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New observations of a very unusual and mysterious star located 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 a so-called magnetar – a rare breed of neutron stars, which forms when supergiant stars fail to supernova, instead of collapsing into incredibly dense nuclei.
Unlike most neutron stars, however, magnetars are known to produce an extremely strong magnetic field. Only about 30 of these strange objects have ever been detected in the Milky Way, but even among their ilk, Swift J1818.0-1607 is an unusual outlier.
This is because only a handful of known magnetars have ever been observed emitting radio waves of a type similar to pulsars – another type of neutron star, much more common than magnetars, but remarkable nonetheless for the way that they emit pulses of radiation from their magnetic. posts.
Amid this exclusive clade of “strong radio” magnetars, however, none have ever been observed pulsing in the same way as Swift J1818.0-1607, leading some in the astronomical community to suggest that it could represent a kind of “missing link” between magnetars and pulsars.
Now, a series of new observations by astronomers at the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav) in Australia do nothing to suggest that Swift J1818.0-1607’s reputation for oddity doesn’t is not deserved.
In eight observations with CSIRO’s Parkes radio telescope over a five-month period in 2020, the researchers observed that the magnetar’s radio pulses changed distinctly in character – resembling pulses of a pulsar in May, then changing to a different shape from bright / weak flicker in June. , before adopting a mysterious mixture of pulsar and magnetar-type 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 sober scientific terms, saying Swift J1818.0-1607 “[exhibited] significant change in the temporal profile over this period “.
But don’t let that language fool you into thinking it wasn’t an extraordinary thing to be a witness.
“This bizarre behavior has never been seen before in any other radio-strong magnetar,” says lead author of the study, Marcus Lower of Swinburne University and CSIRO.
“It appears that this was only a short-lived phenomenon because by our next sighting it had permanently settled into this new magnetar-like state.”
Although the mixed messages relayed by Swift J1818.0–1607 cannot be fully explained, the researchers suggest that the fluctuations may represent a form of stellar evolution that we do not yet fully understand.
In part, this is because while this magnetar may be one of a kind (for the moment), the truth is that magnetars in general, not to mention radio-strong magnetars, are still an area of study. very young.
“This raises a number of questions,” Lower explained to The Sydney Morning Herald.
“Maybe this magnetar has evolved from a more regular pulsar over time … or maybe we are missing other magnetars in the Milky Way because they are so far away from us that the The low frequency radio waves we see are too scattered for us to detect. “
In other words, this seemingly bizarre behavior might be more habitual than we know, it’s just that we’re limited in what we currently understand about magnetars. Yet we are discovering more all the time.
New observations from Swift J1818.0–1607 suggest that the magnetar’s magnetic axis is not aligned with its axis of rotation, but instead plunges into its southern hemisphere. If so, this is a first for a magnetar – whose magnetic fields are generally aligned with their spin axis.
But it could also explain some of the observed changes in the radio emission profile, potentially reflecting the 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 of last year – does not match this 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 briefly turned to a completely different magnetic pole located in the northern hemisphere of the magnetar,” Lower explains.
Researchers say that continuous monitoring of Swift J1818.0–1607 will help us understand what’s really going on here.
“We are looking closely at our data to try to capture one of these reversals while it is happening, because if we can do that, we could potentially 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 the theory of neutron stars and being able to predict how they will evolve over time.”
The results are reported in Monthly notices from the Royal Astronomical Society.
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