CAASTRO Astronomers Make precise measurement of neutron star

The densely packed matter of a pulsar spins at incredible speeds, and emits radio waves that can be observed from Earth, but how neutron stars emit these waves is still a mystery. 

Credit: Swinburne Astronomy Productions /CAASTRO.

An international team of astronomers has made a measurement of a distant neutron star that is one million times more precise than the previous world's best.

The researchers were able to use the interstellar medium (ISM), the 'empty' space between stars and galaxies that is made up of sparsely spread charged particles, as a giant lens to magnify and look closely at the radio wave emission from a small rotating neutron star.

This technique yielded the highest resolution measurement ever achieved, equivalent to being able to see the double-helix structure of our genes from the Moon!

"Compared to other objects in space, neutron stars are tiny – only tens of kilometres in diameter – so we need extremely high resolution to observe them and understand their physics," Dr Jean-Pierre Macquart from the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) in Perth, Australia, said.

Dr Macquart, a member of the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO), said neutron stars were particularly interesting objects to study, as some of them – called pulsars – gave off pulsed radio waves whose beams swept across telescopes at regular intervals.

"More than 45 years since astronomers discovered pulsars, we still don't understand the mechanism by which they emit radio wave pulses," he said.



A spinning neutron star emitting a stream of radio waves that appear as regular pulses when observed from Earth. Simulation credit to Swinburne Astronomy Productions /CAASTRO.

The researchers found they could use the distortions of these pulse signals as they passed through the turbulent interstellar medium (ISM) to reconstruct a close in view of the pulsar from thousands of individual sub-images of the pulsar.

"The best we could previously do was pointing a large number of radio telescopes across the world at the same pulsar, using the distance between the telescopes on Earth to get good resolution," Dr Macquart said.

The previous record using combined views from many telescopes was an angular resolution of 50 microarcseconds, but the team - led by Professor Ue-Li Pen of the Canadian Institute of Theoretical Astrophysics and a CAASTRO Partner Investigator, has now proven their 'interstellar lens' can get down to 50 picoarcseconds, or a million times more detail, resolving areas of less than 5km in the emission region.

"Our new method can take this technology to the next level and finally get to the bottom of some hotly debated theories about pulsar emission," Professor Pen said.

This new technique also opens up the possibilities for precise distance measurements to pulsars that orbit a companion star and 'image' their extremely small orbits, which is ultimately a new and highly sensitive test of Einstein's theory of General Relativity," Professor Pen said.

More information: Ue-Li Pen, Jean-Pierre Macquart, Adam T. Deller, and Walter Brisken. "50 picoarcsec astrometry of pulsar emission." MNRAS (May 01, 2014) Vol. 440 L36-L40 first published online February 14, 2014. DOI: 10.1093/mnrasl/slu010 . 

Also available on arXiv: http://adsabs.harvard.edu/abs/2014MNRAS.440L..36P

SKA and CAASTRO: Forecast Sky bubbling with exploding stars

It is hard to imagine that any astronomical phenomenon could escape our latest and most powerful telescopes, but an international research team has now forecast some of the exotic discoveries that will only be able to be studied with the forthcoming Square Kilometre Array (SKA).

Giancarlo Ghirlanda

The team, led by Dr Giancarlo Ghirlanda at the National Institute for Astrophysics (INAF) in Italy and including CAASTRO members Dr Davide Burlon and Dr Tara Murphy from the University of Sydney, has calculated that the SKA will reveal the lingering footprints of tens of thousands of enigmatic cosmic explosions known as "gamma-ray bursts".

Davide Burlon

"With current telescopes, we see a bright gamma-ray burst somewhere in the Universe around once per day, but new radio telescopes will soon be able to see an afterglow of the explosion after the initial burst has faded away," explains CAASTRO postdoctoral researcher Dr Burlon.

"This afterglow can generally take weeks to gradually decay and teaches us incredible amounts about both the initial explosion and its neighbourhood."

The catch is that a gamma-ray burst is not an explosion that we can see from all directions but is comprised of a very narrow, energetic jet, so we need to be looking down the barrel of the jet at the right time.

Otherwise it is invisible, equivalent to only seeing the beam of a laser pointer when it points directly at us.

The radio afterglow should be visible from any direction though and for long periods of time, even if we missed the burst.

Tara Murphy

These afterglows without a burst are known as "orphan" afterglows, they're a phenomenon that astronomers have until now been looking for without success.

"From the rate at which we detect gamma-ray bursts, we were able to predict that with the power of a sensitive new telescope like the SKA, orphan afterglows should be seen 700 times more often than their gamma-ray bursts." says Dr Burlon.

"The unprecedented sensitivity and wide field-of view of the SKA means that orphan afterglows should be visible for months or even years before eventually disappearing, bubbling across the sky more than ten thousand times per year."

Of course, the SKA's view of the sky will be full of all sorts of objects and events, such as supernova explosions and flaring black holes that are more common than orphan afterglows.

"In this new era of radio astronomy, one of the challenges will be to disentangle these different classes of radio sources." says Dr Tara Murphy, CAASTRO Associate Investigator and project leader of the "Variables and Slow Transients (VAST)" survey with the Australian SKA Pathfinder (ASKAP).

The SKA will join the Australian SKA precursor telescope ASKAP and the South African SKA precursor MeerKAT in painting an entirely new picture of the "radio sky".

"The SKA will not only allow us to finally see these orphan afterglows but help us understand how gamma-ray bursts (GRB) produce such powerful, narrow jets and will cast new light on the big question of just what causes gamma-ray bursts in the first place," concludes Dr Ghirlanda.

More information: G. Ghirlanda, D. Burlon, G. Ghisellini, R. Salvaterra, M. G. Bernardini, S. Campana, S. Covino, P. D'Avanzo, V. D'Elia, A. Melandri, T. Murphy, L. Nava, S. D. Vergani, G. Tagliaferri: "GRB orphan afterglows in present and future radio transient surveys" in The Publications of the Astronomical Society of Australia (PASA). arXiv:1402.6338 [astro-ph.HE] arxiv.org/abs/1402.6338

Critical mass not needed for Type Ia supernovae explosions

A global collaboration of astronomers searching for clues about dark energy, the mysterious force that is speeding up the expansion of the Universe, have uncovered new evidence about the nature of supernovae, finding many are lighter than scientists had expected.

The findings, from an international team from the Nearby Supernova Factory project, overturn previous understanding of white dwarf stars and raise new questions about how these stars explode.

"White dwarfs are dead stars, the corpses of stars that were once like our Sun.'

Richard Scalzo

'They won't explode on their own - they need another star to help blow them up," said ANU astronomer Dr Richard Scalzo, who led the latest research.

"We now know it's much easier to blow them up than we used to think."

A supernova is a star that explodes and shines much more brilliantly as it reaches the end of its life.

By studying "nearby" Type Ia (1a) supernovae - within a billion light years from earth - astronomers can then compare them with older and fainter supernovae even further out in space, allowing them to measure distances in the Universe.

Dr Scalzo said most of the supernovae his team studied had blown up well before dinosaurs walked on Earth.

He said astronomers had previously believed white dwarfs needed to be around 1.4 times the mass of the Sun before they could explode.

Using the University of Hawaii's 2.2-metre telescope, his team studied 19 Type Ia supernovae.

By carefully watching how quickly the supernovae faded away after their brightest point, and comparing to calculations made by computer, the team could then "weigh" each explosion to figure out the white dwarf's mass.

They were surprised to find that as many as half were well below the previously-assumed tipping point for an explosion.

That meant the life the dying stars led, and the cause of their violent deaths, also had to be totally different from what scientists once thought.

Brian Schmidt

Dr. Scalzo said the ultimate aim of the research was to better understand dark energy, for which the 2011 Nobel Prize in Physics was awarded to ANU professor Brian Schmidt, Adam Riess (Johns Hopkins University), and Saul Perlmutter (Lawrence Berkeley National Laboratory - LBNL).

"Brian Schmidt used type Ia supernovae to discover that dark energy exists," he said.

"We're now trying to understand what it is. This new information about how white dwarfs explode is a huge step forward towards that goal."

Cosmologist Greg Aldering, who leads the international Nearby Supernova Factory project in Berkeley, said: "This is a significant advance in furthering Type Ia supernovae as cosmological probes for the study of dark energy."

Dr Scalzo was previously based in the Nearby Supernova Factory headquarters at Lawrence Berkeley National Laboratory in California, and is a member of the ARC Centre of Excellence for All-sky Astrophysics (CAASTRO).

More Information: 'Type Ia supernova bolometric light curves and ejected mass estimates from the Nearby Supernova Factory' arXiv:1402.6842 [astro-ph.CO]