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 from a ‘radio-loud’ magnetar – a rare type neutron star and one of the strongest magnets in the universe.
Their new findings, published in the Monthly Communications from the Royal Astronomical Society (MNRAS), suggest that magnetars have more complex magnetic fields than previously thought – which may challenge theories of how they are born and evolve over time.
Magnetars are a rare type of rotating neutron star with some of the most powerful magnetic fields in the Universe. Astronomers have only thirty of these objects in and around the Milky Way– most were detected by X-ray telescopes after a high energy burst.
However, a handful of these magnetars have also been observed to emit radio pulses similar to pulsars – the less magnetic cousins of magnetars that produce radio waves from their magnetic poles. By monitoring how the pulses of these ‘radio-loud’ magnetars change over time, this provides a unique window into their evolution and geometry.
In March 2020, a new magnetar called Swift J1818.0-1607 (abbreviated J1818) was discovered after it emitted a bright X-ray burst. Rapid follow-up observations detected radio pulses from the magnetar. Oddly, the appearance of J1818’s radio pulses was quite different from that of other radio-loud magnetars.
Most magnetar radio pulses maintain consistent brightness over a wide range of observation frequencies. However, J1818’s pulses were much brighter at low frequencies than high frequencies – similar to what is seen in pulsars, 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) observed it eight times using the CSIRO Parkes radio telescope (aka Murriyang) between May and October 2020.
During this time, they discovered that the magnetar was undergoing a brief identity crisis: in May, it was still broadcasting the unusual pulsar-like pulses previously detected; however, in June it began to flicker between a bright and a dim state. This flickering behavior peaked in July, where they saw it flash back and forth between emitting pulsar-like and magnetar-like radio pulses.
“This bizarre behavior has never been seen before in any other radio-loud magnetar,” explains lead author of the study and Swinburne University / CSIRO PhD student Marcus Lower. “It appears to have been only a momentary phenomenon, because on our next observation it had entered this new magnetar-like state permanently.”
The scientists also looked for changes in pulse shape and brightness at 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 rotating direction of its polarized light.
“From our observations, we found that J1818’s magnetic axis is not aligned with its axis of rotation,” says Lower.
Instead, the radio-emitting magnetic pole appears to be in the southern hemisphere, just below the equator. Most other magnetars have magnetic fields that are aligned with their axes of rotation or are a bit ambiguous. “
“This is the first time we’ve definitively seen a magnetar with a misaligned magnetic pole.”
Remarkably, this magnetic geometry turns out to be stable in most observations. This suggests that changes in the pulse profile are simply due to variations in the height at which the radio pulses are emitted above the surface of the neutron star. However, the August 1, 2020 sighting stands out as a curious exception.
“Our best geometric model for this date suggests that the radio beam flipped briefly to a completely different magnetic pole in the northern hemisphere of the magnetar,” says Lower.
A marked lack of changes 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 is evidence that J1818’s radio pulses come from loops of magnetic field lines connecting two closely spaced poles, such as those seen between the two poles of a horseshoe magnet or sunspots on the sun. This is in contrast to most common neutron stars, which are expected to have north and south poles on either side of the star that are connected by an annular magnetic field.
This peculiar magnetic field configuration is also supported by an independent study of the X-ray pulses from J1818 detected by the NICER telescope aboard the International Space Station. The X-rays appear to come from either a single distorted region of magnetic field lines emerging from the magnetar surface, or two smaller but closely spaced regions.
These discoveries have potential implications for computer simulations of how magnetars are born and evolve over long periods of time, as more complex magnetic field geometries will change how quickly their magnetic fields are expected to decay over time. In addition, theories suggesting that fast radio bursts may come from magnetars will have to consider radio pulses that may come from multiple active locations within their magnetic fields.
Capturing a flip between magnetic poles in action could also provide the first opportunity to map a magnetar’s magnetic field.
“The Parkes telescope will keep a close eye on the magnetar for the next year,” said scientist and co-author Simon Johnston of the 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, December 14, 2020, Monthly Communications from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa3789