Fast Radio Bursts (FRBs) are mysterious pulses of energy that can last from a fraction of a millisecond to about three seconds. Most of them come from outside the galaxy, although one has been detected coming from a source inside the Milky Way. Some of them also repeat, which only adds to their mystery.
Though astrophysicists think that a high-energy astrophysical process is the likely source of FRBs, they aren’t certain how they’re generated. Researchers used gravitational waves (GWs) to observe one nearby, known source of FRBs to try to understand them better.
The only confirmed FRB source in the Milky Way is a neutron star with a powerful magnetic field—a magnetar—named SGR 1935+2154. Its FRB was detected in 2020 and was the first one to be connected to a source. Though SGR 1935+2154 is around 20,000 light-years away, it’s still close enough to be studied.
In new research in The Astrophysical Journal, scientists used the British-German GEO600 gravitational wave detector to probe any connections between the FRBs and gravitational waves. The research is “A Search Using GEO600 for Gravitational Waves Coincident with Fast Radio Bursts from SGR 1935+2154,” and the lead author is A. G. Abac. Abac is from the Max Planck Institute for Gravitational Physics.
FRBs are extraordinarily energetic, and so are magnetars. Connecting an FRB with the magnetar SGR 1935-2154 is a big step in understanding FRBs, although there are still a whole host of unanswered questions. Some magnetars repeatedly emit FRBs and also glow in X-rays. Magnetars can experience powerful star quakes when tension in their crusts is released, and the released energy shakes the magnetar’s magnetic field, releasing the FRBs and X-rays. Researchers have wondered if those same quakes might generate gravitational waves.
Can observing the magnetar for GWs open a window into magnetars and the processes that generate FRBs?
“Observing fast radio bursts and gravitational waves from a magnetar at almost simultaneously would be the evidence we have been looking for for a long time,” said James Lough, lead scientist of the German-British gravitational-wave detector GEO600 at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute; AEI) in Hanover. A simultaneous observation of FRBs and GWs could confirm the common origin in the stellar quakes generated by the neutron star. “That’s why we worked with an international team to analyze data we took with GEO600 while a magnetar on our cosmic doorstep was emitting fast radio bursts,” adds Lough.
If the magnetar is generating GWs, they’ll be strong when they reach our detectors, and their effects should be easier to observe. Between April 2020 and October 2022, SGR 1935+2154 generated three episodes of FRBs, and GEO600 was listening. The GW detector is part of the global network of GW detectors.
“It was essential that GEO600 could continue observing while all the other detectors were in an upgrade phase,” explained Lough. “Otherwise, we would have missed the opportunity of having gravitational-wave data during these fascinating events occurring so close to us.”
Unfortunately, careful analysis of GEO600’s data showed no evidence of GWs. However, the detector’s observations were still valuable. Since the magnetar is so close to us, even the lack of detection provided some new information.
This isn’t the first time that scientists have used GW detectors to search for GWs emitted simultaneously with FRBs, as well as for GWs from magnetar bursts and pulsar glitches. Different researchers have used the more powerful LIGO, Virgo, and KAGRA (LVK) collaborations to find them without success. “While no detections were found in these studies, the searches have established upper limits on GW energy that may have been emitted in association with these events,” the authors write in their research.
The LVK detectors are larger and more powerful than GEO600. Their data shows that the maximum possible gravitational-wave energy that could have been emitted during the magnetar’s 2020 to 2022 FRBs without being detected must have been up to 10,000 times smaller than astronomers had concluded from previous studies.
Different models explain how GWs are produced in FRBs, and the GW observations aren’t yet sensitive enough to distinguish between them. However, by establishing limits for the strength of the GWs, the GW observations are still providing information that is helping scientists refine their models.
The attempt to link GWs and FRBs is really only beginning. While LIGO/Virgo weren’t able to observe the magnetar during its last FRBs, they will hopefully be operational during the next episode. This time, their effectiveness and sensitivity will have been upgraded.
For a long time, astrophysicists have theorized that magnetars are the source of FRBs, and the detection of FRBs from SGR 1935+2154 confirms this, at least for some FRBs. However, the exact mechanism behind their generation remains elusive. “The relationship between these magnetar bursts and FRBs is poorly understood, but are likely to be caused by different physical processes, even if the underlying magnetar behaviour may be related,” the authors write in their conclusion.
If future GW observations of the magnetar with the upgraded LIGO/Virgo and KAGRA observatories can show that GWs are emitted simultaneously with FRBs, that will be a huge development. “Given the increased sensitivity of these detectors compared to GEO600, any SGR 1935+2154 FRB during the remainder of O4 (Observing Run 4) could provide another opportunity to probe the GW-FRB connection,” the authors of the study explain.
“Things could get exciting really soon. We hope that the magnetar, which has been quiet for two years and has not emitted any radio bursts, will become active again in the next few months,” says Karsten Danzmann, director at the AEI and director of the Institute for Gravitational Physics at Leibniz University Hannover. The international detector network is partway through an observing run that will continue until June 2025. “With the data from the more sensitive instruments, we will be able to look even more closely whether the fast radio burst of magnetars are accompanied by gravitational waves and thus perhaps solve a very old mystery,” says Danzmann.