Can a star have a solid surface? It might sound counterintuitive. But human intuition is a response to our evolution on Earth, where up is up, down is down, and there are three states of matter. Intuition fails when it confronts the cosmos.
Magnetars are dead stars with intense magnetic fields, the most intense we know of. They’re a type of neutron star, the stellar remnants of a massive star that exploded as a supernova. Magnetars are not only highly magnetized compared to neutron stars, but they also rotate more slowly. While a magnetar might rotate once or twice every ten seconds, a neutron star can rotate as fast as ten times each second.
Magnetars are one of those cosmic objects that scientists deduced must have existed long before they found one. They were invoked to explain the existence of transient gamma-ray sources called soft gamma repeaters (SGRs.) The hypothesis is that as a magnetar’s intense magnetic field slowly decays, it will emit gamma rays and x-rays. It takes about 10,000 years for the field to decay. Now we know of at least 31 magnetars, and researchers calculate that there are about 30 million inactive magnetars in the Milky Way.
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Magnetars emit powerful x-rays and undergo erratic bursts of activity. A magnetar’s bursts and flares can mit in a single second what our Sun needs an entire year to mit. The extreme magnetic fields are responsible for this behaviour, scientists think, and they can be up to one thousand times more powerful than the magnetic fields around neutron stars.
A new study says that one of these magnetars has a solid surface and no atmosphere. It’s called 4U 0142+61, and it’s about 13,000 light years away from Earth in the Cassiopeia constellation. The study is “Polarized x-rays from a magnetar,” and it’s published in the journal Science. The lead author is Dr. Roberto Taverna, from the University of Padova (Padua), Italy.
A spacecraft launched in December 2021 made this study possible. The Imaging X-ray Polarimetry Explorer (IXPE) is a joint mission between the Italian Space Agency and NASA. As the name makes clear, the spacecraft observes the polarization of x-rays. Exotic objects like black holes, pulsars, neutron stars, and magnetars all have extreme environments that polarize x-rays. IXPE can observe these x-rays and provide insights into the objects and their environments. Understanding the exotic, powerful magnetic fields around magnetars is one of IXPE’s explicit objectives.
As this study shows, it’s paying dividends.
This study marks the first time that scientists have observed polarized x-rays from a magnetar. IXPE observed the magnetar for a total of 840 kiloseconds (about 233 hours) in January and February of 2022. What did those observations show?
First, a bit about polarized light.
Most of the light we encounter is non-polarized. That means that as the light travels, it “vibrates” in multiple planes and travels outward in multiple directions. Sunlight, electric light, and a candle flame all emit non-polarized light.
Polarized light is light that vibrates in only one plane. You’ve probably worn polarized sunglasses at one time or another. They reduce glare by filtering out light vibrating on other planes and only allowing aligned light to reach your eyes.
Since light, including x-rays, is electromagnetic energy, extremely powerful magnetic fields around magnetars can polarize light. By measuring the degree of polarity, scientists can make deductions about the magnetic fields and the objects generating them. That’s at the heart of IXPE’s mission and at the heart of this study. IXPE has three identical imaging X-ray polarimetry systems that operate independently for redundancy. IXPE creates polarization maps that reveal the structure of the magnetic fields around objects like magnetars.
As the paper’s one-sentence summary says, “The IXPE observation of 4U 0142+61 gives the first ever
measurement of polarized emission from a magnetar in x-rays.”
The researchers found a much lower proportion of polarized light than there should be if the x-rays had passed through an atmosphere. An atmosphere around the magnetar would act like a filter and allow only one polarization state of light to pass through.
The team also found that the wiggle, or angle of polarisation, flipped exactly 90 degrees for higher energies when compared to lower energies. Theoretical models of magnetars state that a solid surface surrounded by magnetic fields can produce these observations.
“The most exciting feature we could observe is the change in polarisation direction with energy, with the polarisation angle swinging by exactly 90 degrees,” said lead author Taverna. This is in agreement with what theoretical models predict and confirm that magnetars are indeed endowed with ultra-strong magnetic fields.
“This was completely unexpected,” said co-lead author Professor Silvia Zane (UCL Mullard Space Science Laboratory) and a member of the IXPE science team. “I was convinced there would be an atmosphere. The star’s gas has reached a tipping point and become solid in a similar way that water might turn to ice. This is a result of the star’s incredibly strong magnetic field.”
“But, like with water, temperature is also a factor – a hotter gas will require a stronger magnetic field to become solid,” added Zane. “A next step is to observe hotter neutron stars with a similar magnetic field, to investigate how the interplay between temperature and magnetic field affects the properties of the star’s surface.”
Quantum theory plays a role in these findings. It predicts that when light is propagated in a highly magneticized environment, it’ll be polarized in two directions: parallel to the magnetic field lines and perpendicular to them. By observing both the polarity of the light and the amount of light, scientists can understand the structure of the magnetic field itself, which imprints itself on the light and on the physical state of the matter in the region of the magnetar. According to the study, this is the only way to access that information.
Professor Roberto Turolla from the University of Padova is another of the paper’s co-authors. In a press release, Turolla said, “The polarisation at low energies is telling us that the magnetic field is likely so strong to turn the atmosphere around the star into a solid or a liquid, a phenomenon known as magnetic condensation.”
Theory also predicts that this solid surface is made of ions held together in a lattice by magnetic fields. Rather than spherical like other atoms, these ones would be elongated due to the powerful magnetic force.
Scientists still debate whether or not magnetars and other neutron stars can even have atmospheres. There’s a ton of mystery surrounding these extreme objects and their confounding nature. But at least we know of one magnetar that has no atmosphere, or at least where a solid crust is a suitable explanation.
But the explanation still needs more scrutiny, say the authors.
“It is also worth noting that including quantum electrodynamics effects, as we did in our theoretical modeling, gives results consistent with the IXPE observation,” said co-author Professor Jeremy Heyl of the University of British Columbia. “Nevertheless, we are also investigating alternative models to explain the IXPE data, for which proper numerical simulations are still lacking.”