New observations confirm that a magnetar has a solid surface and no atmosphere

Can a star have a solid surface? This may seem counter-intuitive. 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 confronted with the cosmos.

Magnetars are dead stars with intense magnetic fields, the strongest we know of. It is a type of neutron star, the stellar remnants of a massive star that exploded as a supernova. Magnetars are not only strongly magnetized compared to neutron stars, but they also rotate more slowly. While a magnetar can spin once or twice every ten seconds, a neutron star can spin up to ten times per second.

Magnetars are one of those cosmic objects that scientists have deduced must exist long before they found one. They have been invoked to explain the existence of sources of transient gamma rays called soft gamma repeaters (SGRs). The hypothesis is that when the intense magnetic field of a magnetar slowly decays, it emits gamma rays and X-rays. It takes about 10,000 years for the field to decay. We now know of at least 31 magnetars, and researchers calculate that there are around 30 million inactive magnetars in the Milky Way.

Magnetars emit powerful X-rays and experience erratic bursts of activity. The bursts and flares of a magnetar can emit in a single second what our Sun needs an entire year to emit. Extreme magnetic fields are responsible for this behavior, scientists believe, and they can be up to a thousand times stronger than the magnetic fields around neutron stars.

A new study indicates that one of these magnetars has a solid surface and no atmosphere. It is called 4U 0142+61 and is about 13,000 light-years from Earth in the constellation Cassiopeia. The study is titled “Polarized x-rays from a magnetar” and is published in the journal Science. The lead author is Dr. Roberto Taverna, University of Padova (Padua), Italy.

“The star’s gas reached a tipping point and turned solid in the same way that water might turn into ice. This is the result of the star’s incredibly strong magnetic field.

Co-lead author, Professor Silvia Zane, UCL, member of the IXPE scientific team.

A spacecraft launched in December 2021 made this study possible. Imaging X-ray Polarimetry Explorer (IXPE) is a joint mission between the Italian Space Agency and NASA. As the name suggests, 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 information about objects and their surroundings. Understanding the powerful alien magnetic fields around magnetars is one of IXPE’s explicit goals.

As this study shows, it pays 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 2022. What do these observations show?

First, a bit about polarized light.

Most of the light we encounter is unpolarized. This means that as light travels, it “vibrates” in multiple planes and travels outward in multiple directions. Sunlight, electric light, and a candle flame all emit unpolarized light.

Polarized light is light that vibrates in a single plane. You’ve probably worn polarized sunglasses at one time or another. They reduce glare by filtering out light vibrating on other planes and allowing only aligned light to reach your eyes.

Since light, including X-rays, is electromagnetic energy, extremely strong magnetic fields around magnetars can polarize light. By measuring the degree of polarity, scientists can make inferences about magnetic fields and the objects that generate them. This is 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 magnetic fields around objects like magnetars.

As the one-sentence summary of the paper puts it, “The IXPE observation of 4U 0142+61 yields the first-ever
measurement of the polarized emission of a magnetar in X-rays.

The researchers found a much lower proportion of polarized light than it would have if the X-rays had passed through an atmosphere. An atmosphere around the magnetar would act as a filter and allow only one state of light polarization to pass.

The team also found that the agitation, or angle of polarization, tilted exactly 90 degrees for higher energies compared to lower energies. Theoretical models of magnetars indicate that a solid surface surrounded by magnetic fields can produce these observations.

“The most exciting feature we could observe was the change in direction of polarization with energy, with the angle of polarization oscillating by exactly 90 degrees,” said lead author Taverna. “This is in line with what theoretical models predict and confirms that magnetars are indeed endowed with ultra-strong magnetic fields.”

“It was completely unexpected,” said co-lead author Professor Silvia Zane (UCL Mullard Space Science Laboratory) and member of the IXPE science team. “I was convinced that there would be an atmosphere. The star’s gas reached a tipping point and turned solid in the same way that water might turn into ice. This is the result of the star’s incredibly strong magnetic field.

“But, as with water, temperature is also a factor – a hotter gas will require a stronger magnetic field to become solid,” Zane added. “A next step is to observe hotter neutron stars with a similar magnetic field, to study how the interaction between temperature and magnetic field affects the star’s surface properties.”

Quantum theory plays a role in these discoveries. He predicts that when light travels through a strongly magnetized environment, it will be polarized in two directions: parallel to the magnetic field lines and perpendicular to them. By observing both the polarity of light and the amount of light, scientists can understand the structure of the magnetic field itself, which imprints on light, and the physical state of matter in the magnetar region. . According to the study, this is the only way to access this information.

Professor Roberto Turolla of the University of Padua is another of the co-authors of the article. In a press release, Turolla said: “Low-energy polarization tells us that the magnetic field is likely so strong that it turns the atmosphere around the star into a solid or liquid, a phenomenon known as name of magnetic condensation.”

The theory also predicts that this solid surface is made up of ions held together in a lattice by magnetic fields. Rather than spherical like other atoms, these would be elongated due to the powerful magnetic force.

Scientists are still debating whether magnetars and other neutron stars can even have atmospheres. There’s a ton of mystery surrounding these extreme objects and their puzzling nature. But at least we know of a magnetar that has no atmosphere, or at least where a solid crust is a fitting explanation.

But the explanation requires even more examination, say the authors.

“It should also be noted that including the effects of quantum electrodynamics, as we did in our theoretical modeling, yields results consistent with the IXPE observation,” said co-author Professor Jeremy Heyl from 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.”

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