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

Evidence of Toroidal Magnetic field surrounding Magnetar

Credit: L. Calçada/ESO

Space researchers in Japan working at the JAXA Suzaku observatory have found possible evidence of the existence of a toroidal magnetic field surrounding the magnetar 4U 0142+61.

In their paper published in the journal Physical Review Letters, the research team describe their observation of inconsistent pulses coming from 4U 0142+61, suggesting the presence of an external magnetic field.

Magnetars are a class of neutron star, both trace their origins to a supernova. What sets them apart is the strength of their magnetic fields, magnetars, as their name implies, are very strongly magnetic, with some measured at 1011 Tesla, approximately a billion times stronger than anything found on Earth.

They are also extremely dense, of course, and somewhat small—generally no more than 20 kilometers in diameter.

JAXA Suzaku observatory

For some time, space scientists have theorized that magnetars also have a second doughnut-shaped (torus) magnetic field surrounding their equator, perhaps even stronger than the one measured at their surface.

In this new effort, the researchers appear to have found evidence for just such a magnetic field surrounding 4U 0142+61.

In studying the magnetar, the researchers were analyzing the strong x-ray emissions that come from its poles—pulsating every 8.7 seconds. But then they noticed something else, the pulse was not consistent.

Sometimes the pulse came slightly early, sometimes slightly late, a far cry from pulsars which are as consistent as an atomic clock.

The researchers suggest the inconsistency is due to a toroidal magnetic field over the magnetar's equator.

They believe such a field, with a strength of up to 1012 Tesla would squeeze the magnetar into the shape of a football, and like a football, it could conceivably wobble as it spins, in this case with a period just a fraction of the spin period. That, the researchers suggest, would explain the inconsistent pulse.

If this new theory by the team in Japan proves to be true, than the wobbling of 4U 0142+61 should be causing gravity waves to be generated, and if that is the case, than future gravity wave detectors should be able to prove that magnetars due indeed have extremely strong gravity fields hovering over their equators.

More information: Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61, Phys. Rev. Lett. 112, 171102 – Published 30 April 2014 arxiv.org/abs/1404.3705