Without our even realizing it, the Earth is hit practically daily by gamma radiation coming from powerful distant outbursts. Many of these explosions are mild and last a short time, but sometimes a giant flare reaches our galaxy, with an energy that dwarfs that of the Sun itself. In fact, gamma-ray bursts, GRB for its acronym in English (Gamma Ray Bursts) are among the most powerful and energetic events in the entire Universe.
The truth is that, just a few months ago, our planet received radiation from one of these giant gamma-ray bursts, cataloged as GRB 200415A. And as explained by a team of researchers in Nature Astronomy, this powerful GRB comes from the same place that in the past had already thrown us smaller and shorter GRBs: a magnetar (a type of neutron star), which is, in cosmic terms, much closer to us (about 11.4 million light years) than most GRBs known so far.
Many of the gamma ray bursts do indeed come from distant galaxies, but this one has occurred so close that it could have caused cuts in mobile phone reception on earth. GRBs, however, are interesting for quite another reason: they carry valuable information about the earliest history of the Universe.
“Our Sun is a very common star,” says Soebur Razzaque of the University of Johannesburg and lead author of the study. “When it dies, it will get bigger and become a red giant. After that, it will collapse into a small compact star. called a white dwarf. But stars that are much more massive than the Sun have a very different ending. “
In fact, the researcher continues, “when these massive stars die, they explode in a supernova. What remains after that is a very small compact star, small enough to fit into a valley barely 20 km wide. It is called a neutron star. And it is so dense that a single tablespoon of its material would weigh many tons on Earth. ” Those massive stars, or what’s left of them, are what cause GRBs, the biggest explosions in the entire Universe.
In fractions of a second
Scientists have long known that supernovae emit long GRBs, bursts of gamma rays that last more than two seconds. And in 2017 it was discovered that also two neutron stars orbiting each other in a spiral can emit a short GRB. But the outburst detected in 2017 occurred at a “safe distance” of about 130 million light-years from us. Which couldn’t explain any of the other GRBs that researchers can detect in our sky almost daily.
All of that changed in just a fraction of a second at 4:42 AM, Eastern Time, on April 15, 2020. That day, in effect, a gigantic GRB flare passed through Mars, being felt by many satellites, a spaceship and the International Space Station orbiting our planet.
That was the first known giant flare since the 2008 launch of NASA’s Fermi Gamma-ray Space Telescope. An explosion that lasted just 140 milliseconds, a simple blink during which, however, an energy equivalent to that of millions of suns was released. Now, telescopes and other instruments in orbit have managed to collect much more data from the giant flare last April than was possible 16 years ago.
The elusive GRB was named GRB 200415A. And the Interplanetary Network (IPN), a consortium of astronomers from around the world, managed to discover where that giant flare came from. GRB 200415A exploded from a magnetar located in the galaxy NGC 253, in the constellation of Sculptor. Until now, the vast majority of known GRBs had been traced to very different objects: supernovae or two neutron stars orbiting each other. But very few coming from a magnetar.
“In the Milky Way there are tens of thousands of neutron stars,” Razzaque explains. “And of all of them only 30 are known to be magnetars. Magnetars are up to a thousand times more magnetic than ordinary neutron stars. Most emit X-rays. from time to time. But so far, we only know of a handful of magnetars that have produced giant flares. The brightest we could detect was in 2004. Until GRB 200415A arrived in 2020. “
The galaxy NGC 253, 11.4 million light years away, may seem very far away, but that’s relatively close when it comes to the nuclear firepower of a giant GRB flare.
A gamma ray burst is something difficult to imagine, much more powerful than any solar flare, which, as it is known, already has enough power to sometimes interrupt telecommunications systems and damage electrical networks. The giant flare of 2004 also had the same effect, and this despite being much further away from us than the Sun.
The first detection of a double explosion
Now, almost twenty years later, the instruments available allow GRB events to be detected in many different ways, from gravitational waves to radio waves, visible light, X-rays or gamma rays. For this reason, with GRB 200415A it was found that it was actually a double explosion. And both could be detected.
As early as 2005, Razzaque predicted that more than one explosion could occur during a giant flare event. And in this study, the researcher and his colleagues developed a theoretical model, or prediction, of what a second explosion in a giant GRB flare would look like. This model was then compared with the measured data from GRB 200415A.
“Data from the Fermi Gamma-ray Burst Monitor – says Razzaque – informed us of the first explosion. Data from the Fermi Large Area Telescope (LAT) informed us about the second. The second explosion occurred about 20 seconds after the first and with much higher gamma ray energy. It also lasted longer. However, we still have to understand what happens after a few hundred seconds. “
Messengers from the early Universe
If the next giant GRB explosion occurs closer to our home galaxy, it could be detected by the MeerKAT radio telescope in South Africa, Razzaque believes. “It would be an excellent opportunity to study the relationship between very high-energy gamma ray emissions and radio wave emissions from the second explosion. And that would tell us more about what works and what does not work in our model.”
The truth is that the better we understand the nature of these fleeting and powerful explosions, the better we will understand the Universe in which we live. Everything is connected, and a star that died shortly after the beginning of the Universe could be disrupting the reception of mobile phones today.
“Although gamma-ray bursts happen from a single star,” Razzaque says, “we can detect them very early in the history of the Universe. Even when the Universe was only a few hundred million years old. We are only now detecting gamma-ray bursts. of the stars that died at that time, because light takes time to travel. And that means gamma-ray bursts can tell us much more about how the Universe expands and evolves over time. “