Posted on by Ian O'Neill

After the Storm: Measuring the Structure and Temperature of a Quiescent Neutron Star

Accretion can cause neutron stars to flare violently

[/caption]So how do you take the temperature of one of the most exotic objects in the Universe? A neutron star (~1.35 to 2.1 solar masses, measuring only 24 km across) is the remnant of a supernova after a large star has died. Although they are not massive enough become a black hole, neutron stars still accrete matter, pulling gas from a binary partner, often undergoing prolonged periods of flaring.

Fortunately, we can observe X-ray flares (using instrumentation such as Chandra), but it isn’t the flare itself that can reveal the temperature or structure of a neutron star.

At the AAS conference last week, details about the results from an X-ray observing campaign of MXB 1659-29, a quasi-persistent X-ray transient source (i.e. a neutron star that flares for long periods), revealed some fascinating insights to the physics of neutron stars, showing that as the crust of a neutron star cools, the crustal composition is revealed and the temperature of these exotic supernova remnants can be measured…

During a flare outburst, neutron stars generate X-rays. These X-ray sources can be measured and their evolution tracked. In the case of MXB 1659-29, Ed Cackett (Univ. of Michigan) used data from NASA’s Rossi X-ray Timing Explorer (RXTE) to monitor the cooling of the neutron star crust after an extended period of X-ray flaring. MXB 1659-29 flared for 2.5 years until it “turned off” in September 2001. Since then, the source was periodically observed to measure the exponential decrease in X-ray emissions.

So why is this important? After a long period of X-ray flaring, the crust of a neutron star will heat up. However, it is thought that the core of the neutron star will remain comparatively cool. When the neutron star stops flaring (as the accretion of gas, feeding the flare, shuts off), the heating source for the crust is lost. During this period of “quiescence” (no flaring), the diminishing X-ray flux from the cooling neutron star crust reveals a huge wealth of information about the characteristics of the neutron star.

The cross section of a neutron star
The cross section of a neutron star
During quiescence, astronomers will observe X-rays emitted from the surface of the neutron star (as opposed to the flares), so direct measurements can be made of the neutron star. In his presentation, Cackett examined how the X-ray flux from MXB 1659-29 reduced exponentially and then levelled off at a constant flux. This means the crust cooled rapidly after the flaring, eventually reaching thermal equilibrium with the neutron star core. Therefore, by using this method, the neutron star core temperature can be inferred.

Including the data from another neutron star X-ray transient KS 1731-260, the cooling rates observed during the onset of quiescence suggests these objects have well-ordered crustal lattices with very few impurities. The rapid temperature decrease (from flare to quiescence) took approximately 1.5 years to reach thermal equilibrium with the neutron star core. Further work will now be carried out using Chandra data so more information about these rapidly spinning exotic objects can be uncovered.

Suddenly, neutron stars became a little less mysterious to me in the 10 minute talk last Tuesday, I love conferences…

Related publications:

Posted on by Ian O'Neill

Hunt is on for “Killer” Third Star in BD+20 307 Binary System

Exoplanet collision in BD+20 301. Possibly an Earth-like rocky exoplanet was involved? (Lynette Cook)

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In September, it was announced the Chandra X-ray Observatory had spotted something very odd about BD+20 307. The binary system appeared to have a dusty disk surrounding it, indicative of a young, planet-forming system a fraction of the age of the Solar System. However, it was well known that the binary was actually several billion years old. It turns out that this disk was created by a rare planetary event; a cataclysmic planetary collision.

On Wednesday, at the AAS conference in Long Beach, I attended the “Extrasolar Planets” session to listen in on more results from Hubble about the exciting exoplanet discoveries in November… however, for me, the most captivating talk was about the strange, dusty old binary and the future detective work to be carried out to track down a planet killer…

The talks by astrophysicists working with the optical Hubble data were superb, showing off some of the science behind last years spate of direct observations of exoplanets, particularly the massive planet orbiting the star Fomalhaut, shaping a scattered disk of dust. However, there was no further news to report, apart from some cool numerical models the scientists will be using to characterize Fomalhaut b and a very interesting talk about the predicted lifetimes of exoplanets undergoing tidal stresses (which, unfortunately, I missed the first five minutes of as I got lost in the Long Beach Convention Center).

The one presentation that did pique my interest was Ben Zuckerman’s review of the progress being made in the study of BD+20 307. A few months ago, this piece of research caused a huge amount of interest as it provided the first piece of evidence of a huge, rocky planetary collision in the star system 300 light years away. Naturally, many news sources ran with article titles like: Is this what the Solar System would look like after Earth was hit by another planetary body? As Zuckerman pointed out, the fact that the group used an artist impression of a colliding Earth-like body (plus land masses and oceans, as pictured top) was no accident. BD+20 307 is certainly at an age when oceans might have formed and life–as Zuckerman morbidly conjectured–may have thrived. Not for any longer…

Usually when we observe dust around a star, we can assume that it is a planet-forming star system that is fairly young. Conversely, as I found out to great depth in the conference, very old white dwarf systems can reveal a lot about their past planetary population when their dusty contaminants are studied. However, the dust contained in the BD+20 307 system is a puzzle. Astronomers had discovered a system, of comparable age to ours with a large amount of warm dust (T~500K). A system of that age will have long since expelled (via stellar wind pressure) or accreted any left-over dust from the planet-forming stages. Therefore, the only remaining explanation is that a rocky body collided with another, ejecting a huge amount of micron-sized warm dust particles.

So is this what the Solar System would look like after Earth is shattered by another planet? Possibly.

Zuckerman then pushed into some work being done to understand how the planetary collision could have happened in the first place. After all, the planets in our Solar System have settled into long-term stable orbits, any planet in BD+20 307 will have the same qualities. There were some questions as to whether the binary stars may have contributed to destabilizing the system, but Zuckerman quickly argued against this idea as the binary has such a tight orbit (with an orbital period of only 3.5 days), the destroyed planet will have found a stable orbit far from any gravitational variations.

So what could have caused the carnage in BD+20 307? We know that massive planetary bodies exert a huge gravitational pull on their host star and other planets in a system (i.e. Jupiter in the case of our Solar System), occasionally bullying (and sometimes capturing) them along the way. A small nudge in the wrong direction and planets could be knocked from their orbits, set on a collision course. So, much effort is now being put into a search for a third, faint star in BD+20 307. Perhaps it could be orbiting far away from the dancing binary, occasionally swinging past the planetary bodies, setting up the huge collision event.

This certainly seems reasonable, as 70% of binary star systems are found to have a third star. However, Zuckerman’s team have yet to find the “killer” third star and he appears confident that after careful analysis that there is no other stellar body within a 20 AU radius of the binary pair. Next, he intends to study the “wobble” of the centre of mass of the BD+20 307 binary to see if there is any gravitational anomaly as the mysterious “third star” tugs at the pair.

Posted on by Nancy Atkinson

Cassiopeia A Comes Alive in 3-D Movies

Cassiopeia A from Chandra. Credit: NASA/CXC/D.Berry

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Want to know what it’s like to fly through a supernova remnant? Then, THIS, you have to see. You’ll be able to experience SNR Cassiopeia A (Cas A) as never before, and see it across both time and space. Another time lapse animation shows the remnant’s expansion and changes over time, and still another provides a 3-D model of Cas A. Almost ten years ago, Chandra’s “First Light” image of Cas A revealed previously unseen structures and detail, and now, after eight years of observation, scientists have been able to construct these incredible animations which were presented at today’s American Astronomical Society meeting in Long Beach, California.

The fly-through movie is based on data from Chandra, NASA’s Spitzer Space Telescope, and ground-based optical telescopes. “We have always wanted to know how the pieces we see in two dimensions fit together with each other in real life,” said Tracey Delaney of the Massachusetts Institute of Technology. “Now we can see for ourselves with this ‘hologram’ of supernova debris.”

Delaney said there are two components to the explosion, a spherical component from the outer layers of the star and a flattened component from the inner layers of the star. Most intriguing, said Delaney is that the jets of the explosion are not all over the place but came out of the same plane in the supernova. Plumes, or jets, of silicon appear in the northeast and southwest, while plumes of iron are seen in the southeast and north. Astronomers had known about the plumes and jets before, but did not know that they all came out in a broad, disk-like structure.

Cas A expansion. Credit: NASA/CXC/SAO/D.Patnaude et al.
Cas A expansion. Credit: NASA/CXC/SAO/D.Patnaude et al.


The time-lapse animation tracks the remnant’s expansion and changes over time, measuring the expansion velocity of features in Cas A. “With Chandra, we have watched Cas A over a relatively small amount of its life, but so far the show has been amazing,” said Daniel Patnaude of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “And, we can use this to learn more about the aftermath of the star’s explosion.”

Using estimates of the properties of the supernova explosion, including its energy and dynamics, Patnaude’s group show that about 30% of the energy in this supernova has gone into accelerating cosmic rays, energetic particles that are generated, in part, by supernova remnants and constantly bombard the Earth’s atmosphere. The flickering in the movie provides valuable new information about where the acceleration of these particles occurs.

The researchers found the expansion is slower than expected based on current theoretical models. Patnaude thinks the explanation for this mysterious loss of energy is cosmic ray acceleration.

Cas A in 3-D. Credit: NASA/CXC/MIT/T.Delaney et al.
Cas A in 3-D. Credit: NASA/CXC/MIT/T.Delaney et al.

The 3-D model of Cas A was made possible through a collaboration with the Astronomical Medicine project based at Harvard. The goal of this project is to bring together the best techniques from two very different fields, astronomy and medical imaging.

“Right now, we are focusing on improving three-dimensional visualization in both astronomy and medicine,”said Harvard’s Alyssa Goodman who heads the Astronomical Medicine project. “This project with Cas A is exactly what we have hoped would come out of it.”

3-D visualization and the 3-D expansion model provide researchers with a unique ability to study this remnant. The implication of this work is that astronomers who build models of supernova explosions must now consider that the outer layers of the star come off spherically, but the inner layers come out more disk like with high-velocity jets in multiple directions.

Cassiopeia A is the remains of a star thought to have exploded about 330 years ago, and is one of the youngest remnants in the Milky Way galaxy. The study of Cas A and remnants like it help astronomers better understand how the explosions that generate them seed interstellar gas with heavy elements, heat it with the energy of their radiation, and trigger blast waves from which new stars form.

Source: Chandra site

Posted on by Nancy Atkinson

No “Big Rip” in our Future: Chandra Provides Insights Into Dark Energy

Galaxy cluster Abell 85, seen by Chandra, left, and a model of the growth of cosmic structure when the Universe was 0.9 billion, 3.2 billion and 13.7 billion years old (now). Credit: Chandra

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When you throw a ball up into the air, you expect gravity will eventually slow the ball, and it will come back down again. But what if you threw a ball up into the air and instead of coming back down, it accelerated away from you? That’s basically what is happening with our universe: everything is accelerating away from everything else. This acceleration was discovered in 1998, and scientists believe “dark energy” is responsible, a form of repulsive gravity, and it composes a majority of the universe, about 72%. We don’t know what it is yet, but now, for the first time, astronomers have clearly seen the effects of dark energy. Using the Chandra X-ray Observatory, scientists have tracked how dark energy has stifled the growth of galaxy clusters. Combining this new data with previous studies, scientists have obtained the best clues yet about what dark energy is, confirming its existence. And there’s good news, too: the expanding Universe won’t rip itself apart.

Previous methods of dark energy research measured Type Ia supernovae. The new X-ray results provide a crucial independent test of dark energy, long sought by scientists, which depends on how gravity competes with accelerated expansion in the growth of cosmic structures.

“This result could be described as ‘arrested development of the universe’,” said Alexey Vikhlinin of the Smithsonian Astrophysical Observatory in Cambridge, Mass., who led the research. “Whatever is forcing the expansion of the universe to speed up is also forcing its development to slow down.”

Vikhlinin and his colleagues used Chandra to observe the hot gas in dozens of galaxy clusters, which are the largest collapsed objects in the universe. Some of these clusters are relatively close and others are more than halfway across the universe.

The results show the increase in mass of the galaxy clusters over time aligns with a universe dominated by dark energy. It is more difficult for objects like galaxy clusters to grow when space is stretched, as caused by dark energy. Vikhlinin and his team see this effect clearly in their data. The results are remarkably consistent with those from the distance measurements, revealing general relativity applies, as expected, on large scales.

Previously, it wasn’t known for sure if dark energy was a constant across space, with a strength that never changes with distance or time, or if it is a function of space itself and as space expands dark energy would expand and get stronger. In other words, it wasn’t known if Einstein’s theory of general relativity and his cosmological constant was correct or if the theory would have to be modified for large scales.

But the Chandra study strengthens the evidence that dark energy is the cosmological constant, and is not growing in strength with time, which would cause the Universe to eventually rip itself apart.

“Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that ‘nothing weighs something’,” said Vikhlinin. “A lot more testing is needed, but so far Einstein’s theory is looking as good as ever.”

These results have consequences for predicting the ultimate fate of the universe. If dark energy is explained by the cosmological constant, the expansion of the universe will continue to accelerate, and everything will disappear from sight of the Milky Way and its gravitationally bound neighbor galaxy, Andromeda. This won’t happen soon, but Vikhlinin said, “Double the age of Universe from today, and you will see strong affect. An astronomer would say this may be a good time to fund cosmological research because further down the road there will be nothing to observe!”

Vikhlinin’s paper can be found here.

Source: Chandra Press Release, press conference

Posted on by Nancy Atkinson

Clash of Clusters Separates Dark Matter From Ordinary Matter

Credit: X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)

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A powerful collision of galaxy clusters captured by NASA’s Hubble Space Telescope and Chandra X-ray Observatory provides evidence for dark matter and insight into its properties. Observations of the cluster known as MACS J0025.4-1222 indicate that a titanic collision has separated dark matter from ordinary matter. The images also provide an independent confirmation of a similar effect detected previously in a region called the Bullet Cluster. Like the Bullet Cluster, this newly studied cluster shows a clear separation between dark and ordinary matter.

MACS J0025 formed after an enormously energetic collision between two large clusters. Using visible-light images from Hubble, the team was able to infer the distribution of the total mass — dark and ordinary matter. Hubble was used to map the dark matter (colored in blue) using a technique known as gravitational lensing. The Chandra data enabled the astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (pink).

As the two clusters that formed MACS J0025 (each almost a whopping quadrillion times the mass of the Sun) merged at speeds of millions of miles per hour, the hot gas in the two clusters collided and slowed down, but the dark matter passed right through the smashup. The separation between the material shown in pink and blue therefore provides observational evidence for dark matter and supports the view that dark-matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.

On the Chandra website, there are two animations, one that shows the different views of this cluster viewed by the different observatories, and another depicting how the galaxies may have collided.

Bullet Cluster. Credit: NASA/CXC/CfA/STScI
Bullet Cluster. Credit: NASA/CXC/CfA/STScI

These new results show that the Bullet Cluster is not an anomalous case and helps answers questions about how dark matter interacts with itself.

Sources: HubbleSite, Chandra

Posted on by Ian O'Neill

How do you Weigh a Supermassive Black Hole? Take its Temperature

A composite image of Chandra and Hubble Space Telescope observations of giant elliptical galaxy NGC 4649 (ASA/STScI/NASA/CXC/UCI/P. Humphrey et al.)

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Working out the mass of huge black holes, like the ones hiding in the centre of galactic nuclei, is no easy task and attempts are being made to find novel ways to weigh them. Using data from the Chandra X-ray Observatory, two scientists have confirmed a theory they conceived ten years ago, that the supermassive black holes in the centre of galaxies strongly influence the nature of the gases surrounding them. So, acting like a remote thermometer, Chandra is being used to probe deep into the neighbourhood of these exotic objects, gauging their masses very accurately…

The supermassive black hole at the centre of NGC 4649 is a monster. It is about 3.4 billion times the mass of the Sun and a thousand times bigger than the black hole at the centre of the Milky Way. This fact makes it an ideal candidate to test new methods of measuring the mass of black holes to see how the results correlate with traditional methods. With a high degree of accuracy, scientists have proven that a previously untested theory of weighing black holes works by using the Chandra X-ray telescope.

Until now, supermassive black hole masses have been measured by observing the motions of stars and gas deep inside galactic nuclei, now astronomers are using the gravitational influence of the black hole over the hot gas trapped around the singularity. As the gas is pulled slowly toward the black hole, it is compressed and heated. The bigger the black hole, the higher the peak temperature. Chandra has been used to measure the peak temperature of the gas right in the centre of NGC 4649 to find the derived mass is identical to the mass previously measured by traditional means.

Fabrizio Brighenti from the University of Bologna in Italy, and William Mathews from the University of California at Santa Cruz have been working on this research for the past decade. It is only now, with the availability of a telescope as powerful as Chandra that these observations have been possible.

“It was wonderful to finally see convincing evidence of the effects of the huge black hole that we expected. We were thrilled that our new technique worked just as well as the more traditional approach for weighing the black hole.” – Fabrizio Brighenti

The black hole inside NGC 4649 appears to be in a dormant state; it doesn’t seem to be pulling in material toward its event horizon very rapidly and it isn’t generating much light as it slowly grows. Therefore, using Chandra to indirectly measure its mass by sensing the peak temperature of surrounding matter is required to weigh it. In the early universe, huge black holes such as these will have generated dramatic displays of light. Now, in the local Universe, such black holes lead a more retiring life, making them difficult to observe. This prospect excites the lead scientist on the project, Philip Humphrey. “We can’t wait to apply our new method to other nearby galaxies harboring such inconspicuous black holes,” he said.

Source: Physorg.com

Posted on by Ian O'Neill

X-Ray Flare Echo Reveals Supermassive Black Hole Torus

The echo of X-ray emissions from a black hole swallowing a star can be observed as light echos (MPE/ESA)

The light echo of an X-ray flare from the nucleus of a galaxy has been observed. The flare almost certainly originates from a single star being gravitationally ripped apart by a supermassive black hole in the galactic core. As the star was being pulled into the black hole, its material was injected into the black hole accretion disk, causing a sudden burst of radiation. The resulting X-ray flare emission was observed as it hit local stellar gases, producing the light echo. This event gives us a better insight to how stars are eaten by supermassive black holes and provides a method to map the structure of galactic nuclei. Scientists now believe they have observational evidence for the elusive molecular torus that is thought to surround active supermassive black holes.

Light echoes from distant galaxies have been observed before. The echoes from a supernova that occurred 400 years ago (that is now observed as the supernova remnant SNR 0509-67.5) were only just observed here on Earth, after the supernova emissions bounced off galactic matter. This is the first time however that the energetic emissions from a sudden influx of matter into a supermassive black hole accretion disk has been observed echoing off gases within galactic nuclei. This is a major step toward understanding how stars are consumed by supermassive black holes. Additionally, the echo acts like a searchlight, highlighting the dark stellar matter between the stars, revealing a structure we have never seen before.

This new research was carried out by an international team led by Stefanie Komossa from the Max Planck Institute for extraterrestrial Physics in Garching, Germany, using data from the Sloan Digital Sky Survey. Komossa likens this observation to illuminating a dark city with a firework burst:

“To study the core of a normal galaxy is like looking at the New York skyline at night during a power failure: You can’t learn much about the buildings, roads and parks. The situation changes, for example, during a fireworks display. It’s exactly the same when a sudden burst of high-energy radiation illuminates a galaxy.” – Stefanie Komossa

A strong X-ray burst such as this can be very hard to observe as they are short-lived emissions, but a huge amount of information can be gained by seeing such an event if astronomers are quick enough. By analysing the degree of ionization and velocity data in the spectroscopic emission lines of the echoed light, the Max Planck physicists were able to deduce the flare location. Held within the emission lines are the cosmic “fingerprints” of the atoms at the source of the emission, leading them to the galactic core where a supermassive black hole is believed to live.

A molecular torus surrounding a supermassive black hole (NASA/ESA)

The standard model for galactic nuclei (a.k.a. unified model of active galaxies) predict a “molecular torus” surrounding the black hole accretion disk. These new observations of the galaxy named SDSSJ0952+2143 appear to show the X-ray flare was reflected by the galactic molecular torus (with strong iron emission lines). This is the first time the presence of a possible torus has been seen, and if confirmed, astrophysicists will have their observational evidence of this theoretical possibility, strengthening the standard model. What’s more, using accretion disk flares may aid scientists when attempting to map the structure of other molecular toruses.

Strengthening the observation of echoed X-ray emission from the torus is the possibility of seeing variable infrared emissions. This emission signifies a “last call for help” by the dusty cloud being rapidly heated by the incident X-rays. The dust will have been vaporized soon after.

But how do they know it was a star that fell into the accretion disk? In addition to the strong iron lines, there are strange hydrogen emission lines which have never been seen before. This is a strong piece of evidence that it is the debris from a star that came too close to the black hole, stripping away its hydrogen fuel.

Although the X-ray flare has subsided, the galaxy continues to be observed by the X-ray satellite Chandra. Faint but measurable X-ray emissions are being observed perhaps signifying that the star is still being fed to the accretion disk. It seems possible that measuring this faint emission may also be of use, allowing researchers to continue to map the molecular torus long after the initial strong X-ray emission has ended.

Sources: arXiv, Max Planck Institute for Extraterrestrial Physics

Posted on by Ian O'Neill

Light Echos from 400 Year Old Supernova Observed for the First Time (Time-lapse Video)

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Its observations like these that really give us an idea about how big the cosmos actually is. A star in a small galaxy called the Large Magellanic Cloud (LMC), some 160,000 light years from Earth, exploded as a massive supernova 400 years ago (Earth years that is). Combining the observations from an X-ray observatory and an optical telescope, scientists are currently observing the reflected light off galactic dust, only just reaching the Earth hundreds of years after the explosion…

Shakespeare’s first run the stage production, Hamlet, will have been in full-swing. Galileo might have been experimenting with his first telescope. Guy Fawkes could have been plotting to blow up the British parliament. These events all occurred around the beginning of the 17th Century when a bright point of light may have been seen in the night sky. This point of light, in the Large Magellanic Cloud (LMC), is a massive star exploding, ending its life in a powerful supernova.

Now, 400 years after the event, we can see a “supernova remnant” (SNR), and this particular remnant is known as SNR 0509-67.5 (not very romantic I know). The remnant of superheated gas slowly expands into space and still emits X-rays of various energies. The 400 year old explosion has even been imaged in great detail by the Chandra Observatory currently observing space in X-ray wavelengths. Analysis of the SNR indicates that it was most likely caused by a Type Ia supernova after analysis of the composition of the gases, in particular the quantities of silicon and iron, was carried out. It is understood that the supernova was caused when a white dwarf star in a binary system reached critical mass, became gravitationally unstable (due to fusion reactions in the core stopping) and exploded.

When SNR 0509-67.5 exploded all those years ago, it will have radiated optical electromagnetic radiation (optical light) in all directions of space. Now, for the first time, optical Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory (Chile) has observed reflected light from within the LMC originating from the supernova, 400 years after the event. Using the (reflected) optical light and X-ray emissions directly from the supernova remnant, scientists have been able to learn just how much energy was generated by the explosion.

Astronomers have even assembled a time-lapse video from observations of the light “echo” from 2001 to 2006. Although there are only five frames to the video, you can see the location of the reflected light change shape as different volumes of galactic dust are illuminated by the flash of supernova light. In each progressive frame, the clouds of gas that become illuminated will be further and further away from us, we are effectively looking further back in time as the light “echoes” bounce off the galactic matter.

An amazing discovery.

Source: Chandra X-ray Observatory

Posted on by Ian O'Neill

Pulsars are Exploding Unexpectedly and “Magnetars” Might be to Blame

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Pulsars are fast-spinning, highly radiating neutron stars. Most pulsars emit radio, X-ray and gamma radiation at regular intervals (usually periods of a few milliseconds to a few seconds), in fact many pulses keep the accuracy of the most accurate atomic clocks on Earth. However, occasionally, these rapidly rotating bodies undergo a violent change, blasting massive quantities of energy into space. Although short-lived (a fraction of a second), the observed explosion packs a punch of at least 75,000 Suns. Is this a natural process in the life of a pulsar? Is it a totally different type of cosmic phenomena? Researchers suggest these observations may be a different type of neutron star: magnetars disguised as pulsars (and without an ounce of dark matter in sight!)…

Neutron stars are a product of massive stars after a supernova. The star isn’t big enough to create a black hole (i.e. less than 5 solar masses), but it is big enough to create a tiny, dense and hot mass of neutrons (hence the name). Due to the “Pauli exclusion principal” – a quantum mechanical principal that prevents any two neutrons from having the same quantum characteristics within the same volume – neutron stars are also predicted to be very hot. Intense gravity forces matter into a tiny volume, but quantum effects are repelling the neutrons. After the star has gone supernova, as neutron stars are so small (a radius of only 10 to 20 km), the small mass preserves the stars angular momentum, resulting in a fast-spinning, highly radiating body.

Much of the stars magnetism is also preserved, but in a vastly increased dense state. Neutron stars are therefore expected to have an intense magnetic field. It is in fact this magnetic field that helps to generate jets of emission from the magnetic poles of the rotating body, creating a beam of radiation (much like a lighthouse).

However, one of these flashing lighthouses has surprised observers… it exploded, blasting vast amounts of energy into space, and then continued to spin and flash as if nothing had happened. This phenomenon has recently been observed by NASA’s Rossi X-ray Timing Explorer (RXTE) and has been backed up by data from the Chandra X-ray Observatory.

There are in fact other classes of neutron star out there. Slow-spinning, highly magnetic “magnetars” are considered to be a separate type of neutron star. They are distinct from the less-magnetic pulsar as they sporadically release vast amounts of energy into space and do not exhibit the periodic rotation we understand from pulsars. It is believed that magnetars explode as the intense magnetic field (the strongest magnetic field believed to exist in the Universe) warps the neutron star surface, causing extremely energetic reconnection events between magnetic flux, causing violent and sporadic X-ray bursts.

There is now speculation that known periodic pulsars that suddenly exhibit magnetar-like explosions are actually the highly magnetic cousins of pulsars disguised as pulsars. Pulsars simply do not have enough magnetic energy to generate explosions of this magnitude, magnetars do.

Fotis Gavriil of NASA’s Goddard Space Flight Center in Greenbelt, and his colleagues analysed a young neutron star (called PSR J1846-0258 in the constellation Aquila). This pulsar was often considered to be “normal” due to its fast spin (3.1 revolutions per second), but RXTE observed five magnetar-like X-ray bursts from the pulsar in 2006. Each event lasted no longer than 0.14 seconds and generated the energy of 75,000 Suns. Follow up observations by Chandra confirmed that over the course of six years, the pulsar had become more “magnetar-like”. The rotation of the pulsar is also slowing down, suggesting a high magnetic field may be braking its rotation.

These findings are significant, as it suggests that pulsars and magnetars may be the same creature, just at different periods of a pulsars lifetime, and not two entirely different classes of neutron star…

Results of this research will be published in today’s issue of Science Express.

Source: AAAS Science Express

Posted on by Fraser Cain

Researchers Find a Supernova, Before it Exploded

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The problem with supernovae is that you never know where they’re going to happen. Your only clue is the bright flash in the sky, and then it’s too late. But a team of European researchers think they were lucky enough to have spotted the precursor to supernova.

In an article in the February 14th issue of the journal Nature, a team of European researchers describe how they were trying to find evidence of a binary system after one of the objects detonated as a supernova. In looking back through archived images captured by NASA’s Chandra X-Ray Observatory, they were lucky enough to find one image that actually contained the system.

The supernova, known as SN 2007on exploded as a Type Ia. This is the situation where a white dwarf is in orbit around another star. It’s possible that the white dwarf feeds off material ejected from the other star until it hits a critical amount of mass – approximately 1.4 times the mass of our Sun. Or maybe it’s actually a collision between a white dwarf and another star, or between two white dwarfs.

Whatever the condition, the result is always the same. The white dwarf detonates suddenly with a very specific amount of energy and characteristic light curve. Astronomers use these explosions to measure distance in the Universe, since they’re always exploding with the same amount of energy.

To really figure out what’s going on, astronomers need more examples of these precursors. They need to be able to study a potential Type Ia supernova before it actually explodes.

So, the researchers finally have a target they can study. In the case of SN 2007on, the data gathered by the Chandra X-Ray Telescope strengthens the “mass stealing” theory. X-rays streaming from the system show the kind of fusion you would expect from a white dwarf consuming material from a neighbour.

This isn’t a slam dunk, though. A higher-quality optical image shows the binary system to be in a slightly different position from where the supernova detonated. So maybe this system isn’t the precursor after all.

But followup observations from Chandra show that the X-ray source is gone. Whatever was at that location isn’t there any more. Perhaps it did indeed vaporize in a supernova explosion.

Original Source: Chandra News Release