Fast radio bursts (FRBs) are one of the most enigmatic phenomena in astrophysics today. These incredibly brief, yet intense bursts of energy last only milliseconds to a few seconds but can release as much energy as the Sun does in several days. Most FRBs originate from beyond our galaxy, though a single one has been detected coming from within the Milky Way. The repeating nature of some FRBs only deepens the mystery surrounding their origins.
Although astrophysicists generally agree that FRBs are likely caused by a high-energy astrophysical process, the exact mechanisms behind these bursts remain uncertain. A promising avenue for further investigation lies in the study of gravitational waves (GWs)—ripples in the fabric of space-time that are caused by extremely energetic events. Scientists have recently turned their attention to one specific source of FRBs to explore potential connections between FRBs and gravitational waves, which may provide valuable insights into these mysterious bursts of energy.
The Only Confirmed Source of FRBs in Our Galaxy
One of the most intriguing discoveries in the study of FRBs came in 2020 when researchers connected an FRB to a source within the Milky Way galaxy. This source was a neutron star with an extraordinarily powerful magnetic field, known as a magnetar. The specific magnetar, designated SGR 1935+2154, was located approximately 20,000 light-years away from Earth. This discovery was groundbreaking, as it was the first time that an FRB had been linked to a known object within our galaxy.
Magnetars like SGR 1935+2154 are fascinating because they possess magnetic fields that are about 1,000 times stronger than those of typical neutron stars. These powerful magnetic fields can produce high-energy phenomena, including FRBs. Some magnetars emit FRBs repeatedly, and they also shine brightly in X-rays, further hinting at the connection between their activity and the bursts of energy. It’s believed that magnetars can experience powerful starquakes when tension in their crusts is released. These starquakes can shake the magnetar’s magnetic field, resulting in the emission of both FRBs and X-rays. But could these starquakes also generate gravitational waves, which would provide even more insight into the processes happening inside these bizarre objects?
The Search for Gravitational Waves from FRBs
In a new study published in The Astrophysical Journal, scientists have used the British-German GEO600 gravitational wave detector to investigate any potential connection between FRBs and gravitational waves. The research, titled “A Search Using GEO600 for Gravitational Waves Coincident with Fast Radio Bursts from SGR 1935+2154,” was led by A.G. Abac, a scientist from the Max Planck Institute for Gravitational Physics.
FRBs and magnetars are both extraordinarily energetic, but despite this shared trait, much remains unknown about how they work together. Could the same cosmic events that generate FRBs also produce gravitational waves? If gravitational waves could be detected alongside FRBs, it would provide compelling evidence for a common origin—such as the starquakes occurring within magnetars.
As James Lough, lead scientist of the GEO600 detector and a researcher at the Max Planck Institute for Gravitational Physics, explains, simultaneous detection of FRBs and gravitational waves from a magnetar would be a significant breakthrough. “Observing fast radio bursts and gravitational waves from a magnetar almost simultaneously would be the evidence we have been looking for for a long time,” he stated.
Lough’s team analyzed data from GEO600 during periods when SGR 1935+2154 was emitting FRBs, in hopes of detecting any accompanying gravitational waves. The GEO600 detector is part of a global network of gravitational wave observatories, and it was crucial that GEO600 continued observing while other detectors were undergoing upgrades, ensuring that valuable data wouldn’t be missed.
From April 2020 to October 2022, SGR 1935+2154 generated three episodes of FRBs, and GEO600 was actively listening for any corresponding gravitational waves. The detection of gravitational waves would have been especially significant because SGR 1935+2154 is relatively close to Earth, making any resulting waves stronger and easier to detect compared to more distant sources of gravitational waves.
No Gravitational Waves Detected, But Valuable Information Gained
Unfortunately, after thoroughly analyzing the GEO600 data, the researchers found no evidence of gravitational waves associated with the FRBs from SGR 1935+2154. While this result may seem disappointing at first glance, the absence of gravitational waves provided its own set of valuable insights. Since the magnetar is so close to Earth, the lack of detection suggests that any gravitational waves generated during the FRBs were likely much weaker than previously expected. This finding provides a better understanding of the strength of the gravitational waves that could be associated with such events.
This isn’t the first time scientists have attempted to observe gravitational waves coinciding with FRBs. Previous efforts have been made by more powerful detectors, such as LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and KAGRA. However, like the GEO600 study, these efforts have also yielded no detections. Despite the lack of direct evidence, these past searches have been useful in setting upper limits on the potential energy of gravitational waves emitted during these events.
For example, the LVK (LIGO, Virgo, and KAGRA) detectors are larger and more sensitive than GEO600, and their data revealed that the maximum possible energy of gravitational waves that could have been emitted during the FRBs observed from SGR 1935+2154 must have been up to 10,000 times smaller than previously predicted. This information is helping scientists refine their models and narrow down the range of possible sources of gravitational waves.
What’s Next for Gravitational Wave Observations?
Despite the lack of conclusive evidence, the search for a connection between FRBs and gravitational waves is far from over. The results of this study have provided important upper limits for the strength of gravitational waves associated with FRBs. Although current detectors are not sensitive enough to differentiate between the different models explaining how gravitational waves might be generated during FRB events, the ongoing observations will continue to improve our understanding.
As Lough and his colleagues note, the search for gravitational waves from FRBs is still in its early stages. With future upgrades to the LIGO, Virgo, and KAGRA observatories, the next observations of FRBs from SGR 1935+2154 may provide a clearer picture. The upcoming observations will benefit from enhanced sensitivity, which will allow for better detection of weaker gravitational waves.
The team is hopeful that the magnetar, which has been quiet for the past two years, may become active again in the near future. “Things could get exciting really soon,” says Karsten Danzmann, director at the Albert Einstein Institute (AEI) and the Institute for Gravitational Physics at Leibniz University Hannover. The international network of detectors is currently in the middle of its observing run, which will continue until June 2025. This extended period of observation presents a significant opportunity for scientists to study the connection between FRBs and gravitational waves more closely than ever before.
Conclusion: A Mystery Unfolding
The detection of fast radio bursts from SGR 1935+2154 was a monumental step in understanding the origins of these mysterious bursts of energy. Although much progress has been made in connecting FRBs with magnetars, the exact mechanism behind the generation of FRBs remains unclear. The search for gravitational waves associated with these bursts has added an exciting dimension to this research, with the potential to uncover more about the inner workings of magnetars and the events that lead to the release of FRBs.
While no gravitational waves have been detected so far, the efforts to observe them have been far from fruitless. Researchers continue to refine their models and improve their observational techniques, and with the enhanced sensitivity of upcoming gravitational wave detectors, the possibility of detecting GWs from FRBs is within reach. As we continue to explore the mysteries of the universe, the connection between FRBs and gravitational waves remains one of the most tantalizing puzzles in modern astrophysics. The coming years may well be a period of major discoveries as scientists seek to unlock the secrets behind these powerful bursts of energy.