NuSTAR discovers impossibly bright dead star - First ultraluminous pulsar

High-energy X-rays streaming from a rare and mighty pulsar (magenta), the brightest found to date, can be seen in this new image combining multi-wavelength data from three telescopes. 

The bulk of a galaxy called Messier 82 (M82), or the 'Cigar galaxy,' is seen in visible-light data captured by the National Optical Astronomy Observatory's 2.1-meter telescope at Kitt Peak in Arizona. 

Starlight is white, and lanes of dust appear brown. Low-energy X-ray data from NASA's Chandra X-ray Observatory are colored blue, and higher-energy X-ray data from NuSTAR are pink. 

Credit: NASA/JPL-Caltech/SAO/NOAO

Astronomers working with NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), led by Caltech's Fiona Harrison, have found a pulsating dead star beaming with the energy of about 10 million suns.

The object, previously thought to be a black hole because it is so powerful, is in fact a pulsar, the incredibly dense rotating remains of a star.

"This compact little stellar remnant is a real powerhouse. We've never seen anything quite like it," says Harrison, NuSTAR's principal investigator and the Benjamin M. Rosen Professor of Physics at Caltech.

"We all thought an object with that much energy had to be a black hole."

Dom Walton, a postdoctoral scholar at Caltech who works with NuSTAR data, says that with its extreme energy, this pulsar takes the top prize in the weirdness category. Pulsars are typically between one and two times the mass of the sun.

This new pulsar presumably falls in that same range but shines about 100 times brighter than theory suggests something of its mass should be able to.

"We've never seen a pulsar even close to being this bright," Walton says. "Honestly, we don't know how this happens, and theorists will be chewing on it for a long time."

Besides being weird, the finding will help scientists better understand a class of very bright X-ray sources, called ultraluminous X-ray sources (ULXs).

Harrison, Walton, and their colleagues describe NuSTAR's detection of this first ultraluminous pulsar in a paper that appears in the current issue of Nature.

"This was certainly an unexpected discovery," says Harrison. "In fact, we were looking for something else entirely when we found this."



This animation shows a neutron star, the core of a star that exploded in a massive supernova. 

This particular neutron star is known as a pulsar because it sends out rotating beams of X-rays that sweep past Earth like lighthouse beacons. 

Credit: NASA/JPL-Caltech

Earlier this year, astronomers in London detected a spectacular, once-in-a-century supernova (dubbed SN2014J) in a relatively nearby galaxy known as Messier 82 (M82), or the Cigar Galaxy, 12 million light-years away.

Because of the rarity of that event, telescopes around the world and in space adjusted their gaze to study the aftermath of the explosion in detail.

Besides the supernova, M82 harbours a number of other ULXs. When Matteo Bachetti of the Université de Toulouse in France, the lead author of this new paper, took a closer look at these ULXs in NuSTAR's data, he discovered that something in the galaxy was pulsing, or flashing light.

"That was a big surprise," Harrison says. "For decades everybody has thought these ultraluminous X-ray sources had to be black holes, but black holes don't have a way to create this pulsing."

More information: An Ultraluminous X-ray Source Powered by An Accreting Neutron Star, Nature, dx.doi.org/10.1038/nature13791189

Fermi finds a mysterious 'transformer' pulsar

These artist's renderings show one model of pulsar J1023 before (top) and after (bottom) its radio beacon (green) vanished. 

Normally, the pulsar's wind staves off the companion's gas stream. 

When the stream surges, an accretion disk forms and gamma-ray particle jets (magenta) obscure the radio beam. 

Credit: NASA's Goddard Space Flight Center

In late June 2013, an exceptional binary containing a rapidly spinning neutron star underwent a dramatic change in behavior never before observed.

The pulsar's radio beacon vanished, while at the same time the system brightened fivefold in gamma rays, the most powerful form of light, according to measurements by NASA's Fermi Gamma-ray Space Telescope.

"It's almost as if someone flipped a switch, morphing the system from a lower-energy state to a higher-energy one," said Benjamin Stappers, an astrophysicist at the University of Manchester, England, who led an international effort to understand this striking transformation.

"The change appears to reflect an erratic interaction between the pulsar and its companion, one that allows us an opportunity to explore a rare transitional phase in the life of this binary."

A binary consists of two stars orbiting around their common center of mass. This system, known as AY Sextantis, is located about 4,400 light-years away in the constellation Sextans.

It pairs a 1.7-millisecond pulsar named PSR J1023+0038 (J1023) with a star containing about one-fifth the mass of the sun.

The stars complete an orbit in only 4.8 hours, which places them so close together that the pulsar will gradually evaporate its companion.

When a massive star collapses and explodes as a supernova, its crushed core may survive as a compact remnant called a neutron star or pulsar, an object squeezing more mass than the sun's into a sphere no larger than Washington, D.C.

Young isolated neutron stars rotate tens of times each second and generate beams of radio, visible light, X-rays and gamma rays that astronomers observe as pulses whenever the beams sweep past Earth.

Pulsars also generate powerful outflows, or "winds," of high-energy particles moving near the speed of light.

The power for all this comes from the pulsar's rapidly spinning magnetic field, and over time, as the pulsars wind down, these emissions fade.

More than 30 years ago, astronomers discovered another type of pulsar revolving in 10 milliseconds or less, reaching rotational speeds up to 43,000 rpm.

While young pulsars usually appear in isolation, more than half of millisecond pulsars occur in binary systems, which suggested an explanation for their rapid spin.

"Astronomers have long suspected millisecond pulsars were spun up through the transfer and accumulation of matter from their companion stars, so we often refer to them as recycled pulsars," explained Anne Archibald, a postdoctoral researcher at the Netherlands Institute for Radio Astronomy (ASTRON) in Dwingeloo who discovered J1023 in 2007.


Zoom into an artist's concept of AY Sextantis, a binary star system whose pulsar switched from radio emissions to high-energy gamma rays in 2013. 

This transition likely means the pulsar's spin-up process is nearing its end. 

Credit: NASA FERMI

During the initial mass-transfer stage, the system would qualify as a low-mass X-ray binary, with a slower-spinning neutron star emitting X-ray pulses as hot gas raced toward its surface.

A billion years later, when the flow of matter comes to a halt, the system would be classified as a spun-up millisecond pulsar with radio emissions powered by a rapidly rotating magnetic field.

To better understand J1023's spin and orbital evolution, the system was regularly monitored in radio using the Lovell Telescope in the United Kingdom and the Westerbork Synthesis Radio Telescope in the Netherlands.

These observations revealed that the pulsar's radio signal had turned off and prompted the search for an associated change in its gamma-ray properties.

A few months before this, astronomers found a much more distant system that flipped between radio and X-ray states in a matter of weeks.

Located in M28, a globular star cluster about 19,000 light-years away, a pulsar known as PSR J1824-2452I underwent an X-ray outburst in March and April 2013. As the X-ray emission dimmed in early May, the pulsar's radio beam emerged.

While J1023 reached much higher energies and is considerably closer, both binaries are otherwise quite similar. What's happening, astronomers say, are the last sputtering throes of the spin-up process for these pulsars.

In J1023, the stars are close enough that a stream of gas flows from the sun-like star toward the pulsar. The pulsar's rapid rotation and intense magnetic field are responsible for both the radio beam and its powerful pulsar wind.

When the radio beam is detectable, the pulsar wind holds back the companion's gas stream, preventing it from approaching too closely but now and then the stream surges, pushing its way closer to the pulsar and establishing an accretion disk.

Gas in the disk becomes compressed and heated, reaching temperatures hot enough to emit X-rays. Next, material along the inner edge of the disk quickly loses orbital energy and descends toward the pulsar.

When it falls to an altitude of about 50 miles (80 km), processes involved in creating the radio beam are either shut down or, more likely, obscured.

The inner edge of the disk probably fluctuates considerably at this altitude. Some of it may become accelerated outward at nearly the speed of light, forming dual particle jets firing in opposite directions, a phenomenon more typically associated with accreting black holes.

Shock waves within and along the periphery of these jets are a likely source of the bright gamma-ray emission detected by Fermi.

The findings were published in the July 20 edition of The Astrophysical Journal. The team reports that J1023 is the first example of a transient, compact, low-mass gamma-ray binary ever seen.

The researchers anticipate that the system will serve as a unique laboratory for understanding how millisecond pulsars form and for studying the details of how accretion takes place on neutron stars.

"So far, Fermi has increased the number of known gamma-ray pulsars by about 20 times and doubled the number of millisecond pulsars within in our galaxy," said Julie McEnery, the project scientist for the mission at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"Fermi continues to be an amazing engine for pulsar discoveries."

More information: Paper: "A State Change In The Missing Link Binary Pulsar System PSR J1023+0038" iopscience.iop.org/0004-637X/790/1/39 - Paper: "A Radio Pulsar/X-ray Binary Link" arxiv.org/abs/0905.3397

Remarkable white dwarf star; coldest, dimmest ever detected

This is an artist impression of a white dwarf star in orbit with pulsar PSR J2222-0137. 

It may be the coolest and dimmest white dwarf ever identified. 

Credit: B. Saxton (NRAO /AUI /NSF)

A team of astronomers has identified possibly the coldest, faintest white dwarf star ever detected.

This ancient stellar remnant is so cool that its carbon has crystallized, forming an Earth-size diamond in space.

David Kaplan

"It's a really remarkable object," said David Kaplan, a professor at the University of Wisconsin-Milwaukee. "These things should be out there, but because they are so dim they are very hard to find."

Kaplan and his colleagues found this stellar gem using the National Radio Astronomy Observatory's (NRAO) Green Bank Telescope (GBT) and Very Long Baseline Array (VLBA), as well as other observatories.

White dwarfs are the extremely dense end-states of stars like our Sun that have collapsed to form an object approximately the size of the Earth.

Composed mostly of carbon and oxygen, white dwarfs slowly cool and fade over billions of years. The object in this new study is likely the same age as the Milky Way, approximately 11 billion years old.

Pulsars are rapidly spinning neutron stars, the superdense remains of massive stars that have exploded as supernovas.

As neutron stars spin, lighthouse-like beams of radio waves, streaming from the poles of its powerful magnetic field, sweep through space.

When one of these beams sweeps across the Earth, radio telescopes can capture the pulse of radio waves.

The pulsar companion to this white dwarf, dubbed PSR J2222-0137, was the first object in this system to be detected.

Jason Boyles

It was found using the GBT by Jason Boyles, then a graduate student at West Virginia University in Morgantown.

These first observations revealed that the pulsar was spinning more than 30 times each second and was gravitationally bound to a companion star, which was initially identified as either another neutron star or, more likely, an uncommonly cool white dwarf. The two were calculated to orbit each other once every 2.45 days.

The pulsar was then observed over a two-year period with the VLBA by Adam Deller, an astronomer at the Netherlands Institute for Radio Astronomy (ASTRON).

These observations pinpointed its location and distance from the Earth, approximately 900 light-years away in the direction of the constellation Aquarius.

This information was critical in refining the model used to time the arrival of the pulses at the Earth with the GBT.

By applying Einstein's theory of relatively, the researchers studied how the gravity of the companion warped space, causing delays in the radio signal as the pulsar passed behind it.

These delayed travel times helped the researchers determine the orientation of their orbit and the individual masses of the two stars.

The pulsar has a mass 1.2 times that of the Sun and the companion a mass 1.05 times that of the Sun.

These data strongly indicated that the pulsar companion could not have been a second neutron star; the orbits were too orderly for a second supernova to have taken place.

Knowing its location with such high precision and how bright a white dwarf should appear at that distance, the astronomers believed they should have been able to observe it in optical and infrared light.

Remarkably, neither the Southern Astrophysical Research (SOAR) telescope in Chile nor the 10-meter Keck telescope in Hawaii was able to detect it.

"Our final image should show us a companion 100 times fainter than any other white dwarf orbiting a neutron star and about 10 times fainter than any known white dwarf, but we don't see a thing," said Bart Dunlap, a graduate student at the University of North Carolina at Chapel Hill and one of the team members.

"If there's a white dwarf there, and there almost certainly is, it must be extremely cold."

The researchers calculated that the white dwarf would be no more than a comparatively cool 3,000 degrees Kelvin (2,700 degrees Celsius).

Astronomers believe that such a cool, collapsed star would be largely crystallized carbon, not unlike a diamond.

Other such stars have been identified and they are theoretically not that rare, but with a low intrinsic brightness, they can be deucedly difficult to detect.

Its fortuitous location in a binary system with a neutron star enabled the team to identify this one.

ESA's XMM-Newton: Pulsar encased in a supernova bubble

Credit: ESA /XMM-Newton /L. Oskinova /M. Guerrero; CTIO /R. Gruendl / Y.H. Chu

Massive stars end their lives with a bang: exploding as spectacular supernovas, they release huge amounts of mass and energy into space.

These explosions sweep up any surrounding material, creating bubble remnants that expand into interstellar space.

At the heart of bubbles like these are small, dense neutron stars or black holes, the remains of what once shone brightly as a star.

Since supernova-carved bubbles shine for only a few tens of thousands of years before dissolving, it is rare to come across neutron stars or black holes that are still enclosed within their expanding shell.

Pulsar SXP 1062

This image captures such an unusual scene, featuring both a strongly magnetised, rotating neutron star – known as a pulsar – and its cosmic cloak, the remains of the explosion that generated it.

Small Magellanic Cloud

This pulsar, named SXP 1062, lies in the outskirts of the Small Magellanic Cloud, one of the satellite galaxies of our Milky Way galaxy.

It is an object known as an X-ray pulsar: it hungrily gobbles up material from a nearby companion star and burps off X-rays as it does so.

In the future, this scene may become even more dramatic, as SXP 1062 has a massive companion star that has not yet exploded as a supernova.

Most pulsars whirl around incredibly quickly, spinning many times per second.

However, by exploring the expanding bubble around this pulsar and estimating its age, astronomers have noticed something intriguing: SXP 1062 seems to be rotating far too slowly for its age. It is actually one of the slowest pulsars known.

While the cause of this weird sluggishness is still a mystery, one explanation may be that the pulsar has an unusually strong magnetic field, which would slow the rotation.

The diffuse blue glow at the centre of the bubble in this image represents X-ray emission from both the pulsar and the hot gas that fills the expanding bubble.

The other fuzzy blue objects visible in the background are extragalactic X-ray sources.

This image combines X-ray data from ESA's XMM-Newton (shown in blue) with optical observations from the Cerro Tololo Inter-American Observatory in Chile.

The optical data were obtained using two special filters that reveal the glow of oxygen (shown in green) and hydrogen (shown in red).

The size of the image is equivalent to a distance of 457 light-years on a side.

This image was first published in 2011.

More information: "Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the Wing of the Small Magellanic Cloud." V. Hénault-Brunet, L. M. Oskinova, M. A. Guerrero, et al. Monthly Notices of the Royal Astronomical Society: Letters 420 (1) L13 (2012) DOI: 10.1111/j.1745-3933.2011.01183.x

Smallest speed jump of pulsar caused by billions of superfluid vortices

Artist’s impression of a pulsar. Pulsars are rotating neutron stars -- remnants of massive stars that end their lives in supernova explosions. They act like cosmic lighthouses whose beams sweep through the universe. 

Credit: NASA

A team of astronomers, including Danai Antonopoulou and Anna Watts from the University of Amsterdam, has discovered that sudden speed jumps in the rotational velocity of pulsars have a minimum size, and that they are caused not by the unpinning and displacement of just one sub-surface superfluid vortex, but by billions.

This result is important to our understanding of the behaviour of matter under extreme conditions, and has been published this week in the journal Monthly Notices of the Royal Astronomical Society.

Pulsars are rotating neutron stars - remnants of massive stars that end their lives in supernova explosions. They act like cosmic lighthouses whose beams sweep through the Universe.

Their rotational velocity decreases in time, but can suddenly increase in rare events called glitches.

These glitches are caused by the unpinning and displacement of vortices that connect the crust with the mixture of particles containing superfluid neutrons beneath the crust.

The team of astronomers discovered that the glitches of the Crab Pulsar always involve a decrease in the rotational period of at least 0.055 nanoseconds.

The Crab Pulsar was one of the first pulsars to be discovered and has been observed almost daily with the 42-ft Telescope at the Jodrell Bank Observatory over the last 29 years.

The huge amount of data makes this object the best choice to study glitches.

The smallest glitch is likely to be caused by the unpinning and movement of billions of vortices. "Surprisingly, no one tried to determine a lower limit to glitch size before. Many assumed that the smallest glitch would be caused by a single vortex unpinning.

The smallest glitch is clearly much larger than we expected", says Danai Antonopoulou from the University of Amsterdam (UvA).

"Astronomers would of course like to know whether the smallest glitches of other pulsars are also caused by billions of vortices. The next step is to sift through the data of other pulsars and to continue observing", says first author Cristobal Espinoza (IA-PUC, Chile).

Antonopoulou's supervisor Anna Watts (UvA) adds: "By comparing the observations with theoretical predictions we learn about the behavior of matter in these exotic objects. The precise cause of glitches is still a mystery to us, and this result offers a new challenge to theorists."

More information: "Neutron star glitches have a substantial minimum size," C. M. Espinoza, D. Antonopoulou, B. W. Stappers, A. Watts, A. G. Lyne, Monthly Notices of the Royal Astronomical Society, MNRAS (2014) 440 (3): 2755 dx.doi.org/10.1093/mnras/stu395, preprint: arxiv.org/abs/1402.7219

NASA Chandra observes runaway pulsar firing an extraordinary jet

Credit X-ray: NASA /CXC /ISDC /L.Pavan et al, Radio: CSIRO /ATNF /ATCA Optical: 2MASS /UMass /IPAC-Caltech /NASA /NSF

NASA's Chandra X-ray Observatory has seen a fast-moving pulsar escaping from a supernova remnant while spewing out a record-breaking jet, the longest of any object in the Milky Way galaxy, of high-energy particles.

The pulsar, a type of neutron star, is known as IGR J11014-6103.

IGR J11014-6103's peculiar behaviour can likely be traced back to its birth in the collapse and subsequent explosion of a massive star.

Originally discovered with the European Space Agency satellite INTEGRAL, the pulsar is located about 60 light-years away from the center of the supernova remnant SNR MSH 11-61A in the constellation of Carina.

Its implied speed is between 2.5 million and 5 million mph, making it one of the fastest pulsars ever observed.

Lucia Pavan

"We've never seen an object that moves this fast and also produces a jet," said Lucia Pavan of the University of Geneva in Switzerland and lead author of a paper published Tuesday in the journal Astronomy and Astrophysics.

"By comparison, this jet is almost 10 times longer than the distance between the sun and our nearest star."

The X-ray jet in IGR J11014-6103 is the longest known in the Milky Way galaxy.

In addition to its impressive span, it has a distinct corkscrew pattern that suggests the pulsar is wobbling like a spinning top.

IGR J11014-6103 also is producing a cocoon of high-energy particles that enshrouds and trails behind it in a comet-like tail.

This structure, called a pulsar wind nebula, has been observed before, but the Chandra data show that the long jet and the pulsar wind nebula are almost perpendicular to one another.

Pol Bordas

"We can see that this pulsar is moving directly away from the center of the supernova remnant based on the shape and direction of the pulsar wind nebula," said co-author Pol Bordas, from the University of Tuebingen in Germany. "The question is, why is the jet pointing off in this other direction?"

Usually, the spin axis and jets of a pulsar point in the same direction as they are moving, but IGR J11014-6103's spin axis and direction of motion are almost at right angles.

Gerd Puehlhofer

"With the pulsar moving one way and the jet going another, this gives us clues that exotic physics can occur when some stars collapse," said co-author Gerd Puehlhofer also of the University of Tuebingen.

One possibility requires an extremely fast rotation speed for the iron core of the star that exploded.

A problem with this scenario is that such fast speeds are not commonly expected to be achievable.

The supernova remnant that gave birth to IGR J11014-6013 is elongated from top-right to bottom-left in the image roughly in line with the jet's direction.

These features and the high speed of the pulsar are hints that jets could have been an important feature of the supernova explosion that formed it.

More information: arxiv.org/abs/1309.6792

GBT NRAO: Pulsar in stellar triple system makes unique gravitational laboratory

The pulsar (L) is orbited by a hot white dwarf star (C) both of which are orbited by a cooler, distant white dwarf (R)

Credit: NRAO

Astronomers using the National Science Foundation's Green Bank Telescope (GBT) have discovered a unique stellar system of two white dwarf stars and a superdense neutron star, all packed within a space smaller than Earth's orbit around the Sun.

The results appear in Nature journal and will be presented at the 223rd American Astronomical Society meeting.

The closeness of the stars, combined with their nature, has allowed the scientists to make the best measurements yet of the complex gravitational interactions in such a system.

In addition, detailed studies of this system may provide a key clue for resolving one of the principal outstanding problems of fundamental physics—the true nature of gravity.

"This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work and potentially for detecting problems with General Relativity that physicists expect to see under extreme conditions," said Scott Ransom of the National Radio Astronomy Observatory (NRAO).

West Virginia University graduate student Jason Boyles (now at Western Kentucky University) originally uncovered the pulsar as part of a large-scale search for pulsars with the GBT.

Pulsars are neutron stars that emit lighthouse-like beams of radio waves that rapidly sweep through space as the object spins on its axis.

GALEX satellite

One of the search's discoveries was a pulsar some 4200 light-years from Earth, spinning nearly 366 times per second.

Such rapidly-spinning pulsars are called millisecond pulsars, and can be used by astronomers as precision tools for studying a variety of phenomena, including searches for the elusive gravitational waves.

Subsequent observations showed that the pulsar is in a close orbit with a white dwarf star, and that pair is in orbit with another, more-distant white dwarf.

WIYN NRAO Telescope

"This is the first millisecond pulsar found in such a system, and we immediately recognized that it provides us a tremendous opportunity to study the effects and nature of gravity," Ransom said.

The scientists began an intensive observational program using the GBT, the Arecibo radio telescope in Puerto Rico, and the Westerbork Synthesis Radio Telescope in the Netherlands.

They also studied the system using data from the Sloan Digital Sky Survey, the GALEX satellite, the WIYN telescope on Kitt Peak, Arizona, and the Spitzer Space Telescope.

"The gravitational perturbations imposed on each member of this system by the others are incredibly pure and strong," Ransom said.

"The millisecond pulsar serves as an extremely powerful tool for measuring those perturbations incredibly well," he added.

More information: Nature DOI: 10.1038/nature12917

Crab Nebula's Strange Pulsar Heart Slowly Going Off-Kilter

A composite image of the Crab Nebula showing the X-ray (blue), and optical (red) images superimposed. 

The size of the X-ray image is smaller because the higher energy X-ray emitting electrons radiate away their energy more quickly than the lower energy optically emitting electrons as they move.

Credit: NASA/HST/ASU/J. Hester et al. X-Ray: NASA/CXC/ASU/J. Hester

For the first time, astronomers have tracked the evolution of a pulsar's magnetic field over time, watching as it slowly tilts toward the dead star's equator.

The new observations of the pulsar, located in the Crab Nebula, could offer clues to the long-standing problem of what slows pulsars' rotation.

Andrew Lyne

"Most pulsars are millions or tens of millions of years old," said Andrew Lyne, emeritus professor of physics at the University of Manchester in the U.K., who led the study, which appears in the Nov. 1 issue of the journal Science.

"So we don't expect to see significant changes. But we have been looking at this for a substantial portion of its lifetime, some 40 out of 1,000 years."

The supernova that birthed the pulsar in the Crab Nebula occurred in A.D. 1054. Chinese and Arab astronomers both noted it.

"It's a result we've waited 30 years for," said Vasily Beskin, an astrophysicist at the Russian Academy of Sciences.

Beskin, who was not involved in the study, and his colleagues predicted that pulsar magnetic fields would move to their equators in the 1980s.

The new data also gave other insights. "Normally, magnetic fields don't move through superconductors," Lyne said. "This magnetic field is moving, which suggests the superconductor in the neutron star is not perfect."

It's not likely that astronomers will run across another like the Crab pulsar, because to see one at all, the radio beam has to sweep across the Earth, and the odds of one being in precisely the right orientation are small.

On top of that, the supernova that made the pulsar would have to be less than a few thousand years old, scientists say.

There are several supernovas of the correct age, but they aren't all the right type to produce pulsars, and even if they were, they aren't pointed the right way.

It still isn't completely clear why pulsars' magnetic fields look as they do. "I wouldn't class it as being a simple problem," Lyne said. "We're trying to understand why it should evolve in this way."

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.

Pulsar Stars to act as a Navigation Aid for Spacecraft

Spacecraft could one day navigate through the cosmos using a particular type of dead star as a kind of GPS.

German scientists are developing a technique that allows for very precise positioning anywhere in space by picking up X-ray signals from pulsars.

These dense, burnt-out stars rotate rapidly, sweeping their emission across the cosmos at rates that are so stable they rival atomic clock performance.

This timing property is perfect for interstellar navigation, says the team.

If a spacecraft carried the means to detect the pulses, it could compare their arrival times with those predicted at a reference location.

This would enable the craft to determine its position to an accuracy of just five kilometres anywhere in the galaxy.

"The principle is so simple that it will definitely have applications," said Prof Werner Becker from the Max-Planck Institute for Extraterrestrial Physics in Garching.

"These pulsars are everywhere in the Universe and their flashing is so predictable that it makes such an approach really straightforward," he told BBC News.

Prof Becker has been describing his team's research here at the UK National Astronomy Meeting in Manchester.

The proposed technique is very similar to that employed in the popular Global Positioning System, which broadcasts timing signals to the user from a constellation of satellites in orbit.

But GPS only works on, or just above, the Earth so it has no use beyond our planet.

Currently, mission controllers wanting to work out the position of their spacecraft deep in the Solar System will study the differences in time radio communications take to travel to and from the satellite. It is a complex process and requires several antennas dotted across the Earth.

It is also a technique that is far from precise, and the errors increase the further away the probe moves.

For the most distant spacecraft still in operation - Nasa's Voyager satellites, which are now approaching the very edge of the Solar System, some 18 billion km away - the errors associated with their positions are on the order of several hundred km.

Pulsars: The discovery of deceleration

An artist's impression of an accreting X-ray millisecond pulsar. 

The flowing material from the companion star forms a disk around the neutron star which is truncated at the edge of the pulsar magnetosphere. Credit: NASA / Goddard Space Flight Center / Dana Berry.

Pulsars are among the most exotic celestial bodies known. They have diameters of about 20 kilometres, but at the same time roughly the mass of our sun.

A sugar-cube sized piece of its ultra-compact matter on the Earth would weigh hundreds of millions of tons. A sub-class of them, known as millisecond pulsars, spin up to several hundred times per second around their own axes.

Previous studies reached the paradoxical conclusion that some millisecond pulsars are older than the universe itself.

The astrophysicist Thomas Tauris from the Max Planck Institute for Radio Astronomy and the Argelander Institute for Astronomy in Bonn could resolve this paradox by computer simulations.

Through numerical calculations on the base of stellar evolution and accretion torques, he demonstrated that millisecond pulsars loose about half of their rotational energy during the final stages of the mass-transfer process before the pulsar turns on its radio beam.

This result is in agreement with current observations and the findings also explain why radio millisecond pulsars appear to be much older than the white dwarf remnants of their companion stars - and perhaps why no sub-millisecond radio pulsars exist at all. The results are reported in the February 03 issue of the journal Science.

Millisecond pulsars are strongly magnetized, old neutron stars in binary systems which have been spun up to high rotational frequencies by accumulating mass and angular momentum from a companion star.

Today we know of about 200 such pulsars with spin periods between 1.4-10 milliseconds. These are located in both the Galactic Disk and in Globular Clusters.

Since the first millisecond pulsar was detected in 1982, it has remained a challenge for theorists to explain their spin periods, magnetic fields and ages. For example, there is the "turn-off" problem, i.e. what happens to the spin of the pulsar when the donor star terminates its mass-transfer process?

"We have now, for the first time, combined detailed numerical stellar evolution models with calculations of the braking torque acting on the spinning pulsar", says Thomas Tauris, the author of the present study.

"The result is that the millisecond pulsars loose about half of their rotational energy in the so-called Roche-lobe decoupling phase."

This phase describes the termination of the mass transfer in the binary system. Hence, radio-emitting millisecond pulsars should spin slightly slower than their progenitors, X-ray emitting millisecond pulsars which are still accreting material from their donor star.

This is exactly what the observational data seem to suggest. Furthermore, these new findings help explain why some millisecond pulsars appear to have characteristic ages exceeding the age of the Universe and perhaps why no sub-millisecond radio pulsars exist.

The key feature of the new results is that it has now been demonstrated how the spinning pulsar is able to break out of its so-called equilibrium spin.

At this epoch the mass-transfer rate decreases which causes the magnetospheric radius of the pulsar to expand and thereby expell the collapsing matter like a propeller. This causes the pulsar to loose additional rotational energy and thus slow down its spin rate.

"Actually, without a solution to the "turn-off" problem we would expect pulsars to even slow down to spin periods of 50-100 milliseconds during the Roche-lobe decoupling phase", concludes Thomas Tauris. "That would be in clear contradiction with observational evidence for the existence of millisecond pulsars."

ESA INTEGRAL: reveals new facets of the Vela pulsar wind nebula

This image shows the Vela pulsar wind nebula as observed with ESA's INTEGRAL observatory (blue pixellated image) and with other high-energy astronomical facilities (coloured contours).

The INTEGRAL image shows emission detected at hard X-ray energies, between 18 and 40 keV, with the IBIS imager on board INTEGRAL, after subtraction of the point-like source corresponding to the inner nebula.

The contours show soft X-ray emission detected by the German ROSAT telescope between 0.5 and 2 keV (green) and by the Birmingham Spacelab 2 telescope between 2.5 and 12 keV (cyan), and very-high energy gamma-ray emission detected with the H.E.S.S. Telescopes above 1 TeV (magenta).

The Vela pulsar wind nebula is a cloud of highly energetic electrons and positrons that are injected by the pulsar into its surroundings and radiate across the electromagnetic spectrum. The location of the Vela pulsar is marked with a cross.

The image measures roughly two degrees on the horizontal side. North is up and East is to the left. Copyright: ESA/INTEGRAL/IBIS-ISGRI/F. Mattana et al./ROSAT/H.E.S.S. /Spacelab 2.

ESA XMM-Newton: Cosmic Ornament

Image Credit: NASA/CXC/Univ. of Potsdam/L. Oskinova et al.

A new image from an assembly of telescopes reveals a pulsar that appears like a spinning cosmic ornament. Combined data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton were used in the discovery of a young pulsar in the remains of a supernova located in the Small Magellanic Cloud, or SMC.

This is the first time a pulsar, which is a spinning, ultra-dense star, has been found in a supernova remnant in the SMC, a small satellite galaxy to the Milky Way.

In this composite image, X-rays from Chandra and XMM-Newton have been colored blue and optical data from the Cerro Tololo Inter-American Observatory in Chile are coloured red and green.

The pulsar, known as SXP 1062, is the bright white source located on the right-hand side of the image in the middle of the diffuse blue emission inside a red shell.

The diffuse X-rays and optical shell are both evidence of a supernova remnant surrounding the pulsar. The optical data also displays spectacular formations of gas and dust in a star-forming region on the left side of the image.

SXP 1062 interests astronomers because the Chandra and XMM-Newton data show that it is rotating unusually slowly -- about once every 18 minutes. (In contrast, some pulsars are found to revolve multiple times per second, including most newly born pulsars.)

This relatively leisurely pace of SXP 1062 makes it one of the slowest rotating X-ray pulsars in the SMC.

Scientists have estimated that the supernova remnant around SXP 1062 is between 10,000 and 40,000 years old, as it appears in the image.

This means that the pulsar is very young, from an astronomical perspective, since it was presumably formed in the same explosion that produced the supernova remnant.

Therefore, assuming that it was born with rapid spin, it is a mystery why SXP 1062 has been able to slow down by so much, so quickly.

Work has already begun on theoretical models to understand the evolution of this unusual object.

ESA XMM-Newton: Strangely slow X-Ray pulsar discovered

The X-ray pulsar SXP 1062 embedded in the remnant of the supernova that created it. Credit: ESA/XMM-Newton/ L.Oskinova/ M.Guerrero; CTIO/R.Gruendl/Y.H.Chu.

Astronomers have discovered a very slowly rotating X-ray pulsar still embedded in the remnant of the supernova that created it.

This unusual object was detected on the outskirts of the Small Magellanic Cloud, a satellite galaxy of the Milky Way, using data from a number of telescopes, including ESA's XMM-Newton.

A puzzling mismatch between the fairly young age of the supernova remnant and the slow rotation of the pulsar, which would normally indicate a much older object, raises interesting questions about the origin and evolution of pulsars.

The spectacular supernova explosion that marks the end of a massive star's life also has an intriguing aftermath.

On the one hand, the explosion sweeps up the surrounding interstellar material creating a supernova remnant that is often characterised by a distinctive bubble-like shape, on the other hand, the explosion also leaves behind a compact object - a neutron star or a black hole.

Since supernova remnants shine only for a few tens of thousands of years before dispersing into the interstellar medium, not many compact objects have been detected while still enclosed in their expanding shell.

An international team of astronomers has now discovered one of these rarely observed pairs, consisting of a strongly magnetised, rotating neutron star - a pulsar - surrounded by the remains of the explosion that generated it.

The newly found pulsar, named SXP 1062, is located at the outskirts of the Small Magellanic Cloud (SMC), one of the satellite galaxies of the Milky Way. SXP 1062 is an X-ray pulsar, part of a binary system in which the compact object is accreting mass from a companion star, resulting in the emission of copious amounts of X-rays.

The astronomers first detected the pulsar's X-ray emission using data from ESA's XMM-Newton as well as NASA's Chandra space-based observatories. A later study of optical images of the source and its surroundings revealed the bubble-shaped signature of the supernova remnant around the binary system.

"The most interesting aspect of this pulsar is possibly its extremely long period - 1062 seconds - which makes it one of the slowest pulsars on record," comments Lidia Oskinova from the Institute for Physics and Astronomy in Potsdam, Germany, coordinator of the team that analysed the X-ray data.

Pulsars rotate quite rapidly in their early stages, with periods of only a fraction of a second, and then slow down gradually with age. "Slowly spinning pulsars are particularly difficult to detect. Only a few with periods longer than a thousand seconds have been observed to date," she adds.

The pulsar at the centre of the Crab Nebula

This artist's concept shows the pulsar at the centre of the Crab Nebula, with a Hubble Space Telescope photo of the nebula in the background.

Researchers using the Veritas telescope array have discovered pulses of high-energy gamma rays coming from this object.

Picture: David A. Aguilar (CfA) / NASA / ESA / AFP/Getty

NASA Chandra: A Pulsar and Its Mysterious Tail

A spinning neutron star is tied to a mysterious tail -- or so it seems. Astronomers using NASA's Chandra X-ray Observatory have found that this pulsar, known as PSR J0357+3205 (or PSR J0357 for short), apparently has a long, X-ray bright tail streaming away from it.

This composite image shows Chandra data in blue and Digitized Sky Survey data in yellow.

The position of the pulsar at the upper right end of the tail is seen by mousing over the image.

The two bright sources lying near the lower left end of the tail are both thought to be unrelated background objects located outside our galaxy.

PSR J0357 was originally discovered by the Fermi Gamma Ray Space Telescope in 2009. Astronomers calculate that the pulsar lies about 1,600 light years from Earth and is about half a million years old, which makes it roughly middle-aged for this type of object.

If the tail is at the same distance as the pulsar then it stretches for 4.2 light years in length. This would make it one of the the longest X-ray tails ever associated with a so-called "rotation-powered" pulsar, a class of pulsar that get its power from the energy lost as the rotation of the pulsar slows down. (Other types of pulsars include those driven by strong magnetic fields and still others that are powered by material falling onto the neutron star.)

The Chandra data indicate that the X-ray tail may be produced by emission from energetic particles in a pulsar wind, with the particles produced by the pulsar spiraling around magnetic field lines.

Other X-ray tails around pulsars have been interpreted as bow-shocks generated by the supersonic motion of pulsars through space, with the wind trailing behind as its particles are swept back by the pulsar's interaction with the interstellar gas it encounters.

However, this bow-shock interpretation may or may not be correct for PSR J0357, with several issues that need to be explained. For example, the Fermi data show that PSR J0357 is losing a very small amount of energy as its spin slows down with time. This energy loss is important, because it is converted into radiation and powering a particle wind from the pulsar. This places limits on the amount of energy that particles in the wind can attain, and so might not account for the quantity of X-rays seen by Chandra in the tail.

Space Art: A Pulsar

NASA ESA and The Crab Nebula

The Crab Nebula, the result of a supernova noted by Earth-bound chroniclers in 1054 A.D., is filled with mysterious filaments that are are not only tremendously complex, but appear to have less mass than expelled in the original supernova and a higher speed than expected from a free explosion.

The Crab Nebula spans about 10 light-years. In the nebula's very center lies a pulsar: a neutron star as massive as the Sun but with only the size of a small town. The Crab Pulsar rotates about 30 times each second.


Image Credit: NASA, ESA, J. Hester, A. Loll (ASU)