An international team of astronomers have made a significant breakthrough in understanding the unprecedented behaviour of a previously dormant star with a powerful magnetic field.
Using the Lovell telescope at Jodrell Bank the researchers from the UK, Germany and Australia have shed new light on radio emission coming from a magnetar, known as XTE J1810-197.
Magnetars are a type of neutron star and the strongest magnets in the Universe. At roughly 8,000 light years away, this magnetar is also the closest known to Earth.
The magnetar is emitting light which is strongly polarised and rapidly changing. The scientists say this implies that interactions at the surface of the star are more complex than previous theoretical explanations suggest.
The results are published in two papers in the journal Nature Astronomy today.
Detecting radio pulses from magnetars is already extremely rare; XTE J1810-197 is one of only a handful known to produce them.
XTE J1810-197 was first observed to emit radio signals in 2003 before going silent for well over a decade. The signals were again detected by The University of Manchester’s 76-m Lovell telescope at the Jodrell Bank Observatory in 2018.
Since then, researchers at the University, in collaboration with institutes including the Max Planck Institute for Radio Astronomy in Germany, Australia’s national science agency CSIRO and the University of Southampton have been closely observing the magnetar.
Using the Lovell, Effelsberg and Murriyang telescopes, researchers have since noticed significant changes in the radio signals coming from the magnetar, particularly in the way the light was polarised, indicating that the magnetar’s radio beam was shifting its direction in relation to Earth.
The researchers believed this was caused by an effect called free precession where the magnetar wobbles slightly due to slight asymmetries in its structure, similar to a spinning top.
Unexpectedly, this wobbling motion decreased rapidly over a few months and until it eventually stopped altogether. This contradicts the idea proposed by many astronomers that repeating fast radio bursts could be caused by magnetars undergoing precession.
Gregory Desvignes from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and lead author of one of the two papers, said: “We expected to see some variations in the polarisation of this magnetar’s emission, as we knew this from other magnetars but we did not expect that these variations are so systematic, following exactly the behaviour that would be caused by the wobbling of the star.”
But the reason as to why the circular polarisation changes, where the light appears to spiral as it moves through space, remain uncertain.
Dr Marcus Lower, a postdoctoral fellow at CSIRO, who led the Australian research using Murriyang, CSIRO’s Parkes radio telescope, said: “Our results suggest there is a superheated plasma above the magnetar’s magnetic pole, which is acting like a polarising filter. How exactly the plasma is doing this is still to be determined.”
Papers
Desvignes, G., Weltevrede, P., Gao, Y. et al. A freely precessing magnetar following an X-ray outburst. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02226-7
Lower, M.E., Johnston, S., Lyutikov, M. et al. Linear to circular conversion in the polarized radio emission of a magnetar. Nat Astron (2024). https://doi.org/10.1038/s41550-024-02225-8
