In late 2018, a magnetar known as XTE J1810-197, which had been silent for a decade, suddenly became active again. This city-sized star, born from a supernova explosion, released a tremendous amount of energy in the form of gamma rays, X-rays, and radio waves as its tangled magnetic field snapped and untwisted. Astronomers have been studying magnetars to understand their erratic behavior and potential connections to fast radio bursts from distant galaxies. In two new studies published in Nature Astronomy, researchers used three of the world’s largest radio telescopes to observe never-before-seen changes in the radio waves emitted by XTE J1810-197 in unprecedented detail.
Magnetars are young neutron stars with magnetic fields billions of times stronger than Earth’s most powerful magnets. The slow decay of their magnetic fields causes stress in their outer crust, leading to fractures and the release of energetic X-rays and gamma rays. While most magnetars have only been detected as sources of X-rays and gamma rays, a few have been found to emit radio waves.
XTE J1810-197 was initially discovered as a bright source of X-rays in 2003 and was later found to emit bright pulses of radio waves as it rotated. However, the intensity of the radio pulses quickly dropped, and it remained radio silent for over a decade. In December 2018, astronomers noticed that XTE J1810-197 was emitting bright radio pulses again. This was confirmed by three telescopes, and an intense campaign was launched to track the magnetar’s radio emission.
The reactivated radio pulses were found to be highly linearly polarized, indicating the orientation of the magnetar’s magnetic field and spin direction with respect to Earth. Tracking the polarization direction revealed that the star’s spin was slowly wobbling due to the outburst, causing its surface to become slightly lumpy. The amount of lumpiness gradually disappeared within three months.
During the 2018 outburst, an unusually large amount of circular polarization was detected in XTE J1810-197’s radio waves. This “linear-to-circular conversion” had been predicted to occur when radio waves travel through the super-heated particles in neutron star magnetic fields. However, the observations did not match theoretical predictions, suggesting that there are other factors at play.
The discovery of the wobble and circular polarization in XTE J1810-197’s radio emission provides valuable insights into the behavior of radio-loud magnetars. It also contributes to a better understanding of the 2018 outburst. The cracking of the magnetar’s surface causes it to become distorted and wobble temporarily, while its magnetic field becomes filled with super-hot particles. The wobble could be used to test theories of matter behavior at high densities, and the inconsistency with theory motivates the development of more complex ideas about how radio waves escape from magnetic fields.
While XTE J1810-197 has since settled into a more relaxed state with no further signs of wobbling or linear-to-circular conversion, there are indications that these phenomena may have been observed in other radio-loud magnetars. With upgrades to telescopes, astronomers are better prepared to study magnetars when they awaken again.