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

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

Can a star have a solid surface? It may 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 faced with the cosmos.

Magnetars are dead stars with intense magnetic fields, the most intense known. They are a type of neutron star, the stellar remnants of a massive star that exploded as a supernova. Compared to neutron stars, magnetars are not only highly magnetized, but also rotate more slowly. While a magnetar can rotate once or twice every ten seconds, a neutron star can rotate up to ten times per second.

Magnetars are one of those cosmic objects that scientists concluded must have existed long before they found one. They have been invoked to explain the existence of transient gamma-ray sources called soft gamma repeaters (SGRs). The hypothesis is that a magnetar’s intense magnetic field, as it slowly decays, 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 about 30 million inactive magnetars in the Milky Way.

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Magnetars emit powerful X-rays and are subject to erratic bursts of activity. A magnetar’s explosions and flares can emit in a single second what our sun takes a year to emit. According to scientists, the extreme magnetic fields, which can be up to a thousand times stronger than the magnetic fields around neutron stars, are responsible for this behavior.

According to a new study, one of these magnetars has a solid surface and no atmosphere. It’s called 4U 0142+61 and is about 13,000 light-years from Earth in the constellation of Cassiopeia. The study is called “Polarized X-rays from a Magnetar” and was published in the journal Science. The main author is Dr. Roberto Taverna from the University of Padova (Padua), Italy.

“The star’s gas has reached an inflection point and is solidifying, much like water might become ice. This is due to the star’s incredibly strong magnetic field.”

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

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 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 insights into the objects and their surroundings. Understanding the exotic, strong magnetic fields around magnetars is one of IXPE’s stated goals.

A SpaceX Falcon 9 rocket carries NASA's Imaging X-ray Polarimetry Explorer (IXPE) spacecraft onboard from Launch Complex 39A Thursday, December 9, 2021 at NASA's Kennedy Space Center in Florida.  The IXPE spacecraft is the first satellite dedicated specifically to measuring the polarization of X-rays from a variety of cosmic sources such as black holes and neutron stars.  Launch occurred at 1 a.m. EST.  Credit: NASA/Joel Kowsky
A SpaceX Falcon 9 rocket carries NASA’s Imaging X-ray Polarimetry Explorer (IXPE) spacecraft onboard from Launch Complex 39A Thursday, December 9, 2021 at NASA’s Kennedy Space Center in Florida. The IXPE spacecraft is the first satellite dedicated specifically to measuring the polarization of X-rays from a variety of cosmic sources such as black holes and neutron stars. Launch occurred at 1 a.m. EST. Credit: NASA/Joel Kowsky

As this study shows, it pays off.

This study is the first time 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 did these observations show?

First a bit about polarized light.

Most of the light we encounter is not polarized. This means that the light “vibrates” in multiple planes as it travels, traveling outward in multiple directions. Sunlight, electric light, and a candle flame all emit non-polarized light.

Polarized light is light that oscillates in only one plane. Chances are you’ve worn polarized sunglasses before. They reduce glare by filtering out light that vibrates at other levels and only allowing focused light to reach your eyes.

Because light, including X-rays, is electromagnetic energy, extremely strong magnetic fields around magnetars can polarize light. By measuring the degree of polarity, scientists can draw conclusions about the magnetic fields and the objects generating them. This is at the heart of IXPE’s mission and at the heart of this study. IXPE has three identical X-ray polarimetry imaging systems that work independently of each other for redundancy reasons. IXPE creates polarization maps that show the structure of the magnetic fields around objects like magnetars.

An artist's rendering of the IXPE spacecraft.  Photo credit: HEASARC (High Energy Astrophysics Science Archive Center).
An artist’s rendering of the IXPE spacecraft. Photo credit: HEASARC (High Energy Astrophysics Science Archive Center.)

As the one-sentence summary of the paper says: “The IXPE observation of 4U 0142+61 provides the very first
Measuring the polarized emission of a magnetar in X-rays.”

The researchers found a much lower proportion of polarized light than would have been the case if the X-rays had passed through an atmosphere. An atmosphere around the magnetar would act like a filter, letting only one polarization state of light through.

The team also found that the wobble, or polarization angle, rotated exactly 90 degrees at higher energies 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 have observed is the change in polarization direction with energy, with the polarization angle swinging by exactly 90 degrees,” said lead author Taverna. “This agrees with the predictions of theoretical models and confirms that magnetars are indeed endowed with ultra-strong magnetic fields.”

A diagram of the IXPE spacecraft.  Photo credit: From NASA - https://wwwastro.msfc.nasa.gov/ixpe/for_scientists/presentations/20170601_huntsville.pdf, public domain, https://commons.wikimedia.org/w/index.php?curid=62263364
A diagram of the IXPE spacecraft. Photo credit: From NASA – https://wwwastro.msfc.nasa.gov/ixpe/for_scientists/presentations/20170601_huntsville.pdf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=62263364

“This was totally 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 has reached an inflection point and is solidifying, much like water might turn to ice. This is due to the star’s incredibly strong magnetic field.”

“But like water, temperature is also a factor — a hotter gas requires a stronger magnetic field to solidify,” Zane added. “A next step is to observe hotter neutron stars with a similar magnetic field to study how the interplay of 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 propagates in a highly magnetized environment, it will become 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 on the light and the physical state of matter in the region of the magnetar. According to the study, this is the only way to get this information.

Magnetars can have complex magnetic fields, and IXPE is a powerful tool for observing them.  IXPE creates polarization maps of objects like magnetars, which is the only way to reveal their structure.  This image is an artist's impression of a magnetar with a very complicated magnetic field inside and a simple small dipole field outside.  Credits: ESA - Author: Christophe Carreau
Magnetars can have complex magnetic fields, and IXPE is a powerful tool for observing them. IXPE creates polarization maps of objects like magnetars, which is the only way to reveal their structure. This image is an artist’s impression of a magnetar with a very complicated magnetic field inside and a simple small dipole field outside. Credits: ESA – Author: Christophe Carreau

Professor Roberto Turolla from the University of Padova is another co-author of the paper. In a press release, Turolla said, “The polarization at low energies 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 magnetic condensation.”

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

Scientists are still debating whether or not magnetars and other neutron stars can have atmospheres at all. There is a lot of mystery surrounding these extreme objects and their perplexing nature. But at least we know of a magnetar that has no atmosphere, or at least where a solid crust is a reasonable explanation.

But the explanation needs further investigation, the authors say.

“It’s also worth noting that including quantum electrodynamic effects, as we did in our theoretical modeling, yields results that are compatible 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 suitable numerical simulations are still lacking.”

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