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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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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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