Posted on by Vanessa Janek

Gamma Ray Bursts Limit The Habitability of Certain Galaxies, Says Study

An artistic image of the explosion of a star leading to a gamma-ray burst. (Source: FUW/Tentaris/Maciej Fro?ow)

Gamma ray bursts (GRBs) are some of the brightest, most dramatic events in the Universe. These cosmic tempests are characterized by a spectacular explosion of photons with energies 1,000,000 times greater than the most energetic light our eyes can detect. Due to their explosive power, long-lasting GRBs are predicted to have catastrophic consequences for life on any nearby planet. But could this type of event occur in our own stellar neighborhood? In a new paper published in Physical Review Letters, two astrophysicists examine the probability of a deadly GRB occurring in galaxies like the Milky Way, potentially shedding light on the risk for organisms on Earth, both now and in our distant past and future.

There are two main kinds of GRBs: short, and long. Short GRBs last less than two seconds and are thought to result from the merger of two compact stars, such as neutron stars or black holes. Conversely, long GRBs last more than two seconds and seem to occur in conjunction with certain kinds of Type I supernovae, specifically those that result when a massive star throws off all of its hydrogen and helium during collapse.

Perhaps unsurprisingly, long GRBs are much more threatening to planetary systems than short GRBs. Since dangerous long GRBs appear to be relatively rare in large, metal-rich galaxies like our own, it has long been thought that planets in the Milky Way would be immune to their fallout. But take into account the inconceivably old age of the Universe, and “relatively rare” no longer seems to cut it.

In fact, according to the authors of the new paper, there is a 90% chance that a GRB powerful enough to destroy Earth’s ozone layer occurred in our stellar neighborhood some time in the last 5 billion years, and a 50% chance that such an event occurred within the last half billion years. These odds indicate a possible trigger for the second worst mass extinction in Earth’s history: the Ordovician Extinction. This great decimation occurred 440-450 million years ago and led to the death of more than 80% of all species.

Today, however, Earth appears to be relatively safe. Galaxies that produce GRBs at a far higher rate than our own, such as the Large Magellanic Cloud, are currently too far from Earth to be any cause for alarm. Additionally, our Solar System’s home address in the sleepy outskirts of the Milky Way places us far away from our own galaxy’s more active, star-forming regions, areas that would be more likely to produce GRBs. Interestingly, the fact that such quiet outer regions exist within spiral galaxies like our own is entirely due to the precise value of the cosmological constant – the factor that describes our Universe’s expansion rate – that we observe. If the Universe had expanded any faster, such galaxies would not exist; any slower, and spirals would be far more compact and thus, far more energetically active.

In a future paper, the authors promise to look into the role long GRBs may play in Fermi’s paradox, the open question of why advanced lifeforms appear to be so rare in our Universe. A preprint of their current work can be accessed on the ArXiv.

Posted on by Brian Koberlein

How CERN’s Discovery of Exotic Particles May Affect Astrophysics

The difference between a neutron star and a quark star (Chandra)

You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430).  A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers.  The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark.  The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.

A periodic table of elementary particles. Credit: Wikipedia
A periodic table of elementary particles.
Credit: Wikipedia

The building blocks of matter are made of leptons (such as the electron and neutrinos) and quarks (which make up protons, neutrons, and other particles).  Quarks are very different from other particles in that they have an electric charge that is 1/3 or 2/3 that of the electron and proton.  They also possess a different kind of “charge” known as color.  Just as electric charges interact through an electromagnetic force, color charges interact through the strong nuclear force.  It is the color charge of quarks that works to hold the nuclei of atoms together. Color charge is much more complex than electric charge.  With electric charge there is simply positive (+) and its opposite, negative (-).  With color, there are three types (red, green, and blue) and their opposites (anti-red, anti-green, and anti-blue).

Because of the way the strong force works, we can never observe a free quark.  The strong force requires that quarks always group together to form a particle that is color neutral. For example, a proton consists of three quarks (two up and one down), where each quark is a different color.  With visible light, adding red, green and blue light gives you white light, which is colorless. In the same way, combining a red, green and blue quark gives you a particle which is color neutral.  This similarity to the color properties of light is why quark charge is named after colors.

Combining a quark of each color into groups of three is one way to create a color neutral particle, and these are known as baryons.  Protons and neutrons are the most common baryons.  Another way to combine quarks is to pair a quark of a particular color with a quark of its anti-color.  For example, a green quark and an anti-green quark could combine to form a color neutral particle.  These two-quark particles are known as mesons, and were first discovered in 1947.  For example, the positively charged pion consists of an up quark and an antiparticle down quark.

Under the rules of the strong force, there are other ways quarks could combine to form a neutral particle.  One of these, the tetraquark, combines four quarks, where two particles have a particular color and the other two have the corresponding anti-colors.  Others, such as the pentaquark (3 colors + a color anti-color pair) and the hexaquark (3 colors + 3 anti-colors) have been proposed.  But so far all of these have been hypothetical.  While such particles would be color neutral, it is also possible that they aren’t stable and would simply decay into baryons and mesons.

There has been some experimental hints of tetraquarks, but this latest result is the strongest evidence of 4 quarks forming a color neutral particle.  This means that quarks can combine in much more complex ways than we originally expected, and this has implications for the internal structure of neutron stars.

Very simply, the traditional model of a neutron star is that it is made of neutrons.  Neutrons consist of three quarks (two down and one up), but it is generally thought that particle interactions within a neutron star are interactions between neutrons.  With the existence of tetraquarks, it is possible for neutrons within the core to interact strongly enough to create tetraquarks.  This could even lead to the production of pentaquarks and hexaquarks, or even that quarks could interact individually without being bound into color neutral particles.  This would produce a hypothetical object known as a quark star.

This is all hypothetical at this point, but verified evidence of tetraquarks will force astrophysicists to reexamine some the assumptions we have about the interiors of neutron stars.

Addendum: It has been pointed out that CERN’s results are not an original discovery, but rather a confirmation of earlier results by the Belle Collaboration.  The Belle results can be found in a 2008 paper in Physical Review Letters, as well as a 2013 paper in Physical Review D.  So credit where credit is due.

Posted on by Elizabeth Howell

Home Computers Discover Gamma-Ray Pulsars

Gamma-ray pulsars in the Milky Way's plane, found by volunteers using Einstein@Home. The sky map is from Fermi's Large Area Telescope. The brighter the color you see, the more intense the radiation in that spot. The small flags show the nationality of the volunteers whose computers spotted the pulsars. Credit: Knispel/Pletsch/AEI/NASA/DOE/Fermi LAT Collaboration

Imagine that you’re innocently running your computer in pursuit of helping data crunch a huge science project. Then, out of the thousands of machines running the project, yours happens to stumble across a discovery. That’s what happened to several volunteers with Einstein@Home, which seeks pulsars in data from the Fermi Gamma-Ray Space Telescope, among other projects.

“At first I was a bit dumbfounded and thought someone was playing a hoax on me. But after I did some research,” everything checked out. That someone as insignificant as myself could make a difference was amazing,” stated Kentucky resident Thomas M. Jackson, who contributed to the project.

Pulsars, a type of neutron star, are the leftovers of stars that exploded as supernovae. They rotate rapidly, with such precision in their rotation periods that they have sometimes been likened to celestial clocks. Although the discovery is exciting to the eight volunteers because they are the first to find these gamma-ray pulsars as part of a volunteer computing project, the pulsars also have some interesting scientific features.

Artist's illustration of a neutron star, a tiny remnant that remains after its predecessor star explodes. Here, the 12-mile (20-kilometer) sphere is compared with the size of Hannover, Germany. Credit: NASA's Goddard Space Flight Center
Artist’s illustration of a neutron star, a tiny remnant that remains after its predecessor star explodes. Here, the 12-mile (20-kilometer) sphere is compared with the size of Hannover, Germany. Credit: NASA’s Goddard Space Flight Center

The four pulsars were discovered in the plane of the Milky Way in an area that radio telescopes had looked at previously, but weren’t able to find themselves. This means that the pulsars are likely only visible in gamma rays, at least from the vantage point of Earth; the objects emit their radiation in a narrow direction with radio, but a wider stripe with gamma rays. (After the discoveries, astronomers used the Max Planck Institute for Radio Astronomy’s 100-meter Effelsberg radio telescope and the Australian Parkes Observatory to peer at those spots in the sky, and still saw no radio signals.)

Two of the pulsars also “hiccup” or exhibit a pulsar glitch, when the rotation sped up and then fell back to the usual rotation period a few weeks later. Astronomers are still learning more about these glitches, but they do know that most of them happen in young pulsars. All four pulsars are likely between 30,000 and 60,000 years old.

Artist's conception of a gamma-ray pulsar. Gamma rays are shown in purple, and radio radiation in green. Credit: NASA/Fermi/Cruz de Wilde
Artist’s conception of a gamma-ray pulsar. Gamma rays are shown in purple, and radio radiation in green. Credit: NASA/Fermi/Cruz de Wilde

“The first-time discovery of gamma-ray pulsars by Einstein@Home is a milestone – not only for us but also for our project volunteers. It shows that everyone with a computer can contribute to cutting-edge science and make astronomical discoveries,” stated co-author Bruce Allen, principal investigator of Einstein@Home. “I’m hoping that our enthusiasm will inspire more people to help us with making further discoveries.”

Einstein@Home is run jointly by the Center for Gravitation and Cosmology at the University of Wisconsin–Milwaukee and the Albert Einstein Institute in Hannover, Germany. It is funded by the National Science Foundation and the Max Planck Society. As for the volunteers, their names were mentioned in the scientific literature and they also received certificates of discovery for their work.

Source: Max Planck Institute for Gravitational Physics

Posted on by Fraser Cain

What is a Pulsar?

What is a Pulsar?

They are what is known as the “lighthouses” of the universe – rotating neutron stars that emit a focused beam of electromagnetic radiation that is only visible if you’re standing in it’s path. Known as pulsars, these stellar relics get their name because of the way their emissions appear to be “pulsating” out into space.

Not only are these ancient stellar objects very fascinating and awesome to behold, they are very useful to astronomers as well. This is due to the fact that they have regular rotational periods, which produces a very precise internal in its pulses – ranging from milliseconds to seconds.

Description:

Pulsars are types of neutron stars; the dead relics of massive stars. What sets pulsars apart from regular neutron stars is that they’re highly magnetized, and rotating at enormous speeds. Astronomers detect them by the radio pulses they emit at regular intervals.

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

Formation:

The formation of a pulsar is very similar to the creation of a neutron star. When a massive star with 4 to 8 times the mass of our Sun dies, it detonates as a supernova. The outer layers are blasted off into space, and the inner core contracts down with its gravity. The gravitational pressure is so strong that it overcomes the bonds that keep atoms apart.

Electrons and protons are crushed together by gravity to form neutrons. The gravity on the surface of a neutron star is about 2 x 1011 the force of gravity on Earth. So, the most massive stars detonate as supernovae, and can explode or collapse into black holes. If they’re less massive, like our Sun, they blast away their outer layers and then slowly cool down as white dwarfs.

But for stars between 1.4 and 3.2 times the mass of the Sun, they may still become supernovae, but they just don’t have enough mass to make a black hole. These medium mass objects end their lives as neutron stars, and some of these can become pulsars or magnetars. When these stars collapse, they maintain their angular momentum.

But with a much smaller size, their rotational speed increases dramatically, spinning many times a second. This relatively tiny, super dense object, emits a powerful blast of radiation along its magnetic field lines, although this beam of radiation doesn’t necessarily line up with it’s axis of rotation. So, pulsars are simply rotating neutron stars.

And so, from here on Earth, when astronomers detect an intense beam of radio emissions several times a second, as it rotates around like a lighthouse beam – this is a pulsar.

History:

The first pulsar was discovered in 1967 by Jocelyn Bell Burnell and Antony Hewis, and it surprised the scientific community by the regular radio emissions it transmitted. They detected a mysterious radio emission coming from a fixed point in the sky that peaked every 1.33 seconds. These emissions were so regular that some astronomers thought it might be evidence of communications from an intelligent civilization.

Although Burnell and Hewis were certain it had a natural origin, they named it LGM-1, which stands for “little green men”, and subsequent discoveries have helped astronomers discover the true nature of these strange objects.

Astronomers theorized that they were rapidly rotating neutron stars, and this was further supported by the discovery of a pulsar with a very short period (33-millisecond) in the Crab nebula. There have been a total of 1600 found so far, and the fastest discovered emits 716 pulses a second.

Later on, pulsars were found in binary systems, which helped to confirm Einstein’s theory of general relativity. And in 1982, a pulsar was found with a rotation period of just 1.6 microseconds. In fact, the first extrasolar planets ever discovered were found orbiting a pulsar – of course, it wouldn’t be a very habitable place.

Interesting Facts:

When a pulsar first forms, it has the most energy and fastest rotational speed. As it releases electromagnetic power through its beams, it gradually slows down. Within 10 to 100 million years, it slows to the point that its beams shut off and the pulsar becomes quiet.

When they are active, they spin with such uncanny regularity that they’re used as timers by astronomers. In fact, it is said that certain types of pulsars rival atomic clocks in their accuracy in keeping time.

Pulsars also help us search for gravitational waves, probe the interstellar medium, and even find extrasolar planets in orbit. In fact, the first extrasolar planets were discovered around a pulsar in 1992, when astronomers Aleksander Wolszczan and Dale Frail announced the discovery of a multi-planet planetary system around PSR B1257+12 – a millisecond pulsar now known to have two extrasolar planets.

Artist's impression of the planets orbiting PSR B1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)
Artist’s impression of the planets orbiting PSR B1257+12. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

It has even been proposed that spacecraft could use them as beacons to help navigate around the Solar System. On NASA’s Voyager spacecraft, there are maps that show the direction of the Sun to 14 pulsars in our region. If aliens wanted to find our home planet, they couldn’t ask for a more accurate map.

We have written many articles about stars here on Universe Today. Here’s an article about a newly discovered gamma ray pulsar, and here’s an article about how millisecond pulsars spin so fast.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We have recorded several episodes of Astronomy Cast about stars. Here are two that you might find helpful: Episode 12: Where Do Baby Stars Come From, and Episode 13: Where Do Stars Go When they Die?

Posted on by Shannon Hall

Newly Discovered Fast Radio Bursts May be Colliding Neutron Stars

An artist's conception of two neutron stars, moments before they collide. Image Credit: NASA

The Universe is sizzling with undiscovered phenomena. Only last month astronomers heard four unexpected bumps in the night. These Fast Radio Bursts released torrents of energy, each occurred only once, and lasted a few thousandths of a second. Their origin has since mystified astronomers.

Dismissing my first guess, which includes a feverish Jodie Foster verifying the existence of extraterrestrial life, astronomers have found a more likely answer. Two neutron stars collide, but before doing so produce a quick burst of radio emission, which we later observe as a Fast Radio Burst.

Our first hint? These Fast Radio Bursts are extra-galactic in origin.  The exact distance is quantifiable from a “dispersion measure – the frequency dependent time delay of the radio signal,” Dr. Tomonori Totani, lead author on the paper, told Universe Today. “This is proportional to the number of electrons along the line of sight.”

For all bursts, the short-wavelength component arrived at the telescope a fraction of a second before the longer wavelengths.  This is due to an effect known as interstellar dispersion: through any medium, longer-wavelength light moves slightly slower than short-wavelength light.

Light from extra-galactic objects will have to travel through intergalactic space, which is teeming with electrons in clouds of cold plasma. The farther the light travels, the more electrons it will have to travel through, and the greater the time delay between arriving wavelength components. By the time light reaches the Earth, it has been dispersed, and the amount of dispersion is directly correlated with distance.

These Fast Radio Bursts are likely to have originated anywhere from 5 to 10 billion light years away.

While the exact source of these Fast Radio Bursts has been highly debated, a recent hypothesis concludes that they are the result of merging neutron stars in the distant Universe.

In the final milliseconds before merging, the rotation periods of the two neutron stars synchronize – they become tidally locked to one another as the Moon is tidally locked to the Earth. At this point their magnetic fields also synchronize. Energetic charged particles spiral along the strong magnetic field lines and emit a beam of radio synchrotron emission.

Known neutron star magnetic field strengths are consistent with the radio flux observed in these Fast Radio Bursts.  The emission then ceases in a few milliseconds when the two neutron stars have collided, which explains the short duration of these Fast Radio Bursts.

Not only does this mechanism describe both the high energy and the time duration of these bursts, but they’re inferred occurrence rate as well. It’s likely that 100,000 Fast Radio Bursts occur each day. This matches the likely neutron star merger rate.

Merging neutrons stars will also create gravitational waves – ripples in the curvature of spacetime that propagate away from the event. Dr. Totani emphasized that the next step will be to perform a correlated search of gravitational waves and Fast Radio Bursts. Such a fast rate estimate is certainly good news for scientists hoping to detect gravitational waves in the nearby future.

The Universe is bursting with energy – literally – every 10 seconds, and until recently we simply had no idea. This recently discovered phenomenon is likely to be the center of a new active area of research. And I have no doubt that it will lead to exciting discoveries that just may break trends and burst into new territories.

The discovery paper may be found here, while the paper analyzing neutron stars as a likely source may be found here.

Posted on by Jason Major

Earth’s Gold Came From Colliding Stars

Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)

Are you wearing a gold ring? Or perhaps gold-plated earrings? Maybe you have some gold fillings in your teeth… for that matter, the human body itself naturally contains gold — 0.000014%, to be exact! But regardless of where and how much of the precious yellow metal you may have with you at this very moment, it all ultimately came from the same place.

And no, I don’t mean Fort Knox, the jewelry store, or even under the ground — all the gold on Earth likely originated from violent collisions between neutron stars, billions of years in the past.

Recent research by scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts has revealed that considerable amounts of gold — along with other heavy elements — are produced during impacts between neutron stars, the super-dense remains of stars originally 1.4 to 9 times the mass of our Sun.

The team’s investigation of a short-duration gamma-ray outburst that occurred in June (GRB 130603B) showed a surprising residual near-infrared glow, possibly from a cloud of material created during the stellar merger. This cloud is thought to contain a considerable amount of freshly-minted heavy elements, including gold.

“We estimate that the amount of gold produced and ejected during the merger of the two neutron stars may be as large as 10 moon masses – quite a lot of bling!” said lead author Edo Berger.

"With this remnant of a dead neutron star, I thee wed." (FreeDigitalPhotos.net/bigjom)
“With this remnant of a dead neutron star, I thee wed.” (FreeDigitalPhotos.net/bigjom)

The mass of the Moon is 7.347 x 1022 kg… about 1.2% the mass of Earth. The collision between these neutron stars then, 3.9 billion light-years away, produced 10 times that much gold based on the team’s estimates.

Quite a lot of bling, indeed.

Gamma-ray bursts come in two varieties – long and short – depending on the duration of the gamma-ray flash. GRB 130603B, detected by NASA’s Swift satellite on June 3rd, lasted for less than two-tenths of a second.

Although the gamma rays disappeared quickly, GRB 130603B also displayed a slowly fading glow dominated by infrared light. Its brightness and behavior didn’t match the typical “afterglow” created when a high-speed jet of particles slams into the surrounding environment.

Instead, the glow behaved like it came from exotic radioactive elements. The neutron-rich material ejected by colliding neutron stars can generate such elements, which then undergo radioactive decay, emitting a glow that’s dominated by infrared light – exactly what the team observed.

“We’ve been looking for a ‘smoking gun’ to link a short gamma-ray burst with a neutron star collision,” said Wen-fai Fong, a graduate student at CfA and a co-author of the paper. “The radioactive glow from GRB 130603B may be that smoking gun.”

The team calculates that about one-hundredth of a solar mass of material was ejected by the gamma-ray burst, some of which was gold. By combining the estimated gold produced by a single short GRB with the number of such explosions that have likely occurred over the entire age of the Universe, all the gold in the cosmos – and thus on Earth – may very well have come from such gamma-ray bursts.

Watch an animation of two colliding neutron stars along with the resulting GRB below (Credit: Dana Berry, SkyWorks Digital, Inc.):

How much gold is there on Earth, by the way? Since most of it lies deep inside Earth’s core and is thus unreachable, the total amount ever retrieved by humans over the course of history is surprisingly small: about 172,000 tonnes, or enough to make a cube 20.7 meters (68 feet) per side (based on the Thomson Reuters GFMS annual survey.) Some other estimates put this amount at slightly more or less, but the bottom line is that there really isn’t all that much gold available in Earth’s crust… which is partly what makes it (and other “precious” metals) so valuable.

And perhaps the knowledge that every single ounce of that gold was created by dead stars smashing together billions of years ago in some distant part of the Universe would add to that value.

“To paraphrase Carl Sagan, we are all star stuff, and our jewelry is colliding-star stuff,” Berger said.

The team’s findings were presented today in a press conference at the CfA in Cambridge. (See the paper here.)

Source: Harvard-Smithsonian CfA

Posted on by Tammy Plotner

Neutron Stars: A Cataclysmic Conception

This "SWASI" phenomenon is an analogue of the SASI instability occurring in the supernova core, but it is one million times smaller and about one hundred times slower than its astrophysical counterpart. (Image copyrights: Thierry Foglizzo, Laboratoire AIM Paris-Saclay, CEA)

It’s one of the most intense and violent of all events in space – a supernova. Now a team of researchers at the Max Planck Institute for Astrophysics have been taking a very specialized look at the formation of neutron stars at the center of collapsing stars. Through the use of sophisticated computer simulations, they have been able to create three-dimensional models which show the physical effects – intense and violent motions which occur when stellar matter is drawn inward. It’s a bold, new look into the dynamics which happen when a star explodes.

As we know, stars which have eight to ten times the mass of the Sun are destined to end their lives in a massive explosion, the gases blown into space with incredible force. These cataclysmic events are among the brightest and most powerful events in the Universe and can outshine a galaxy when they occur. It is this very process which creates elements critical to life as we know it – and the beginnings of neutron stars.

Neutron stars are an enigma unto themselves. These highly compact stellar remnants contain as much as 1.5 times the mass of the Sun, yet are compressed to the size of a city. It is not a slow squeeze. This compression happens when the stellar core implodes from the intense gravity of its own mass… and it takes only a fraction of a second. Can anything stop it? Yes. It has a limit. Collapse ceases when the density of the atomic nuclei is exceeded. That’s comparable to around 300 million tons compressed into something the size of a sugar cube.

Studying neutron stars opens up a whole new dimension of questions which scientists are keen to answer. They want to know what causes stellar disruption and how can the implosion of the stellar core revert to an explosion. At present, they theorize that neutrinos may be a critical factor. These tiny elemental particles are created and expelled in monumental numbers during the supernova process and may very well act as heating elements which ignite the explosion. According to the research team, neutrinos could impart energy into the stellar gas, causing it to build up pressure. From there, a shock wave is created and as it speeds up, it could disrupt the star and cause a supernova.

As plausible as it might sound, astronomers aren’t sure if this theory could work or not. Because the processes of a supernova cannot be recreated under laboratory conditions and we’re not able to directly see into the interior of a supernovae, we’ll just have to rely on computer simulations. Right now, researchers are able to recreate a supernova event with complex mathematical equations which replicate the motions of stellar gas and the physical properties which happen at the critical moment of core collapse. These types of computations require the use of some of the most powerful supercomputers in the world, but it has also been possible to use more simplified models to get the same results. “If, for example, the crucial effects of neutrinos were included in some detailed treatment, the computer simulations could only be performed in two dimensions, which means that the star in the models was assumed to have an artificial rotational symmetry around an axis.” says the research team.

With the support of the Rechenzentrum Garching (RZG), scientists were able to create in a singularly efficient and fast computer program. They were also given access to most powerful supercomputers, and a computer time award of nearly 150 million processor hours, which is the greatest contingent so far granted by the “Partnership for Advanced Computing in Europe (PRACE)” initiative of the European Union, the team of researchers at the Max Planck Institute for Astrophysics (MPA) in Garching could now for the first time simulate the processes in collapsing stars in three dimensions and with a sophisticated description of all relevant physics.

“For this purpose we used nearly 16,000 processor cores in parallel mode, but still a single model run took about 4.5 months of continuous computing”, says PhD student Florian Hanke, who performed the simulations. Only two computing centers in Europe were able to provide sufficiently powerful machines for such long periods of time, namely CURIE at Très Grand Centre de calcul (TGCC) du CEA near Paris and SuperMUC at the Leibniz-Rechenzentrum (LRZ) in Munich/Garching.

Turbulent evolution of a neutron star for six moments (0.154, 0.223, 0.240, 0.245, 0.249 and 0.278 seconds) after the beginning of the neutron star formation in a threedimensional computer simulation. The mushroom-like bubbles are characteristic of "boiling" neutrino-heated gas, whereas simultaneously the "SASI" instability causes wild sloshing and rotational motions of the whole neutrino-heated layer (red) and of the enveloping supernova shock (blue) . (Images by Elena Erastova and Markus Rampp, RZG)
Turbulent evolution of a neutron star for six moments (0.154, 0.223, 0.240, 0.245, 0.249 and 0.278 seconds) after the beginning of the neutron star formation in a threedimensional computer simulation. The mushroom-like bubbles are characteristic of “boiling” neutrino-heated gas, whereas simultaneously the “SASI” instability causes wild sloshing and rotational motions of the whole neutrino-heated layer (red) and of the enveloping supernova shock (blue) . (Images by Elena Erastova and Markus Rampp, RZG)
Given several thousand billion bytes of simulation data, it took some time before researchers could fully understand the implications of their model runs. However, what they saw both elated and surprised them. The stellar gas performed in a manner very much like ordinary convection, with the neutrinos driving the heating process. And that’s not all… They also found strong sloshing motions which transiently change to rotational motions. This behavior has been observed before and named Standing Accretion Shock Instability. According to the news release, “This term expresses the fact that the initial sphericity of the supernova shock wave is spontaneously broken, because the shock develops large-amplitude, pulsating asymmetries by the oscillatory growth of initially small, random seed perturbations. So far, however, this had been found only in simplified and incomplete model simulations.”

“My colleague Thierry Foglizzo at the Service d’ Astrophysique des CEA-Saclay near Paris has obtained a detailed understanding of the growth conditions of this instability”, explains Hans-Thomas Janka, the head of the research team. “He has constructed an experiment, in which a hydraulic jump in a circular water flow exhibits pulsational asymmetries in close analogy to the shock front in the collapsing matter of the supernova core.” Known as Shallow Water Analogue of Shock Instability, the dynamic process can be demonstrated in less technicalized manners by eliminating the important effects of neutrino heating – a reason which causes many astrophysicists to doubt that collapsing stars might go through this type of instability. However, the new computer models are able to demonstrate the Standing Accretion Shock Instability is a critical factor.

“It does not only govern the mass motions in the supernova core but it also imposes characteristic signatures on the neutrino and gravitational-wave emission, which will be measurable for a future Galactic supernova. Moreover, it may lead to strong asymmetries of the stellar explosion, in course of which the newly formed neutron star will receive a large kick and spin”, describes team member Bernhard Müller the most significant consequences of such dynamical processes in the supernova core.

Are we finished with supernova research? Do we understand everything there is to know about neutron stars? Not hardly. At the present time, the scientist are ready to further their investigations into the measurable effects connected to SASI and refine their predictions of associated signals. In the future they will further their understanding by performing more and longer simulations to reveal how instability and neutrino heating react together. Perhaps one day they’ll be able to show this relationship to be the trigger which ignites a supernova explosion and conceives a neutron star.

Original Story Source: Max Planck Institute for Astrophysics News Release.

Posted on by Tammy Plotner

Recycling Pulsars – The Millisecond Matters…

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

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It’s a millisecond pulsar… a rapidly rotating neutron star and it’s about to reach the end of its mass gathering phase. For ages the vampire of this binary system has been sucking matter from a donor star. It has been busy, spinning at incredibly high rotational speeds of about 1 to 10 milliseconds and shooting off X-rays. Now, something is about to happen. It is going to lose a whole lot of energy and age very quickly.

Astrophysicist Thomas Tauris of Argelander-Institut für Astronomie and Max-Planck-Institut für Radioastronomie has published a paper in the February 3 issue of Science where he has shown through numerical equations the root of stellar evolution and accretion torques. In this model, millisecond pulsars are shown to dissipate approximately half of their rotational energy during the last phase of the mass-transfer process and just before it turns into a radio source. Dr. Tauris’ findings are consistent with current observations and his conclusions also explain why a radio millisecond pulsar appears age-advanced over their companion stars. This may be the answer as to why sub-millisecond pulsars don’t exist at all!

“Millisecond pulsars are old neutron stars that have been spun up to high rotational frequencies via accretion of mass from a binary companion star.” says Dr. Tauris. “An important issue for understanding the physics of the early spin evolution of millisecond pulsars is the impact of the expanding magnetosphere during the terminal stages of the mass-transfer process.”

By drawing mass and angular momentum from a host star in a binary system, a millisecond pulsar lives its life as a highly magnetized, old neutron star with an extreme rotational frequency. While we might assume they are common, there are only about 200 of these pulsar types known to exist in galactic disk and globular clusters. The first of these millisecond pulsars was discovered in 1982. What counts are those that have spin rates between 1.4 to 10 milliseconds, but the mystery lay in why they have such rapid spin rates, their strong magnetic fields and their strangely appearing ages. For example, when do they switch off? What happens to the spin rate when the donor star quits donating?

“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 lose about half of their rotational energy in the so-called Roche-lobe decoupling phase. This phase is describing 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 can 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.”

Thanks to this new study we’re now able to see how a spinning pulsar could possibly brake out of an equilibrium spin. At this age, the mass-transfer rate slows down and affects the magnetospheric radius of the pulsar. This in turn expands and forces the incoming matter to act as a propeller. The action then causes the pulsar to slow down its rotation and – in turn – slow its spin rate.

“Actually, without a solution to the “turn-off” problem we would expect the 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.”

Original Story Source: Max-Planck-Institut für Radioastronomie News Release>. For Further Reading: Spin-Down of Radio Millisecond Pulsars at Genesis.

Posted on by Tammy Plotner

NASA’s Rossi X-Ray Timing Explorer Retires

Technicians work on RXTE in 1995. Credit: NASA/Goddard

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For more than 16 years, 2,200 papers in refereed journals, 92 doctoral theses, and more than 1,000 rapid notifications alerting astronomers around the globe to new astronomical activity, the NASA Rossi X-Ray Timing Explorer is now retired. It sent the last of its data on January 4th of this year and on January 5th the plucky little satellite was decommissioned. If you’re not familiar with Rossi’s activities, then picture sending back images and data on the extreme environments around white dwarfs, neutron stars and black holes… because that’s what made the mission famous.

On December 30, 1995, the mission was launched as XTE from Cape Canaveral, Florida on board a Delta II 7920 rocket. Within weeks it was named in honor of Bruno Rossi, an MIT astronomer and a pioneer of X-ray astronomy and space plasma physics who died in 1993. However, the mission itself didn’t die – it excelled with honors. The entire scientific community recognized the importance of RXTE research and bestowed it with five awards – four Rossi Prizes (1999, 2003, 2006 and 2009) from the High Energy Astrophysics Division of the AAS and the 2004 NWO Spinoza prize, the highest Dutch science award, from the Netherlands Organization for Scientific Research.

On board, the Rossi was three scientific instruments housed in one unit. The first was the Proportional Counter Array (PCA), which was centered on the lower end of the energy band and was crafted by Goddard. The second instrument was the High Energy X-Ray Timing Experiment (HEXTE) that could be aimed at very specific targets and was manufactured by the University of California at San Diego for exploring the upper energy range. The last of the trio was the All-Sky Monitor developed by the Massachusetts Institute of Technology (MIT) in Cambridge. It took in about 80% of the sky during each orbit, delivering astronomers with an unprecedented amount of data on the wide variances of X-Ray sky and allowing them to record bright sources over a period of time as short as a few microseconds up to months. All of this information was taken in over a broad span of energy ranging from 2,000 to 250,000 electron volts.

The Rossi X-Ray Timing Explorer asked little and returned much. Over its operating lifetime it gave us new insight in the life cycles of neutron stars and black holes. Through its eyes we learned about magnetars and discovered the first accreting millisecond pulsar. But that’s not all. The RXTE provided hard evidence which supported Einstein’s theory by observing “frame dragging” in the neighborhood of a black hole. Even though the instrumentation would be considered antique by today’s standards, it certainly served its purpose. “The spacecraft and its instruments had been showing their age, and in the end RXTE had accomplished everything we put it up there to do, and much more,” said Tod Strohmayer, RXTE project scientist at Goddard.

According to the NASA news release, the decision to decommission RXTE followed the recommendations of a 2010 review board tasked to evaluate and rank each of NASA’s operating astrophysics missions. The three and a half ton satellite is expected to return to Earth sometime between the years 2014 and 2023, depending on solar activity. It will have a fiery end… burning out like the superstar that it was. To celebrate its career, the scientific community will hold a special session on RXTE during the 219th meeting of the American Astronomical Society (AAS) in Austin, Texas. The session is scheduled for Tuesday, January 10, at 3 p.m. CST. A press conference on new RXTE results will also be held at the meeting on January 10 at 1:45 p.m. EST. The decision to decommission RXTE followed the recommendations of a 2010 review board tasked to evaluate and rank each of NASA’s operating astrophysics missions. “After two days we listened to verify that none of the systems we turned off had autonomously re-activated, and we’ve heard nothing,” said Deborah Knapp, RXTE mission director at Goddard.

On the contrary… We heard a lot from Rossi!

Original Story Source: NASA News Release.