Artist’s impression of the active magnetar Swift J1818.0-1607. Credit: Carl Knox, OzGrav
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-loud’ magnetar – a rare species neutron star and one of the strongest magnets in the universe.
Their new findings, published 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 develop 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 thirty of these objects in and around the country Milky Way—Most of them are detected by X-ray telescopes after a high-energy burst.
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-loud’ 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 that of other radio-hard magnets.
Most radio pulses from 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 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 pulsar– such as pulses previously detected; by June, however, it began to flicker between a bright and a weak state. This flickering behavior peaks in July, where they are seen swinging back and forth between the emission of pulsating and magnetar-like radio pulses.
“This bizarre behavior has never been seen before in any other radio-hard magnetar,” explains the study’s lead author and Swinburne University / CSIRO PhD student Marcus Lower. “It appears to have been only a transient phenomenon, as it was permanently established in this new magnetarous state by our next observation.”
The scientists also looked at pulse shape and brightness changes on different radio frequencies and compared their observations with a 50-year 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 have 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 magnetares have magnetic fields that are in line with their axis of rotation or slightly ambiguous. ”
“This is the first time we’re definitely seeing a magnetar with the wrong magnetic pole.”
It is striking that this magnetic geometry is stable in most observations. This indicates that any changes in the pulse profile are simply due to variations in the height that the radio pulses radiate above the neutron star surface. However, the observation on 1 August 2020 stands out 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 with magnetic field lines connecting two closely spaced 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 that are expected to have north and south poles on either side of the star that are 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 appear to come from a single distorted region of magnetic field lines emerging from the magnetar surface, or from two smaller but narrowly 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.
Reference: “The dynamic magnetosphere of Swift J1818.0–1607” by ME Lower, S Johnston, RM Shannon, M Bailes and F Camilo, 14 December 2020, Monthly notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa3789