Satellite X-ray observations: Neutron star with doughnut-shaped magnetic field and axial wobble

An artist's impression of a magnetar with an intense torroidal magnetic field in its core. 

Credit: NASA /CXC /M.Weiss

When a massive star dies, it can collapse under its own gravity with such force that it produces a supernova, leaving behind an extremely dense remnant consisting almost entirely of neutrons, a neutron star.

Some neutron stars, known as magnetar, possess powerful magnetic fields, which are stronger than any other known magnetism in the Universe.

These intense magnetic fields somehow produce high-energy x-ray pulses, but this process is not well understood.

Kazuo Makishima from RIKEN's MAXI Team and Teruaki Enoto from the RIKEN Nishina Center for Accelerator-Based Science in collaboration with the University of Tokyo and NASA have now found evidence that the magnetar 4U 0142+61 'wobbles' about its rotational axis, implying that the sphericity of the star is distorted due to an intense donut-shaped magnetic field at its core.

"Magnetars emit high-energy 'hard' x-rays, but the origins of these emissions are unknown," explains Makishima.

"We observed 4U 0142+61 using the Suzaku x-ray astronomy satellite (formerly known as Astro-E2) to find out whether the magnetar's emissions change over time."

The magnetar had previously been measured to spin at a rate of one revolution in about 8 seconds and to produce x-ray pulses of the same period, but Makishima and his co-workers noticed slow fluctuations in the arrival times of the x-ray pulses.

They attributed these fluctuations to axial wobble, known as free precession.

The star's axis precesses with a period that differs very slightly from the star's rotation period, and the slow beat between the two periods changes the observed emissions.

"The idea of free precession was not in my mind when we started the data analysis," says Makishima, "but I was familiar with it through my long experience with spinning satellites."

"The precession is most likely caused by a slight deformation of the magnetar, and the deformation is possibly due in turn to internal magnetic fields that are even stronger than the external visible fields."

The findings suggest that the magnetar is deformed from a perfect sphere due to an extremely strong, tightly wound toroidal magnetic field buried deep in the star's core.

The results therefore support the hypothesis that the hard-x-ray pulses are produced by consuming magnetic energy.

Makishima's team plans to analyze a third dataset from 4U 0142+61 and search the Suzaku data for other magnetars that might show similar effects.

"We will also propose observations of these objects with ASTRO-H, the powerful successor to Suzaku, which will be launched in 2015," he says.

More information: Makishima, K., Enoto, T., Hiraga, J. S., Nakano, T., Nakazawa, K., Sakurai, S., Sasano, M. & Murakami, H. Possible evidence for free precession of a strongly magnetized neutron star in the magnetar 4U 0142+61. Physical Review Letters 112, 171102 (2014). DOI: 10.1103/PhysRevLett.112.171102

SISSA: Simple, like a neutron star

For astrophysicists neutron stars are extremely complex astronomical objects.

Research conducted with the collaboration of SISSA and published in the journal Physical Review Letters demonstrates that in certain respects these stars can instead be described very simply and that they show similarities with black holes.

In how many ways can one describe an object?

Take an apple: by just looking at it we can easily estimate its weight, shape and colour but we are unable to describe it at any other level, for example, to evaluate the chemical composition of its flesh.

Something similar also applies to astronomical objects: until today one of the challenges facing scientists was to describe neutron stars at the nuclear physics level.

The matter these stars are made up of is in fact extremely complex, and several complicated equations of state have been proposed.

However, to date there is no agreement as to which is the correct (or the best) one.

A theoretical study conducted by SISSA (the International School for Advanced Studies of Trieste), in collaboration with Athens University, has demonstrated that neutron stars can also be described in relatively simple terms, by observing the structure of the space-time surrounding them.

"Neutron stars are complex objects owing to the matter that composes them. We can picture them as enormous atomic nuclei with a radius of about ten kilometres", explains Georgios Pappas, first author of the study carried out at SISSA.

"A neutron star is what remains of the collapse of a massive star: the matter inside it is extremely dense and mostly consisting of neutrons".

"The nuclear physics required to understand the nature of the matter contained in these astronomical objects generally makes their description very complicated and difficult to formulate," continues Pappas.

"What we have demonstrated, by using numerical methods, is that there are properties that can provide a description of some aspects of neutron stars and the surrounding space-time in a simple manner, similar to the description used for black holes".

Black holes are truly unique objects: they have lost all matter and are only made up of space and time. Just like neutron stars they are the result of the collapse of a bigger star (in this case much bigger than the stars giving rise to neutron stars) and in the implosion all the matter has been swept away.

"They are considered to be the most perfect objects in the Universe and the expression 'hairless' that was coined by John Archibald Wheeler to indicate their simplicity has become famous. According to our calculations even neutron stars can be depicted in a very similar manner".

More Information: 'Effectively universal behaviour of rotating neutron stars in general relativity makes them even simpler than their Newtonian counterparts' Authors: George Pappas and Theocharis A. Apostolatos - Phys. Rev. Lett.

New discovery could be a Thorne-Zytkow object

A neutron star. Image: NASA

Speaking at this year's American Astronomical Society meeting, Hubble Fellow, Emily Levesque reported that she and her colleagues at the University of Colorado have discovered a star that just might qualify as a Thorne-Zytkow object.

The object has not been named as yet, however, as the team has not yet published its results.

A Thorne-Zytkow object, Kip Throne and Anna Zytkow theorized back in 1975, could come to exist when a dying red giant star swallows an orbiting neutron star.

The result would be, the researchers suggested, a star with another smaller star embedded in its core and which would overall resemble other known types of stars but would emit a different and unique chemical signature.

Emily Levesque

Since that time, many space scientists have scoured the heavens looking for such an object—many candidates have been found, but thus far none have been confirmed.

In this latest effort, the found object appears to closely resemble what Thorne and Zytkow predicted.

The object is was found in the Small Magellanic Cloud—Levesque reported that thus far, the research team has confirmed that it emits molybdenum, lithium and rubidium—all elements predicted by theory to exist in abundant amounts in the theoretical object.

The original researchers suggested such elements would have to forge unusual pathways to burn their way through the dying stars outer parts due to an interruption of the fusion process in the red giant.

The object was found, Levesque also reported, as part of a survey the team was conducting on 22 objects in the cloud using one of the Magellan telescopes (and its 21 foot diameter mirror) located in Chile's Atacama Desert.

Space scientists have speculated that if theory holds, there should be several Thorne-Zytkow objects in the Milky Way, though no one has found evidence yet.

Commenting on the find, co-theorist Thorne suggested that the new discovery is the most promising yet found.

More work will have to be done before it will become known if the newly discovered specimen is truly a Thorne-Zytkow object.

Specifically, scientists will focus on the elements found in the object as thus far there appears to be a little less of it than theory suggests.

NASA Image: Neutron Star Collision

Nasa have released an artist's impression of two neutron stars colliding to produce a gamma ray burst. 

Earth was blasted by a high-energy burst of radiation from space in the 8th century, scientists believe. 

Gamma ray bursts are the most powerful explosions known in the universe. 

Each one corresponds to around a thousand Earths being vapourised into pure energy in seconds. 

Picture: Nasa

Chandra Movie of Vela Pulsar: Neutron Star Action

The Vela pulsar, a neutron star that was formed when a massive star collapsed. (X-ray: NASA/CXC/Univ of Toronto/M.Durant et al; Optical: DSS/Davide De Martin).

This movie from NASA's Chandra X-ray Observatory shows a fast moving jet of particles produced by a rapidly rotating neutron star, and may provide new insight into the nature of some of the densest matter in the universe.

The star of this movie is the Vela pulsar, a neutron star that was formed when a massive star collapsed.

The Vela pulsar is about 1,000 light years from Earth, spansis about 12 miles in diameter, and makes over 11 complete rotations every second, faster than a helicopter rotor.

As the pulsar whips around, it spews out a jet of charged particles that race out along the pulsar’s rotation axis at about 70% of the speed of light.

In this still image from the movie, the location of the pulsar and the 0.7-light-year-long jet are labeled.

The Chandra data shown in the movie, containing 8 images obtained between June and September 2010, suggest that the pulsar may be slowly wobbling, or precessing, as it spins.

"It's like having an unsecured fire hose and a flow of water at high pressure," said co-author George Pavlov, principal investigator of the Chandra proposal at Pennsylvania State University in University Park. "All you need is a small bend in the hose and violent motion can result."

"We think the Vela pulsar is like a rotating garden sprinkler, except with the water blasting out at over half the speed of light," said Martin Durant of the University of Toronto in Canada, who is the first author of the paper describing these results.

ESA XMM-Newton: Aftermath of a Supernova

Suspended in time and space, the aftermath of a massive star’s dramatic ending in a supernova explosion is captured by ESA’s XMM-Newton space observatory.

Nested knots of hot gas glowing green at X-ray wavelengths – equivalent to millions of degrees celsius – fill the structured central region of this expanding supernova remnant.

Supernova remnants are the glowing fireballs created after a massive star – greater than eight of our Suns – has exhausted its fuel supply and collapses in on itself, ejecting its remaining layers of gas in a blinding explosion.

A neutron star or black hole may remain at the heart of the explosion, obscured by the expanding shell of ejected material that also contains material swept up from the interstellar medium – the space between stars.

In this image, two bright spots at the right edge of the shell are lit up by the interaction of shock waves with the surrounding medium. This supernova remnant is only a few thousand years old – the expansion of the shock will take hundreds of thousands of years to slow down.

By studying supernova remnants at X-ray wavelengths, astronomers can identify the abundance and distribution of different elements forged during the last stages of the star’s life.

This information can provide clues about the mass of the progenitor star and the dynamics of the explosion.

Blue and white specks in and around the remnant are foreground and background stellar objects.