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

Supernova Legacy Survey: Powerful ancient explosions explain new class of supernovae

A small portion of one of the fields from the Supernova Legacy Survey showing SNLS-06D4eu and its host galaxy (arrow). 

The supernova and its host galaxy are so far away that both are a tiny point of light that cannot be clearly differentiated in this image. 

The large, bright objects with spikes are stars in our own galaxy. 

Every other point of light is a distant galaxy. 

Credit: UCSB

Astronomers affiliated with the Supernova Legacy Survey (SNLS) have discovered two of the brightest and most distant supernovae ever recorded, 10 billion light-years away and a hundred times more luminous than a normal supernova. Their findings appear in the Dec. 20 issue of the Astrophysical Journal.

These newly discovered supernovae are especially puzzling because the mechanism that powers most of them—the collapse of a giant star to a black hole or normal neutron star—cannot explain their extreme luminosity.

Discovered in 2006 and 2007, the supernovae were so unusual that astronomers initially could not figure out what they were or even determine their distances from Earth.

"At first, we had no idea what these things were, even whether they were supernovae or whether they were in our galaxy or a distant one," said lead author D. Andrew Howell, a staff scientist at Las Cumbres Observatory Global Telescope Network (LCOGT) and adjunct faculty at UC Santa Barbara.

"I showed the observations at a conference, and everyone was baffled. Nobody guessed they were distant supernovae because it would have made the energies mind-bogglingly large. We thought it was impossible."

One of the newly discovered supernovae, named SNLS-06D4eu, is the most distant and possibly the most luminous member of an emerging class of explosions called superluminous supernovae.

These new discoveries belong to a special subclass of superluminous supernovae that have no hydrogen.

The new study finds that the supernovae are likely powered by the creation of a magnetar, an extraordinarily magnetized neutron star spinning hundreds of times per second.

Magnetars have the mass of the sun packed into a star the size of a city and have magnetic fields a hundred trillion times that of the Earth.

While a handful of these superluminous supernovae have been seen since they were first announced in 2009, and the creation of a magnetar had been postulated as a possible energy source, the work of Howell and his colleagues is the first to match detailed observations to models of what such an explosion might look like.

Co-author Daniel Kasen from UC Berkeley and Lawrence Berkeley National Lab created models of the supernova that explained the data as the explosion of a star only a few times the size of the sun and rich in carbon and oxygen.

The star likely was initially much bigger but apparently shed its outer layers long before exploding, leaving only a smallish, naked core.

More information: dx.doi.org/10.1088/0004-637X/779/2/98

The Magnetar at the Heart of Our Milky Way

This is an artist's impression of PSR J1745-2900, a pulsar with a very high magnetic field ("magnetar") in direct vicinity of the central source of our Galaxy, a supermassive black hole of approximately 4 million times the mass of our sun. 

Measurements of the pulsar imply that a strong magnetic field exists in the vicinity around the black hole. 

Credit: MPIfR/Ralph Eatough.

Astronomers have discovered a magnetar at the centre of our Milky Way. This pulsar has an extremely strong magnetic field and enables researchers to investigate the direct vicinity of the black hole at the heart of the galaxy.

An international team of scientists headed by the Max Planck Institute for Radio Astronomy in Bonn have, for the first time, measured the strength of the magnetic field around this central source and were able to show that the latter is fed by magnetic fields.

These control the inflow of mass into the black hole, also explaining the x-ray emissions of this gravity trap.

The discovery of a pulsar closely orbiting the candidate supermassive black hole at the centre of the Milky Way (called Sagittarius A*, or Sgr A* in short) has been one of the main aims of pulsar astronomers for the last 20 years.

Pulsars, those extremely precise cosmic clocks, could be used to measure the properties of space and time around this object, and to see if Einstein's theory of General Relativity could hold up to the strictest tests.

Shortly after the announcement of a flaring X-ray source in the direction of the Galactic centre by NASA's Swift telescope, and the subsequent discovery of pulsations with a period of 3.76 seconds by NASA's NuSTAR telescope, a radio follow-up program was started at the Effelsberg radio observatory of the Max Planck Institute for Radio Astronomy (MPIfR).

"As soon as we heard about the discovery of regular pulsations with the NuSTAR telescope we pointed the Effelsberg 100-m dish in the direction of the Galactic centre," says Ralph Eatough from MPIfR's Fundamental Physics Research department, the lead author of the study.

"On our first attempt the pulsar was not clearly visible, but some pulsars are stubborn and require a few observations to be detected. The second time we looked, the pulsar had become very active in the radio band and was very bright.

I could hardly believe that we had finally detected a pulsar in the Galactic centre!" Because this pulsar is so special, the research team spent a lot of effort to prove that it was a real object in deep space and not due to human-made radio interference created on Earth.

Additional observations were performed in parallel and subsequently with other radio telescopes around the world (Jodrell Bank, Very Large Array, Nançay). "We were too excited to sleep in between observations!"

"We were calculating flux densities at 6am on Saturday morning and we could not believe that this magnetar had just turned on so bright." says Evan Keane from the Jodrell Bank Observatory.

Other collaborations worked at different telescopes (Australia Telescope/ATCA, Parkes and Green Bank Telescope).

A research paper on the ATCA results by Shannon & Johnston appears in this week's issue of the Oxford journal MNRAS.

"The Effelsberg radio telescope was built such that it could observe the Galactic centre. And 40 years later it detects the first radio pulsar there," explains Heino Falcke, professor at Radboud Universiteit Nijmegen.

"Sometimes we have to be patient. It was a laborious effort, but finally we succeeded."

Journal References: R. P. Eatough, H. Falcke, R. Karuppusamy, K. J. Lee, D. J. Champion, E. F. Keane, G. Desvignes, D. H. F. M. Schnitzeler, L. G. Spitler, M. Kramer, B. Klein, C. Bassa, G. C. Bower, A. Brunthaler, I. Cognard, A. T. Deller, P. B. Demorest, P. C. C. Freire, A. Kraus, A. G. Lyne, A. Noutsos, B. Stappers, N. Wex. A strong magnetic field around the supermassive black hole at the centre of the Galaxy. Nature, 2013; DOI: 10.1038/nature12499

ESA XMM-Newton: Mysterious magnetar boasts one of strongest magnetic fields in Universe

Artist's impression of a magnetar Credit: ESA /ATG Medialab

A team of astronomers including two researchers from UCL's Mullard Space Science Laboratory has made the first ever measurement of the magnetic field at a specific spot on the surface of a magnetar.

Magnetars are a type of neutron star, the dense and compact core of a giant star which has blasted away its outer layers in a supernova explosion.

Magnetars have among the strongest magnetic fields in the Universe. Until now, only their large scale magnetic field had been measured.

However, using a new technique and observations of a magnetar in X-rays, the astronomers have now revealed a strong, localised surface magnetic field on one.

Magnetars are very puzzling neutron stars. Astronomers discovered them through their unusual behaviour when observed in X-ray wavelengths, including sudden outbursts of radiation and occasional giant flares.

These peculiar features of magnetars are caused by the evolution, dissipation and decay of their super-strong magnetic fields, which are hundreds or thousands of times more intense than those of the more common type of neutron stars, the radio pulsars.

The magnetic field of a magnetar can have a complex structure. The most obvious, and easy-to-measure, component is the large scale external magnetic field, which is shaped (and behaves) much like a regular bar magnet's. This is known as the dipolar field.

The study was carried out on a magnetar called SGR 0418+5729. A few years ago, this star was discovered to have a relatively gentle dipolar magnetic field compared to other magnetars.

However, the star was showing the typical flaring and bursting activities seen in other magnetars, leading scientists to suggest that the star's magnetic activity might be caused by a field hidden beneath its surface.

Sometimes, the surface breaks and the hidden magnetic field leaks out (artist's impression) Credit: ESA/ATG Medialab

This new study, based on observations from ESA's XMM-Newton X-ray space telescope, has finally found evidence that SGR 0418+5729 is indeed concealing a very strong magnetic field in its interior.

"This magnetar has a strong magnetic field inside it, but it is hidden beneath the surface. The only way you can detect that is to find a flaw on the surface, where the concealed magnetic field can leak out," says Silvia Zane (UCL Mullard Space Science Laboratory), one of the co-authors of the study.

More information: "A variable absorption feature in the X-ray spectrum of a magnetar," by A. Tiengo et al is published in Nature, 15 August 2013.

ESA XMM-Newton X-ray Image: A magnetic monster’s dual personality

Click on the Image to watch an animation.

Is it a magnetar or is it a pulsar? A second member of a rare breed of dead, spinning star has been identified thanks to an armada of space-based X-ray telescopes, including ESA’s XMM-Newton.

Its curious behaviour is illustrated in this animation.

Magnetars are a type of neutron star, the dead cores of massive stars that have collapsed in on themselves after burning up all their fuel and exploding as dramatic supernovas.

They typically display bright, persistent X-ray emission and the most intense magnetic fields known in the Universe.

Pulsars meanwhile are spinning neutron stars with much lower magnetic fields than magnetars that appear to pulse radio waves as they rotate rapidly.

The pulses are seen when beams of radiation rotate through our line of sight from Earth, rather like the sweeping beam of a lighthouse.

The recently discovered star appears to be a hybrid of these two stellar breeds: the spinning stellar skeleton appears as a pulsar while hiding an intense internal magnetic field much like a magnetar.

The internal field is many times stronger than its external magnetic field, leading to its entry into the new class of ‘low-field magnetars’.

As this animation illustrates, the turbulent interior arises as a result of twisted magnetic field lines.

As the field lines unwind, energy is released as a steady burst of X-rays through fractures in the star’s ‘crust’.

Only two examples of low-field magnetars are known. The first was discovered in 2010 and the second in July 2011, given away by short X-ray bursts that were detected by NASA’s Swift space telescope.

NASA’s Rossi X-Ray Timing Explorer and Chandra X-ray Observatory, ESA’s XMM-Newton and Japan’s Suzaku satellite, as well as the ground-based Gran Telescopio Canarias and the Green Bank Telescope, were alerted and the star’s activity was monitored until April 2012, during which time the outburst began to decay.

The discovery of a second member of this rare family of star strengthens the idea that magnetar-like behaviour may be much more widespread than believed in the past.