On the outskirts of the Milky Way, one of the rarest kinds of stars in the galaxy has just become even more mysterious than it was before.
Astronomers have used the Hubble and Gaia telescopes to study the surroundings of SGR 0501+4516, a type of neutron star known as a magnetar. The investigation reveals that we still have no clear idea of how magnetars form – the lead we thought we had on their birth mechanism is completely unrelated to SGR 0501+4516.
However, what the researchers did, or rather, did not find, suggests that we may have been wrong about how we thought magnetars came about.
Neutron stars are among the densest objects in the Universe, beaten out only by black holes, and they form in a similar way. When a massive star runs out of fuel to fuse in its core, its core can no longer support itself by the outward pressure of fusion, and collapses under gravity in a violent event known as a core-collapse supernova.
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A magnetar is pretty much the same thing, with an added distinction: the magnetic field of a magnetar is the most powerful known in the Universe, around a thousand times more powerful than a normal neutron star’s magnetic field, and a quadrillion times more powerful than Earth’s.
It’s not clear how magnetars form, but, because they are a subspecies of neutron star, astronomers had thought that they must form from core-collapse supernovae too. SGR 0501+4516 appeared to be proof of this.
When massive stars go supernova, the evidence hangs around for some time after in the form of a supernova remnant. SGR 0501+4516’s position is very close to a supernova remnant called HB9. In addition, no other neutron stars have been detected in HB9’s vicinity. So astronomers had thought that the two objects were related, which is honestly a pretty fair assumption.
Now, the combined observations of the Hubble Space Telescope and the recently retired Gaia mission have cast significant doubt on this assumption.
Gaia was a space telescope whose mission was to precisely map the objects within the Milky Way galaxy using precision parallax measurements, including positions in three dimensions and proper motions. Hubble images taken using Gaia data as a reference frame enabled a research team led by astronomer Ashley Chrimes of the European Space Agency to very finely map the movement of SGR 0501+4516 in the sky.
The velocity and proper motion of the magnetar were such that there is no way it could be associated with HB9. In addition, there are no other supernova remnants nearby that could be related to SGR 0501+4516.
This could mean one of several things.
The first is that the magnetar, thought to be around 20,000 years old, is actually far older – old enough for its associated supernova remnant to have dissipated. The problem with this is that magnetars are thought to be a temporary phase in the life of a neutron star, lasting a few tens of thousands of years before settling down into a more staid existence.
The other option is that SGR 0501+4516 did not form via core-collapse supernova, but a merger of some kind. This could involve two low-mass neutron stars colliding; or it could be something else, a white dwarf. That’s a step down from neutron stars on the density scale, an object that forms from the collapsed core of a low-mass star, rather than a massive one.
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White dwarfs commonly have binary companions from which they slurp mass. If a white dwarf slurps up too much mass, it becomes unstable.
“Normally, this scenario leads to the ignition of nuclear reactions, and the white dwarf exploding, leaving nothing behind,” explains astronomer Andrew Levan of Radboud University in the Netherlands and the University of Warwick in the UK.
“But it has been theorised that under certain conditions, the white dwarf can instead collapse into a neutron star. We think this might be how SGR 0501 was born.”
It’s difficult to gauge, really. What does seem clear, however, is that a core-collapse supernova is now the least likely explanation for the magnetar’s formation, making SGR 0501+4516 the best candidate out of the fewer-than-30 magnetars in the Milky Way for a non core-collapse formation pathway.
And that is incredibly cool.
“Magnetar birth rates and formation scenarios are among the most pressing questions in high-energy astrophysics,” says astronomer Nanda Rea of the Institute of Space Sciences in Spain, “with implications for many of the Universe’s most powerful transient events, such as gamma-ray bursts, superluminous supernovae, and fast radio bursts.”
The findings have been published in Astronomy & Astrophysics.
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