Astronomers discover bizarre, never-before-seen activity of one of the universe’s strongest magnets

Astronomers at the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav) and CSIRO just observed bizarre and never-before-seen behavior of a radio-strong magnetar – a rare type of neutron star and one of the most powerful magnets in the universe.

Their new findings, published today in the Monthly notices from the Royal Astronomical Society (MNRAS), suggest that magnetars have more complex magnetic fields than previously thought, which may challenge the theories about how they were born and evolve over time.

Magnetars are a rare type of rotating neutron star with some of the strongest magnetic fields in the universe. Astronomers have detected only 30 of these objects in and around the Milky Way – most of them detected by X-ray telescopes following a high-energy explosion.

However, a handful of these magnetars also emitted radio pulses similar to pulsars – the less magnetic cousins ​​of magnetars that produce beams of radio waves from their magnetic poles. Monitoring the evolution of the pulses of these radio-strong magnetars over time offers a unique window on their evolution and geometry.

In March 2020, a new magnetar named Swift J1818.0-1607 (J1818 for short) was discovered after emitting a burst of light x-rays. Rapid follow-up observations detected radio pulses from the magnetar. Oddly enough, the appearance of the J1818’s radio pulses was quite different from those detected by other radio strong magnetars.

Most magnetar radio pulses maintain constant brightness over a wide range of observation frequencies. However, the pulses from J1818 were much brighter at low frequencies than at high frequencies – similar to what is seen in pulsars, another type of radio-emitting neutron star that is more common.

To better understand how J1818 would evolve over time, a team led by scientists from the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav) observed it eight times using the CSIRO Parkes radio telescope. (also known as Murriyang) between May and October 2020.

During this time, they discovered that the magnetar had suffered a brief identity crisis: in May, it was still emitting the unusual pulsar-like pulses that had been detected previously; however, by June he had started to vacillate between a clear state and a weak state. This flickering behavior peaked in July, when astronomers saw it wobble between pulsar and magnetar-like radio pulses.

“This bizarre behavior has never been seen before in any other radio-strong magnetar,” says the study’s lead author and Swinburne University / CSIRO PhD. student Marcus Lower. “It appears that this was only a short-lived phenomenon, for by our next observation it had permanently settled into this new magnetar-like state.”

Scientists also looked for pulse shapes and changes in brightness at different radio frequencies and compared their observations to a 50-year-old theoretical model. This model predicts the expected geometry of a pulsar, based on the direction of twist of its polarized light.

“From our observations, we found that the magnetic axis of the J1818 is not aligned with its axis of rotation,” says Lower. “Instead, the radio transmitting magnetic pole appears to be in its southern hemisphere, located just below the equator. Most other magnetars have magnetic fields aligned with their spin axes or a little ambiguous. is the first time we have definitely seen a magnetar with a misaligned magnetic pole. ”

Remarkably, this magnetic geometry seems stable on most observations. This suggests that any change in the pulse profile is simply due to variations in the height of the radio pulses emitted above the surface of the neutron star. However, on August 1^st The 2020 observation stands out as a curious exception.

“Our best geometric model for this date suggests that the radio beam briefly turned to a completely different magnetic pole located in the northern hemisphere of the magnetar,” Lower explains.

A distinct lack of any change in the shape of the magnetar’s pulse profile indicates that the same magnetic field lines that trigger the “normal” radio pulses must also be responsible for the pulses seen from the other magnetic pole.

The study suggests that this proves that J1818’s radio pulses originate from loops of magnetic field lines connecting two closely spaced poles, like those seen connecting the two poles of a horseshoe magnet or sunspots on the sun. This is different from most ordinary neutron stars, which should have north and south poles on opposite sides of the star which are connected by a ring-shaped magnetic field.

This particular configuration of the magnetic field is also supported by an independent study of the X-ray pulses of J1818 which were detected by the NICER telescope on board the International Space Station. The x-rays appear to come from either a single distorted region of magnetic field lines emerging from the surface of the magnetar, or two smaller but closely spaced regions.

These findings have potential implications for computer simulations of how magnetars were born and evolve over long periods of time, as more complex magnetic field geometries will change the rate at which their magnetic fields are expected to decay over time. . Additionally, theories suggesting that rapid radio bursts may originate from magnetars will need to account for radio pulses potentially originating from multiple sites active in their magnetic fields.

Catching a flip-flop between the magnetic poles in action could also offer the first opportunity to map the magnetic field of a magnetar.

“The Parkes Telescope will be keeping a close eye on the magnetar over the next year or so,” says scientist and study co-author Simon Johnston of CSIRO Astronomy and Space Science.