Fast Radio Bursts (FRBs) are millisecond-duration events detected from beyond our Milky Way Galaxy. FRB emission characteristics favor magnetars as their sources, as evidenced by FRB-like bursts from a magnetar in the Milky Way, and the star-forming nature of FRB host galaxies. However, the processes that produce FRB sources remain unknown. According to new research, FRBs are more likely to occur in massive star-forming galaxies. The study also suggests that magnetars, whose magnetic fields are 100 trillion times stronger than Earth’s, often form when two stars merge and later blow up in a supernova.
This photo montage shows the antennas of the Deep Synoptic Array-110, which are used to discover and pinpoint the locations of fast radio bursts (FRBs). Above the antennas are images of some of the FRB host galaxies as they appear on the sky. The galaxies are remarkably large, challenging models that describe FRB sources. Image credit: Annie Mejia / Caltech.
“The immense power output of magnetars makes them some of the most fascinating and extreme objects in the Universe,” said lead author Kritti Sharma, a graduate student at Caltech.
“Very little is known about what causes the formation of magnetars upon the death of massive stars. Our work helps to answer this question.”
To search for FRBs, Sharma and colleagues used the Deep Synoptic Array-110 (DSA-110) at the Owens Valley Radio Observatory near Bishop, California.
To date, the sprawling radio array has detected and localized 70 FRBs to their specific galaxy of origin (only 23 other FRBs have been localized by other telescopes).
In the current study, the researchers analyzed 30 of these localized FRBs.
“DSA-110 has more than doubled the number of FRBs with known host galaxies. This is what we built the array to do,” said Caltech’s Dr. Vikram Ravi.
Although FRBs are known to occur in galaxies that are actively forming stars, the authors, to their surprise, found that the FRBs tend to occur more often in massive star-forming galaxies than low-mass star-forming galaxies.
This alone was interesting because the astronomers had previously thought that FRBs were going off in all types of active galaxies.
With this new information, they started to ponder what the results revealed about FRBs.
Massive galaxies tend to be metal-rich because the metals in our Universe — elements that are manufactured by stars — take time to build up over the course of cosmic history.
The fact that FRBs are more common in these metal-rich galaxies implies that the source of FRBs, magnetars, are also more common to these types of galaxies.
Stars that are rich in metals — which in astronomical terms means elements heavier than hydrogen and helium — tend to grow larger than other stars.
“Over time, as galaxies grow, successive generations of stars enrich galaxies with metals as they evolve and die,” Dr. Ravi said.
What is more, massive stars that explode in supernovae and can become magnetars are more commonly found in pairs.
In fact, 84% of massive stars are binaries. So, when one massive star in a binary is puffed up due to extra metal content, its excess material gets yanked over to its partner star, which facilitates the ultimate merger of the two stars.
These merged stars would have a greater combined magnetic field than that of a single star.
“A star with more metal content puffs up, drives mass transfer, culminating in a merger, thus forming an even more massive star with a total magnetic field greater than what the individual star would have had,” Sharma said.
In summary, since FRBs are preferentially observed in massive and metal-rich star-forming galaxies, then magnetars (which are thought to trigger FRBs) are probably also forming in metal-rich environments conducive to the merging of two stars.
The results therefore hint that magnetars across the Universe originate from the remnants of stellar mergers.
In the future, the team hopes to hunt down more FRBs and their places of origin using DSA-110, and eventually the DSA-2000, an even bigger radio array planned to be built in the Nevada desert and completed in 2028.
“This result is a milestone for the whole DSA team. A lot of the authors on this paper helped build the DSA-110,” Dr. Ravi said.
“And the fact that the DSA-110 is so good at localizing FRBs bodes well for the success of DSA-2000.”
The findings were published today in the journal Nature.
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K. Sharma et al. 2024. Preferential occurrence of fast radio bursts in massive star-forming galaxies. Nature 635, 61-66; doi: 10.1038/s41586-024-08074-9
