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Asteroids are pounding a pulsar

SCIENTISTS USING CSIRO’s Parkes telescope and another telescope in South Africa have found evidence that a tiny star called PSR J0738-4042 is being pounded by asteroids – large lumps of rock in space.

“One of these rocks seems to have had a mass of about a billion tons,” CSIRO astronomer and member of the research team Dr Ryan Shannon said.

PSR J0738-4042 lies 37,000 light-years from Earth in the constellation of Puppis. The environment around the star is especially harsh, full of radiation and violent winds of particles.

“If a large rocky object can form here, planets could form around any star. That’s exciting,” Dr Shannon said.

The star is a special one, a ‘pulsar’ that emits a beam of radio waves. As it spins, its radio beam flashes over Earth again and again with the regularity of a clock.

An artist's impression of an asteroid breaking up

An artist’s impression of an asteroid breaking up. Credit: NASA/JPL-Caltech

Formed from shattered remains

In 2008 Dr Shannon and a colleague predicted how an infalling asteroid would affect a pulsar. It would, they said, alter the slowing of the pulsar’s spin rate and the shape of the radio pulse that we see on Earth.

“That is exactly what we see in this case,” Dr Shannon said. “We think the pulsar’s radio beam zaps the asteroid, vaporising it. But the vaporised particles are electrically charged and they slightly alter the process that creates the pulsar’s beam.”

Asteroids circling a pulsar could have been formed by the remains of the exploding star that produced the pulsar itself, the scientists say. The material blasted out from the explosion could fall back towards the pulsar, developing into a swirling cloud of dusty debris that circles it. Astronomers call it a ‘disc’.

Not the only one

Astronomers have found a dust disc around another pulsar called J0146+61.

Parkes radio telescope

The CSIRO’s Parkes radio telescope. Photo courtesy Shaun Amy.

“This sort of dust disc could provide the ‘seeds’ that grow into larger asteroids,” said Paul Brook, a PhD student co-supervised by the University of Oxford and CSIRO who led the study of PSR J0738-4042.

In 1992 two planet-sized objects were found around a pulsar called PSR 1257+12. But these were probably formed by a different mechanism, the astronomers say.

The new study has been published in The Astrophysical Journal Letters, a leading journal of astronomical research.

Adapted from information issued by CSIRO.

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The ‘missing link’ pulsar

AN INTERNATIONAL TEAM of astronomers using CSIRO radio telescopes in Australia and other ground and space-based instruments, has caught a small star called a pulsar undergoing a radical transformation, described in a paper in the journal Nature.

“For the first time we see both X-rays and extremely fast radio pulses from the one pulsar. This is the first direct evidence of a pulsar changing from one kind of object into another – like a caterpillar turning into a butterfly,” said Dr Simon Johnston, Head of Astrophysics at CSIRO’s Astronomy and Space Science division.

The pulsar and its companion star

The pulsar and its companion star. The ageing pulsar rotates slower and slower, then matter from its companion spins it up again. As the pulsar is spun up, it alternates between emitting X-rays (white) and radio waves (pink). Credit: ESA

The cosmic drama is being played out 18,000 light-years away, in a small cluster of stars (called M28) in the constellation of Sagittarius.

The pulsar (called PSR J1824-2452I) has a tiny companion star, with about a fifth the mass of the Sun. Although small, the companion is fierce, pounding the pulsar with streams of matter.

Normally the pulsar shields itself from this onslaught, its magnetic field deflecting the matter stream into space.

But sometimes the stream swells to a flood, overwhelming the pulsar’s protective ‘force field.’ When the stream hits the pulsar’s surface its energy is released as blasts of X-rays.

Eventually the torrent slackens. Once again the pulsar’s magnetic field re-asserts itself and fends off the companion’s attacks.

“We’ve been fortunate enough to see all stages of this process, with a range of ground and space telescopes. We’ve been looking for such evidence for more than a decade,” said Dr Alessandro Papitto, the paper’s lead author. Dr Papitto is an astronomer of the Institute of Space Studies (ICE, CSIC-IEEC) of Barcelona, Spain.

‘Teenage’ behaviour

The pulsar and its companion form what is called a ‘low-mass X-ray binary’ system. In such a system, the matter transferred from the companion lights up the pulsar in X-rays and makes it spin faster and faster, until it becomes a ‘millisecond pulsar’ that spins at hundreds of times a second and emits radio waves. The process takes about a billion years, astronomers think.

In its current state the pulsar is exhibiting behaviour typical of both kinds of systems: millisecond X-ray pulses when the companion is flooding the pulsar with matter, and radio pulses when it is not.

“It’s like a teenager who switches between acting like a child and acting like an adult,” said Mr. John Sarkissian, who observed the system with CSIRO’s 64-m (210-ft) Parkes radio telescope in eastern Australia.

“Interestingly, the pulsar swings back and forth between its two states in just a matter of weeks.”

This video shows an artist’s impression of the pulsar and its companion star. Credit: ESA

A global effort

The pulsar was initially detected as an X-ray source with the INTEGRAL satellite. X-ray pulsations were seen with another satellite, ESA’s XMM-Newton; further observations were made with NASA’s Swift. NASA’s Chandra X-ray telescope got a precise position for the object.

Then, crucially, the object was checked against the pulsar catalogue generated by CSIRO’s Australia Telescope National Facility, and other pulsar observations. This established that it had already been identified as a radio pulsar.

The source was detected in the radio with CSIRO’s Australia Telescope Compact Array, and then re-observed with CSIRO’s Parkes radio telescope, NRAO’s Robert C. Byrd Green Bank Telescope in the USA, and the Westerbork Synthesis Radio Telescope in The Netherlands. Pulses were detected in a number of these later observations, showing that the pulsar had ‘revived’ as a normal radio pulsar only a couple of weeks after the last detection of the X-rays.

The astronomers involved in these investigations work at institutions in Australia, Canada, Germany Italy, The Netherlands, Spain, Switzerland, and the USA.

Adapted from information issued by CSIRO.

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‘The Dish’ finds a ‘diamond planet’

Artist's visualisation of the pulsar and its orbiting planet

An artist's visualisation of the pulsar and its orbiting planet, which astronomers think could be made partly of diamond or a diamond-like substance. The blue squiggly line represents the beams of radio waves emanating from the pulsar. The orange bubble represents the size of the Sun, showing that the the planet's orbit has about the same radius as the Sun (about 600,000 km), yet it whizzes around in just two hours!

  • Planet detected orbiting a pulsar 4,000 light-years away
  • It’s actually the remnant core of what was once a star
  • Probably made of compressed carbon—diamond!

ASTRONOMERS USING ‘THE DISH’—CSIRO’s radio telescope near Parkes, NSW—believe they’ve found a small planet made of diamond, orbiting an unusual star.

The discovery was made by an international research team, led by Professor Matthew Bailes of Swinburne University of Technology in Melbourne, Australia, and is reported today in the journal Science.

“Although bizarre, this planet is evidence that we’ve got the right understanding of how these binary systems evolve,” said Dr Michael Keith of CSIRO Astronomy and Space Science, one of the research team members.

Not fitting the pattern

The researchers, from Australia, Germany, Italy, the UK and the USA, first found an unusual star called a pulsar, now named PSR J1719-1438, using the 64-m Parkes radio telescope in eastern Australia.

Pulsars are small spinning stars about 20 km in diameter—the size of a small city—that emit a beam of radio waves. As the star spins and the radio beam sweeps repeatedly over Earth, radio telescopes detect a regular pattern of radio pulses.

The researchers followed up their discovery with the Lovell radio telescope in the UK and one of the Keck telescopes in Hawaii, and noticed that the arrival times of the pulsar’s pulses were systematically altered—in a way that must be caused by the gravitational pull of a small planet orbiting the pulsar.

The CSIRO's Parkes radio telescope

The CSIRO's Parkes radio telescope

Small, heavy and fast

The modulations of the radio pulses reveal several things about the planet.

First, it orbits the pulsar in just two hours and ten minutes, and the distance between the two objects is 600,000 km—a little less than the radius of our Sun.

Second, the companion must be small, less than 60,000 km (that’s about five times the Earth’s diameter). The planet is so close to the pulsar that, if it were any bigger, it would be ripped apart by the pulsar’s gravity.

But despite its small size, the planet has slightly more mass than Jupiter.

A stripped-down dwarf

“This high density of the planet provides a clue to its origin,” Professor Bailes said.

The team thinks that the ‘diamond planet’ is all that remains of a once-massive star, most of whose matter was siphoned off towards the pulsar.

But pulsar J1719-1438 and its companion are so close together that the companion can only be a very stripped-down ‘white dwarf’ star, one that has lost its outer layers and over 99.9 per cent of its original mass.

“This remnant is likely to be largely carbon and oxygen, because a star made of lighter elements like hydrogen and helium would be too big to fit the measured orbit,” said CSIRO’s Dr Keith.

The density means that this material is certain to be crystalline—that is, a large part of the star may be similar to a diamond.

The pulsar and its planet lie 4,000 light-years away in the constellation of Serpens (the Snake). The system is about an eighth of the way towards the Galactic Centre from the Earth.

Diamond planet Easy Q&A

What have they found?

  • They’ve spotted a system that comprises a weird kind of star, called a pulsar, and a medium-sized planet that is probably made of almost pure carbon…which is most likely in the form of diamond or a diamond-like substance.
  • The system is 4,000 light-years from Earth—that’s 40 thousand trillion kilometres away!
  • The pulsar emits radio waves in a regular pattern as it spins, like a lighthouse, which is what the CSIRO’s Parkes radio telescope picked up.
  • The planet itself cannot be seen as it is too small and too far away.

If they can’t see the planet, how do they know it’s really there?

  • Its presence is inferred by the distorting effect it has on the pulsar’s powerful radio emissions.
  • It whizzes around its star in just two hours (compared to one year for Earth around the Sun).
  • The data was analysed using an incredible supercomputer at Swinburne University in Melbourne.
  • The planet is about 5 times as wide as the Earth, but much, much heavier.

So why do they think it is made of diamond?

  • Now here’s the interesting bit, because the planet actually seems to be the dense, remnant core of a star, rather than a traditional planet.
  • Many stars, as they burn up their hydrogen fuel, end up having cores made of carbon.
  • The star changed into a planet, with only it’s core remaining.

How did it change from a star into a planet?

  • Because the pulsar has a huge gravitational pull and is a cosmic cannibal!
  • The pulsar and the other star would have been orbiting very close to each other.
  • The pulsar would have pulled all the outer gas layers off the other star—99.9 percent of its mass—eventually leaving it with just its carbon core.
  • If we could have seen it happening, it would have looked like a huge whirlpool of gas coming off the doomed star and spiralling onto the neighbouring pulsar.

What do astronomers hope to learn from these types of star systems?

  • For one thing, pulsars are the “end points”—the dying stages—in the lives of many kinds of big stars, so learning more about them tells us about the evolution and life cycle of those stars and the wider universe.
  • But pulsars also are important for understanding and testing laws of physics.
  • Astronomers can use them as “natural laboratories” for testing theories, such as Einstein’s theory of gravity.
  • That’s because you can only go so far testing some theories in the laboratory—to really put them to the test, you need to study massive objects travelling at high speed, and that’s what pulsar systems are.

Main text adapted from information issued by CSIRO. Q&A by Jonathan Nally, SpaceInfo.com.au Images courtesy Swinburne Astronomy Productions and David McClenaghan, CSIRO.

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Happy birthday, Crab Nebula!

Crab Nebula

The Crab Nebula is a supernova remnant, the aftermath of a titanic stellar explosion that was seen on Earth in the year 1054 CE.

ALMOST A THOUSAND YEARS AGO, on July 4 in the year 1054 CE, astronomers in China and the Middle East noticed a bright new star in the night sky. It appeared in the constellation Taurus, and remained visible for roughly two years.

They couldn’t have known what it really was—a supernova, a titanic stellar explosion that occurred when a massive star reached the end of its life. The explosion was matched by an implosion, which crushed the star’s core and produced a neutron star—made of matter so dense that the atoms’ electrons were forced into their nuclei and combined with the protons to form more neutrons.

The density is so high, that a teaspoon of neutron star material has as much mass as 900 Great Pyramids.

And the neutron star is spinning, making it a pulsar…so called because the natural radiation it emits is sent out along beams, which sweep across our field of view like a lighthouse, seeming to pulse on and off.

So much for the implosion. The explosion hurled enormous quantities of gas into space, eventually forming the glowing cloud, or supernova remnant, we see today. On early photographs, which didn’t show much detail, the nebula looked very crab-like, hence its name.

The distance to the Crab Nebula is a bit uncertain, but it’s probably around 6,300 light-years away. And the cloud is expanding at a speed of 1,500 kilometres per second!

Download a 1280 x 1280 wallpaper image of the Crab Nebula here.

Story by Jonathan Nally. Image courtesy NASA, ESA and Allison Loll/Jeff Hester (Arizona State University). Image acknowledgement: Davide De Martin (ESA/Hubble).

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The Case of the Cosmic Crab

A NEW MOVIE FROM NASA’S Chandra X-ray Observatory shows a sequence of images of the Crab Nebula, taken over an interval of seven months and showing dramatic variations.

The Crab Nebula is one of the most famous objects in the sky. It is the remnant cloud from a supernova (exploding star) that was seen by astronomers in China and other countries in the year 1054.

At the centre of the nebula is a pulsar, a rapidly spinning neutron star. It has a mass greater than our Sun but is only tens of kilometres wide, and is spinning at the rate of 30 times per second.

The pulsar’s spin is gradually slowing down, and as it does so large amounts of energy are injected into its surroundings. In particular, a high-speed wind of matter and anti-matter particles ploughs into the surrounding nebula, creating a shock wave that forms the expanding ‘ring’ seen in the movie.

In addition, ‘jets’ shooting out from the poles of the pulsar spew X-ray emitting matter and antimatter particles in a direction at right angles to the ring.

The goal of the latest Chandra observations was to pinpoint the location of remarkable flares spotted by NASA’s Fermi Gamma Ray Observatory satellite and Italy’s AGILE satellite.

A strong gamma-ray flare was detected from the Crab in September 2010, followed by an even stronger series of “superflares” in April 2011. The gamma-ray satellites were not able to locate the source of the flares within the nebula, but it was hoped that Chandra, with its high-resolution images, would.

Scientists have put together a short sequence of the images taken by Chandra, showing the remarkable changes in the nebula:

Chandra began observing the Crab on monthly intervals beginning six days after the discovery of the gamma-ray flare in September 2010. This established a baseline of seven images before the superflare was seen last month.

What was unexpected, though, was that nothing significant showed up in the Chandra observations as compared with the Fermi observations. Astronomers are now trying to figure out why that is so.

One possible explanation is that the gamma-ray flares picked up by Fermi happened very close to the pulsar, in which case they would have been missed by Chandra, because the Crab pulsar is so bright that the detectors are in essence “overexposed” so variations from that region cannot be observed. (Note that in the movie an artificial source of constant brightness is included to show the position of the pulsar.)

Adapted from information issued by CXC. Crab Nebula image courtesy (X-ray) NASA / CXC / SAO / F. Seward; (optical) NASA / ESA / ASU / J. Hester & A. Loll; (infrared) NASA / JPL-Caltech / Univ. Minn. / R. Gehrz.

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Taking the pulse of the universe

Artist's impression of a pulsar

Artist's impression of a pulsar, a rapidly spinning neutron star that emits streams of radio waves and sometimes gamma rays.

  • Pulsars are small, spinning neutron stars
  • They emit radio waves or gamma rays, and sometimes both
  • Parkes radio telescope working jointly with NASA space telescope

USING THE PARKES radio telescope, CSIRO astronomers are working closely with NASA to unlock one of astronomy’s great enigmas—the science behind pulsars.

The team are using the world-class facilities at Parkes, in combination with NASA’ s Fermi Gamma-Ray Space Telescope, to understand how these small, spinning stars make their beams of radiation.

The project has tracked down 25 ultra-fast ‘millisecond’ pulsars in just two years—the same number discovered in the previous 20 years.

“This has been a hugely productive collaboration, and it is generating unprecedented returns for physics and astronomy,” said the leader of the Parkes observations, CSIRO’s Dr Simon Johnston.

The study of pulsars demands highly advanced scientific infrastructure and expertise.

Pulsars emit beams of radio waves, gamma waves, or both. Sensitive radio telescopes such as the CSIRO facility at Parkes can detect the radio waves as they sweep across the Earth.

But gamma rays—which carry billions of times more energy than the light our eyes can see—are blocked by the Earth’s atmosphere. We can study them only by using telescopes in space.

Space and ground working together

The CSIRO-NASA collaboration shows we get the best results by combining land and space-based detectors.

First, the Fermi space telescope is finding unidentified gamma-ray sources, which the Parkes telescope can investigate for radio wave pulses.

“That’s how we were able to find those 25 millisecond pulsars, an incredible haul,” Dr Johnston said.

Simon Johnston in the control room at Parkes

Dr Simon Johnston (foreground) in the control room of CSIRO's Parkes radio telescope.

Second, Parkes is doing very precise timing of 168 radio pulsars that Fermi might be able to study.

“We work out exactly when the pulsar’s radio beam sweeps over us. That tells us how fast the pulsar is rotating,” Dr Johnston said.

“That knowledge helps us make use of the gamma-ray photons that Fermi detects. If Parkes can get the timing precisely right through the radio wave pulses, we can build up a picture of the gamma-ray pulses by collecting a few photons every time the pulsar beam sweeps past.”

Intriguing results

The collaboration has thrown up some intriguing results. Of the 60 objects Fermi has found that emit gamma-ray pulses, about twenty lack detectable radio pulses.

“The most likely explanation is that these pulsars do have radio beams, but they are just not sweeping across the Earth, so we can’t detect them,” Dr Johnston said.

“In other words, we think the beam of gamma rays is a big fat beam, which is easier to detect, and the radio beam is more tightly directed, less spread out.

“This suggests certain things about where on the pulsar the two beams come from, and how they are made. It’s only when we work together that we can crack these long-standing mysteries.”

CSIRO's Parkes radio telescope

CSIRO's Parkes radio telescope

International collaboration

Innovation Minister Senator Kim Carr said the research exemplified the sorts of international collaboration that the Australian Government was fostering across the board.

“We have a proud history of cooperation and involvement with NASA on a number of fronts, from assisting with communicating with the Apollo missions to the moon, to deep space exploration, and understanding how our universe works,” Senator Carr said.

“It’s all about exploring new frontiers and building Australian capacity as a research intensive and innovative nation.

“While this might seem remote from everyday life, experience has shown that space exploration in all its forms has unforeseen spin-offs that provide wide-reaching benefits through new technologies and new approaches to a range of challenges.”

Adapted from information issued by CSIRO. Images courtesy CSIRO and NASA.

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Crushed star puts Einstein to the test

Artist's impression of the J1749 system

Artist's impression of the J1749 system, which comprises a superdense pulsar (a spinning neutron star) and a normal star closely orbiting each other.

  • Stellar pair includes pulsar and a normal star
  • X-rays from the pulsar picked up by space telescope
  • Measurements can test an aspect of Einstein’s theories

A space telescope that “sees” X-rays could be used to test a key prediction of Einstein’s relativity theory.

Scientists using NASA’s Rossi X-ray Timing Explorer (RXTE) have studied a pair of stars that orbit each other so closely that one of them moves in front of the other and causes regular eclipses.

The astronomers can use these eclipses, along with standard physics laws, to estimate the size and mass of one of the stars.

Known collectively as Swift J1749.4-2807—or J1749 for short—one of the objects is a super-dense body called a pulsar, while the other is a normal star. The system is 22,000 light-years from Earth.

Pulsars are spinning neutron stars, the remnant cores left over after a giant star explodes at the end of its life. The matter in a neutron star is so heavily squashed that electrons have been forced into their atoms’ cores and combine with protons to form neutrons, leaving just a huge mass of neutrons.

Neutron stars pack more than the Sun’s mass into a ball just 20 to 25 kilometres across. In fact, their matter is so densely compressed that just one teaspoonful would have a staggering mass of 4,500 million tonnes.

Pulsar is eating its neighbour

Pulsars emit lots of radiation in tight beams, and as they spin they can appear to pulse or flash on and off like lighthouses.

Artist's impression of a pulsar

Artist's impression of a pulsar dragging gas from its companion star.

Astronomers can learn a lot about a pulsar from those flashes, such as how fast it is spinning. The J1749 pulsar spins at 518 times per second!

With J1749, the RXTE satellite spotted three eclipses as well as three pulses of X-rays as the pulsar experienced a series of outbursts.

The X-rays came from hotspots on the pulsar, where gas—sucked (or accreted) from the outer atmosphere of the companion star—had spiralled down and crashed onto the pulsar’s surface. The pulsar is slowly eating its neighbour.

It was these bright X-ray flashes that drew the astronomers’ attention to the J1749 system.

Small variations in the flashes arise from the pulsar’s orbital motion with the companion star, and indicate that the pulsar whizzes around its companion in just 8.8 hours.

The duration of the eclipses have enabled the astronomers to calculate that the companion is about 70% as massive as our Sun, but about 20% bigger than it would normally be for a star of this type—this is because the energy emitted by the pulsar is heating the companion’s outer layers, making them puff out further into space.

“This is the first time we’ve detected X-ray eclipses from a fast pulsar that is also accreting gas,” said Craig Markwardt of NASA’s Goddard Space Flight Centre. “Using this information, we now know the size and mass of the companion star with unprecedented accuracy.”

Artist's impression of RXTE

Artist's impression of the Rossi X-ray Timing Explorer space telescope, which made the observations.

Einstein to the rescue

What the astronomers don’t yet have is an accurate measure of the mass of the pulsar. The standard way to get it would be to use other telescopes to make optical and infrared observations of the companion star’s motion, from which they could work backward mathematically and deduce the pulsar’s mass.

But there is another way. Einstein’s relativity says that massive bodies distort space and slow down time. So what the astronomers hope to do is measure delays in the pulsar flashes as they travel past the companion star, something that RXTE is easily capable of doing.

This will be a good test of Einstein’s theory under extreme stellar conditions.

“High-precision measurements of the X-ray pulses just before and after an eclipse would give us a detailed picture of the entire system,” said Tod Strohmayer, RXTE’s project scientist at Goddard.

But for this, they’ll have to wait for RXTE to spot more X-ray outbursts…so you can be sure they’ll be keeping a close eye on this dynamic stellar duo in the months and years to come.

Story by Jonathan Nally, editor, SpaceInfo.com.au

Images courtesy NASA / GSFC.

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