New observations of a highly unusual and mysterious star about 15,000 light-years from Earth have revealed a bizarre pattern of stellar activity that astronomers say they have never witnessed before.
The star in question is called Swift J1818.0–1607 and was only discovered last year. Swift J1818.0–1607 is what’s known as a magnetar – a rare race of neutron stars that form when supergiant stars don’t supernova but instead collapse into incredibly dense cores.
Unlike most neutron stars, magnetars are known to produce an extremely powerful magnetic field. Only about 30 of these strange objects have ever been detected in the Milky Way, but even among their strange kind, Swift J1818.0–1607 is an unusual outlier.
That’s because only a handful of known magnetars have ever been observed emitting radio waves similar to pulsars – a different type of neutron star, much more common than magnetars, but nevertheless notable for the way they emit pulses of radiation from their magnetic poles.
However, in the midst of this exclusive clade of ‘radio-loud’ magnetars, it has never been observed to pulse in the same way as Swift J1818.0-1607, leading some in the astronomy community to suggest it might be some sort of ‘missing link’. between magnetars and pulsars.
Now, a series of new observations led by astronomers at the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav) in Australia do not suggest that Swift J1818.0–1607’s reputation for strangeness is undeserved.
In eight observations with the CSIRO’s Parkes radio telescope over a five-month period in 2020, the researchers saw the magnetar’s radio pulses clearly change in character – resembling the pulses of a pulsar in May, then changing to a different form of bright / flicker faintly in June. , before adopting a mysterious mix of both pulsar-like and magnetar-like radio pulses in July.
Artist’s impression of Swift J1818.0–1607. (OzGrav / Carl Knox)
In the researchers’ new study, they describe this apparent identity crisis in rather austere scientific terms, saying Swift J1818.0–1607 “[exhibited] significant evolution of the temporal profile during this period “.
But don’t let that language fool you into thinking that this wasn’t something out of the ordinary that you witnessed.
“This bizarre behavior has never been seen before in any other radio-loud magnetar,” explains lead author of the study, Marcus Lower of Swinburne University and CSIRO.
“It appears to have been only a momentary phenomenon, because on our next observation it had entered this new magnetar-like state permanently.”
While the mixed messages relayed by Swift J1818.0–1607 cannot be fully explained, the researchers suggest the fluctuations may represent a form of stellar evolution that we do not yet fully understand.
In part, that’s because, while this magnetar may be unique (for now), the truth is that magnetars in general, let alone radio-loud magnetars, still represent a very young field of study.
“This raises a number of questions,” explains Lower The Sydney Morning Herald.
“Perhaps this magnetar evolved over time from a more regular pulsar … or maybe we miss other magnetars in the Milky Way because they are so far from us that the low-frequency radio waves we see are too scattered for us to understand. to detect. “
In other words, this seemingly bizarre behavior may be more common than we know, it’s just that we are limited in what we currently understand about magnetars. Yet we are discovering more and more.
Swift’s new observations J1818.0–1607 suggest that the magnetic axis of the magnetar is not aligned with its axis of rotation, but is instead diving towards the Southern Hemisphere. If so, that’s a first for a magnetar – whose magnetic fields are usually aligned with their axis of rotation.
But it could also explain some of the observed changes in the radio emission profile, possibly due to radio pulses emitted at different heights around the surface of the neutron star.
However, at least one data point – an observation called MJD 59062 on August 1 last year – doesn’t match that version of events. But the team also has a hypothesis for MJD 59062.
“Our best geometric model for this date suggests that the radio beam changed briefly to a completely different magnetic pole in the northern hemisphere of the magnetar,” says Lower.
The researchers say continued monitoring of Swift J1818.0–1607 will help us find out what’s really going on here.
“We’re looking closely at our data to try to capture one of those flips as it happens, because if we can do that, we might be able to map the magnetic field between the magnetic poles,” Lower said. The Sydney Morning Herald.
“Knowing the actual geometry of magnetars is also very important in neutron star theory and being able to predict how they will evolve over time.”
The findings are reported in Monthly Communications from the Royal Astronomical Society.