They generate as much energy in a millisecond as the Sun does in a year. They are called fast radio bursts (FRB) and they are so brief - they disappear in much less than the blink of an eye - that it is a challenge for researchers to track where they come from and, above all, the objects capable of causing pulses. so extraordinary. Most of the time, they don't even know where to look. Their origin is so enigmatic that there was even speculation with the daring idea that they are the signs of another advanced civilization.
But there are unique occasions. Astronomers using NASA's Hubble Space Telescope have traced the location of five brief, powerful radio bursts to the spiral arms of five distant galaxies billions of light years away. The research helps rule out some of the possible causes of these outbursts considered so far and supports the theory that they are caused by magnetars, a type of neutron star with powerful magnetic fields.
The first known FRB was discovered in archived data recorded by the Parkes Radio Observatory on July 24, 2001. Since then, astronomers have discovered a thousand, but have only been able to associate about fifteen of them with particular galaxies.
«Our results are new and exciting. "This is the first high-resolution image of a population of FRBs, and Hubble reveals that five of them are located near or in the spiral arms of a galaxy," says lead author Alexandra Mannings of the University of California, Santa Cruz. of the study. «Most galaxies are massive, relatively young and are still forming stars. "The images allow us to get a better idea of the general properties of the host galaxy, such as its mass and rate of star formation, as well as investigate what is happening properly in the FRB because Hubble has such high resolution."
Galaxy from which the FRB 180924 emission comes (on the right, digitally enhanced image)
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NASA, ESA, Alexandra Mannings (UC Santa Cruz), Wen-fai Fong (Northwestern), Alyssa Pagan (STScI)
In the Hubble study, astronomers not only linked the bursts to galaxies, but also identified the type of locations where they originated. One of them was located in 2017 and the other seven in 2019 and 2020. "We don't know what causes FRBs, so it's really important to use context when we have it," says Wen-fai Fong, of Northwestern University in Evanston. , Illinois.
The galaxies in Hubble's study existed billions of years ago. Astronomers, therefore, are seeing galaxies as they appeared when the universe was about half its current age. Many of them are as massive as our Milky Way. The observations were made in ultraviolet and near-infrared light.
Ultraviolet light traces the glow of young stars along the winding arms of a spiral galaxy. The researchers used the near-infrared images to calculate the mass of galaxies and find where the oldest populations of stars reside.
The images show a diversity of spiral arm structures, from the tightest to the most diffuse, revealing how stars are distributed along these prominent features. The spiral arms of a galaxy trace the distribution of young, massive stars. However, Hubble images reveal that the FRBs found near the spiral arms do not come from the brightest regions, which glow with the light of large stars. The images help support the idea that these pulses probably do not originate from the youngest, most massive stars.
These clues helped researchers rule out some of the possible triggers for these bright flares, including the explosive deaths of younger, more massive stars, which generate gamma-ray bursts and some types of supernovae. Another unlikely source is neutron star mergers, the crushed cores of stars that end their lives in supernova explosions. These mergers take billions of years to occur and are usually found far from the spiral arms of older galaxies that are no longer forming stars.
magnetic monsters
However, the results are consistent with the main model that FRBs originate from explosions of young magnetars, called the 'strongest magnets in the universe', since they have a magnetic field that is 10 billion times more powerful. than the magnet on a refrigerator door. Last year, astronomers linked observations of an FRB detected in our Milky Way galaxy to a region where a known magnetar resides.
"Due to their strong magnetic fields, magnetars are quite unpredictable," explains Fong. 'In this case, the FRBs are believed to come from the flares of a young magnetar. Massive stars undergo stellar evolution and become neutron stars, some of which can become strongly magnetized, leading to flares and magnetic processes on their surfaces, which can emit radio light. “Our study fits with that image and rules out very young or very old parents of FRBs,” he adds. The observations also helped researchers strengthen the association of the bursts with massive star-forming galaxies.
Although the results are exciting, researchers say they need more observations to better understand these enigmatic flashes and locate their origin more precisely. "Finding these events is an important piece of the puzzle, and a unique piece compared to what has been done before. “This is a unique contribution from Hubble,” says Fong.