Understanding mysterious magnetars and radio bursts with gravitational waves
Data from the German-British GEO600 detector helps to understand the formation processes of these extreme cosmic events.
A flash of radio waves lasting a few thousandths of a second, as bright as millions or billions of stars, and it’s all over: even almost 20 years after their discovery, fast radio bursts remain one of the most mysterious phenomena in our Universe. Scientists believe that neutron stars – very small and extremely dense stellar remnants with huge magnetic fields – emit these bursts. An international team has now used gravitational waves to study a nearby neutron star that has emitted several radio bursts. The researchers analyzed data from the German-British GEO600 detector to learn more about the origin of these events. Their results contribute to a better understanding of these extreme events and their theoretical description.
Special radio telescopes regularly observe fast radio bursts from the depths of the cosmos, far outside the Milky Way. Astronomers believe that neutron stars are the source of these bursts. Neutron stars are the remnants of supernova explosions and pack a lot of mass of their parent star into a sphere only about 20 kilometers in diameter. This means that a teaspoonful of such stellar remnants would weigh more than a billion tons on Earth.
How neutron stars emit fast radio bursts is not yet fully understood, but researchers believe that their extremely strong magnetic fields play a central role. Among the neutron stars with the strongest fields – called magnetars – there are some that repeatedly emit radio bursts while also glowing in X-rays. This is thought to be caused by quakes that shake the star when tensions in its crust are released. The energy liberated in the process has a double effect: it not only shakes the neutron star's magnetic field, triggering the radio bursts and X-rays, but also causes the entire stellar remnant to vibrate.
According to Einstein’s general theory of relativity, accelerated motion of mass such as that of the vibrating neutron star generates gravitational waves. These ripples in space-time travel at the speed of light and minutely stretch and squeeze space. For more than nine years, kilometer-sized detectors around the globe have been regularly and routinely recording these tiny effects with laser light, observing our Universe in a new way.
“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,” says 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. This could confirm the common origin in the neutron star’s stellar quakes. “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 fasts radio bursts,” adds Lough.
The magnetar called SGR 1935+2154 is located in our Milky Way about 20,000 light-years away. This close to Earth, possible gravitational waves are comparatively strong and their effects are easier to observe. Between the late April 2020 and mid-October 2022, SGR 1935+2154 emitted fast radio bursts in three episodes. At all times, GEO600, the technology forge of the international collaboration, was listening into space.
“It was crucial that GEO600 continued to observe while all the other detectors were in an upgrade phase,” explains Lough. “This was the only way we could get data from a gravitational-wave detector during periods of strong magnetar activity.”
After carefully analyzing all the GEO600 data around the times of the fast radio bursts, the research team found no evidence of gravitational waves. However, because the distance to SGR 1935+2154 is so small, the lack of detection also provided new insights: The maximum possible gravitational-wave energy that could have been emitted during the fast radio bursts without being detected must have been up to 10,000 times smaller than astronomers had concluded from previous studies. These were based on data from the larger and more sensitive LIGO and Virgo detectors.
The gravitational-wave observations are not yet sensitive enough to distinguish between the different models for the generation of gravitational waves in fast radio bursts. But they are already providing information that is helping theoretical physicists to refine their models of these extreme cosmic events.
“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 current observing run of the international detector network 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.