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One of the most powerful magnets in the Cosmos came to life – and our theories can’t quite explain it

After remaining silent for a decade, one of the most powerful magnets in the cosmos abruptly burst back to life in late 2018. The reawakening of this “magnetar” named XTE J1810-197, a city-sized star born from a supernova explosion, was an extraordinarily violent affair.The snapping and untwisting of the scrambled magnetic field exerted enormous amounts of energy as X-rays, gamma rays and radio waves.By observingmagnetar outbursts like this in action, astrophysicists are beginning to understand what drives their uneven behaviour. We are also discovering potential links to mysterious flashes of radio light seen from distant galaxies called fast radio bursts

Understanding Magnetars that are virtually Magnetic monsters

Magnetars are young neutron stars, with magnetic fields billions of times powerful compared to strongest of Earth-based magnets. The gradual decay of their magnetic fields causes an enormous amount of stress in their hard outer crust until it eventually fractures. This twists the magnetic field and emits large amounts of energetic gamma rays and X-rays as it unwinds.

These exotic stars were at first detected back in 1979 when an intense gamma-ray burst released by one was picked up by spacecraft across the Solar System. Since then, we’ve discovered another 30 magnetars, the vast majority of which have only been detected as sources of gamma raysand X-rays. But, a rare few have since been found to also discharge flashes of radio waves.

The first of these “radio-loud” magnetarsis named XTE J1810-197. Astronomers at first discovered it as a bright source of X-rays post an outburst in 2003, then found out that it emitted bright pulses of radio waves as it rotated every 5.54 seconds.Unfortunately, the strength of the radio pulses fell rapidly, and in less than two years it had completely faded from view. XTE J1810-197 stayed in this radio silent state for over a decade.

A wobbly start                                                                 

On December 11 2018, astrophysicists employing the University of Manchester’s 76-metre Lovell telescope at the Jodrell Bank Observatory observed that XTE J1810-197 was once again releasing bright radio pulses. This was in no time confirmed by both Murriyang, CSIRO’s 64-metre Parkes radio telescope in Australia and Max-Planck-Institute’s 100-metre Effelsberg radio telescope in Germany.

Following corroboration, all three telescopes started an intense campaign to track how the XTE J1810-197’s radio emission then evolved over time.The reactivated radio pulses from this magnetarwere found to be highly linearly polarised, appearing to wiggle either up and down, left to right, or some blend of the two. Cautious measurements of the polarisation direction let astrophysicists to determine how the magnetar’s spin directionand magnetic field are oriented w.r.t. the Earth.

Astronomers’ meticulous tracking of the polarisation direction revealed something extraordinary: the direction of the magnetar’s spin was gradually wobbling. By comparing the measured wobble against simulations, they were able to determine that the magnetar’s surface had become somewhat lumpy due to the outburst.The extent of lumpiness was tiny, only about a millimetre off from being a perfect sphere, and slowly disappeared within three months of it waking up.

Twisted light

Usually, magnetars only release very small amounts of circularly polarised radio waves, which travel in a spiral pattern. Strangely, astrophysicists detected an enormous amount of circular polarisation in XTE J1810-197 during the 2018 outburst. Their observations with Murriyang revealed that the usually linearly polarised radio waves were being transformed into circularly polarised waves.

This “linear-to-circular conversion” had long been prophesied to occur when radio waves travel through the super-heated soup of particles that exist in neutron star magnetic fields.But, the theoretical predictions for how the effect should change with observing frequency did not match observations of astrophysicists, though that was not a surprise. The setting around a magnetar in outburst is a complex place, and there are quite a few effects that can be at play that relatively simple theories aren’t designed to account for.


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