Astronomers believe they have discovered a rare giant flare erupting from an extremely magnetic dead star, or magnetar, that is bright enough to light up an entire galaxy. If true, the discovery would represent the first sighting of gamma rays from “recently dead” neutron stars exploding outside the Milky Way.
The flare, first discovered by the Center for Integrated Scientific Data in Geneva, took the form of a short burst of high-energy gamma rays that lasted just a tenth of a second. Integral alerted astronomers, who realized just 13 seconds after the flare that the gamma rays appeared to come from the bright Messier 82 (M82) galaxy, nicknamed the “cigar” because of its elongated shape “Galaxy”; the Cigar Galaxy is about 12 million light-years away from Earth.
However, this all leaves astronomers with a mystery to be solved. Is this a fairly common gamma-ray burst they’re seeing from this galaxy, which also has intense star formation, or does it definitely represent a rare flare from a highly magnetic magnetar?
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“We immediately recognized that this was a special alarm,” Sandro Mereghetti, research leader and scientist at the National Institute for Astrophysics and Astronomy (INAF-IASF), said in a statement. “Gamma-ray bursts come from anywhere in the distant sky, but this burst came from a nearby bright galaxy.”
To study gamma-ray flares, Mereghetti and colleagues quickly conducted follow-up observations of the source of the explosion using the XMM-Newton space telescope. They reasoned that if this gamma-ray burst was a brief gamma-ray burst produced by a powerful event such as the collision and merger of two neutron stars, then an associated afterglow should also appear in X-rays and visible light. The event also causes space-time to “ring” and create ripples called “gravitational waves.”
“The XMM-Newton observations only show hot gas and stars in the galaxy,” said team member and INAF researcher Michela Rigoselli. “If this explosion was a brief gamma-ray burst, we would see the X-ray source from its location gradually weakening, but this afterglow is not present.”
Integral’s role in helping researchers quickly investigate gamma-ray flashes is – apologies in advance – indispensable Determine its true origin and trace it back to the magnetar eruption of M82.
“Integral and XMM-Newton have the flexibility to schedule when unexpected observations like this are received, which is crucial for time-critical discoveries like this,” Integral project scientist Jan-Uwe Ness explained in a statement. In this case, even if we observed it a day later, we would not have such strong evidence that this is indeed a magnetar and not a gamma-ray burst.”
A dead and twinkling magnetar
A magnetar is a type of neutron star known for its extremely powerful magnetic field. Like all neutron stars, magnetars are born when a star at least eight times the mass of the Sun exhausts the fuel needed for nuclear fusion in its core. This ends the outward force associated with radiation pressure that protects these stars from collapsing under the influence of their own gravity for millions or even billions of years.
As this protection ends, the core of the dying star collapses, and the outer layers, which represent most of the star’s mass, are blown away in a supernova explosion. The result is a dead star with a core between one and twice the mass of the Sun, squeezed into a width no more than 12 miles (20 kilometers).
This rapid collapse results in neutron stars composed of the densest material known in the universe, with just a tablespoon weighing a billion tons if brought to Earth. The crash had two other extreme consequences.
Just like an ice skater on Earth takes advantage of conservation of angular momentum by contracting their arms to increase their rotational speed, the rapid radial reduction of a dying star’s 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.
In addition, collapse also causes the magnetic field lines of the star’s core magnetic field to move closer together. The closer the magnetic field lines are, the stronger the magnetic field. This means that some neutron stars have the strongest magnetic fields in the entire universe. As these stellar remnants age, the neutron star’s rotational speed and strong magnetism dim.
“Some young neutron stars have extremely strong magnetic fields, more than 10,000 times that of a typical neutron star. These are called magnetars. They emit energy in the form of flares, and sometimes these flares are very large,” said ESA researcher Ashley Ashley Chrimes said.
Magnetar flares are thought to be caused by “starquakes” on the surfaces of these highly magnetic young neutron stars that disrupt their strong magnetic fields. Magnetar flares are both huge and extremely rare.
In 50 years of observing the universe with gamma rays, humans have previously observed only three flares. These were discovered in 1979, 1998 and 2004, all from magnetars located in the Milky Way.
Fortunately, however, flare magnetars are rare. The example seen in December 2004 was caused by a magnetar 30,000 light-years away from Earth, which was so powerful that it actually affected our planet’s upper atmosphere. The effect is similar to that caused by a solar flare, but the sun is 1.9 billion It is twice as close to Earth as the magnetar behind the 2004 gamma-ray flare. Let it sink in.
Integral’s discovery represents the first detection of a flare from an extragalactic magnetar. However, the team believes that some of the other short gamma-ray bursts Integral observed were actually flares from extragalactic magnetars.
“However, bursts of such short duration can only be captured by chance if the observatory is already pointed in the right direction,” Jan-Uwe said. “This makes Integral’s field of view more than 3,000 times larger than the area of the sky covered by the moon, which is very important for these detections.”
The position of the magnetar in M82 is important because this bright galaxy is home to massive star formation. This confirms that in such starburst regions, massive stars “live fast and die young,” turning young neutron stars into turbulent, rapidly rotating magnetars.
The team will now search for more magnetars in starburst galaxies to help better understand the life and death of massive stars in these regions, and to better understand how neutron stars evolve over time.
“This discovery opens up our search for other extragalactic magnetars,” Krims said. “If we can find more, we can start to understand how often these flares occur and how these stars lose energy in the process.”
The team’s findings were published in the journal Nature on Wednesday (April 24).
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