from Earth, was so powerful that it actually affected our planet’s upper atmosphere. The effect was similar to that caused by solar flares, but the sun was 1.9 magnitude. billion It is many times closer to Earth than the magnetar behind the 2004 gamma-ray burst. Let that sink in.
The integral discovery represents the first time a flare from a magnetar outside the Milky Way has been detected. But the team thinks that some of the other short gamma-ray bursts seen by Integral are actually flares from extragalactic magnetars.
“However, such short-duration bursts can only be caught by chance when an observatory is already pointing in the right direction,” Jan-Uwe said. “This makes Integral, with its wide field of view, 3,000 times larger than the area of sky covered by the moon, which is crucial for these detections.”
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The location of the magnetar in M82 is important because this bright galaxy is home to an intense burst of star formation. This confirms that massive stars in such starburst regions “live fast and die young,” leaving young neutron stars as turbulent, rapidly rotating magnetars.
The team will now look for more magnetars in starburst galaxies to better understand the lives and deaths of massive stars in these regions and how neutron stars evolve over time.
“This discovery opens our search for other extragalactic magnetars,” Chrimes said. “If we can find more, we can begin to understand how often these explosions occur and how these stars lose energy in the process.”
The team’s research was published in the journal Nature on Wednesday, April 24.