Astronomers see bizarre, never-before-seen activities of one of the strongest magnets in the universe

Astronomers from the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav) and CSIRO have just observed bizarre, never-before-seen behavior of a radio-hard magnetar – a rare type of neutron star and one of the strongest magnets in the universe.

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

Magnetare is a rare type of rotating neutron star with some of the most powerful magnetic fields in the universe. Astronomers have detected only 30 of these objects in and around the Milky Way – most of which have been detected by X-ray telescopes after a high-energy eruption.

However, it has also been seen that a handful of these magnetars emit radio pulses similar to pulsars – the less magnetic cousins ​​of magnetars that produce radiant radio waves from their magnetic poles. Tracking how the pulses of these radio-hard magnets change over time provides a unique window into their evolution and geometry.

In March 2020, a new magnetar called Swift J1818.0-1607 (J1818 for short) was discovered after it emitted a bright X-ray burst. Rapid follow-up observations detected radio pulses emanating from the magnetar. Oddly enough, the appearance of the radio pulses from J1818 was completely different from those detected by other radio-hard magnets.

Most radio pulses of magnetars hold a constant brightness over a wide range of sensing frequencies. However, the pulses of J1818 were much brighter at low frequencies than high frequencies – similar to what is seen in pulses, another more common type of radio emitting neutron star.

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

During this time, they found that the magnetar had undergone a brief identity crisis: in May, it still radiated the unusual pulsating pulses previously detected; by June, however, it began to flicker between a bright and a weak state. This flickering behavior peaks in July, when astronomers see it flickering back and forth between pulsating and magnetic radio pulses.

“This bizarre behavior has never been seen before in any other radiohard magnetar,” explains lead author of the study and Swinburne University / CSIRO Ph.D. student Marcus Laer. “It appears to have been only a transient phenomenon, for by our next observation it has permanently settled in this new magnetar-like state.”

The scientists also searched for pulse shape and brightness changes on different radio frequencies and compared their observations with a 50-year-old theoretical model. This model predicts the expected geometry of a pulsar, based on the direction of rotation of its polarized light.

“From our observations, we found that the magnetic axis of J1818 is not in line with its axis of rotation,” says Lower. “Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, just below the equator. Most other magnetars have magnetic fields that are in line with their axis of rotation or a bit ambiguous. This is the first time we definitely saw a magnetar with a misinterpreted magnetic pole. ‘

It is striking that this magnetic geometry is stable in most observations. This indicates that changes in the pulse profile are simply due to variations in the height that the radio pulses radiate above the neutron star surface. The 1st of August^st Observation in 2020 stands as a strange exception.

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

A clear lack of changes in the shape of the pulse profile of the magnetar indicates that the same magnetic field lines that cause the ‘normal’ radio pulses must also be responsible for the pulses from the other magnetic pole.

According to the study, this is evidence that the radio pulses of J1818 come from loops of magnetic field lines connecting two close to each other poles, such as those seen connecting the two poles of a horseshoe magnet or sunspots on the sun. It is different from most common neutron stars, which are expected to have north and south poles on either side of the star connected by a donut-shaped magnetic field.

This peculiar magnetic field configuration is also supported by an independent study of the J1818 X-ray pulses detected by the NICER telescope aboard the International Space Station. The X-rays apparently come from a single distorted region of magnetic field lines emerging from the magnetar surface, or from two smaller but closely spaced regions.

These discoveries have potential implications for computer simulations of how magnetars are born and developed over long periods of time, as more complex magnetic field geometry changes how quickly their magnetic fields will decay over time. In addition, theories suggesting that rapid radio bursts originate from magnetars should take into account radio pulses that may have originated from various active sites within their magnetic fields.

Capturing a rotation between magnetic poles in action can also provide the first opportunity to map the magnetic field of a magnetar.

“The Parkes telescope will be closely monitoring the magnetar next year,” said scientist and co-author Simon Johnston of CSIRO Astronomy and Space Science.