Pulsars Help the Search for Gravitational Waves - Video

In a new video from Physics World, scientists from the Jodrell Bank Observatory near Manchester in the United Kingdom discuss how natural time keepers called pulsars, which are actually the condensed left-over material from burned-out stars, could help the search for gravitational waves.

Einstein predicted that very massive, energetic events (like two stars merging together) could create ripples in the fabric of space, the way a stone creates ripples on the surface of a pond.

The ripples aren't made of light or matter, but of space itself. These are called gravitational waves.

So far, astronomers have not been able to detect gravitational waves directly, and some astronomers are choosing to take an indirect path.

Pulsars earned their name because their light appears to pulse on and off. In some cases, the pulsar's blinking is so regular, it exceeds the precision of any clock that can be built by humans.

An interruption in that regularity, therefore, must come from an external event. Check out the video to see how interruptions to these regular pulses of light could indicate the presence of gravitational waves.

Fermi bubbles defy explanation, Despite extensive analysis

This artist's representation shows the Fermi bubbles towering above and below the galaxy. 

Credit: NASA's Goddard Space Flight Center

Scientists from Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have analyzed more than four years of data from NASA's Fermi Gamma-ray Space Telescope, along with data from other experiments, to create the most detailed portrait yet of two towering bubbles that stretch tens of thousands of light-years above and below our galaxy.

The bubbles, which shine most brightly in energetic gamma rays, were discovered almost four years ago by a team of Harvard astrophysicists led by Douglas Finkbeiner who combed through data from Fermi's main instrument, the Large Area Telescope (FGST).

The new portrait, described in a paper that has been accepted for publication in The Astrophysical Journal, reveals several puzzling features, said Dmitry Malyshev, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology who co-led on the analysis.

For example, the outlines of the bubbles are quite sharp, and the bubbles themselves glow in nearly uniform gamma rays over their colossal surfaces, like two 30,000-light-year-tall incandescent bulbs screwed into the center of the galaxy.

Their size is another puzzle. The farthest reaches of the Fermi bubbles boast some of the highest energy gamma rays, but there's no discernible cause for them that far from the galaxy.

Finally, although the parts of the bubbles closest to the galactic plane shine in microwaves as well as gamma rays, about two-thirds of the way out the microwaves fade and only gamma rays are detectable.

Not only is this different from other galactic bubbles, but it makes the researchers' work that much more challenging, said Malyshev's co-lead, KIPAC postdoctoral researcher Anna Franckowiak.

"Since the Fermi bubbles have no known counterparts in other wavelengths in areas high above the galactic plane, all we have to go on for clues are the gamma rays themselves," she said.

What Made The Bubbles?
Soon after the initial discovery theorists jumped in, offering several explanations for the bubbles' origins.

For example, they could have been created by huge jets of accelerated matter blasting out from the supermassive black hole at the center of our galaxy.

Or they could have been formed by a population of giant stars, born from the plentiful gas surrounding the black hole, all exploding as supernovae at roughly the same time.

"There are several models that explain them, but none of the models is perfect," Malyshev said. "The bubbles are rather mysterious."

Creating the portrait wasn't easy.

"It's very tricky to model," said Franckowiak. "We had to remove all the foreground gamma-ray emissions from the data before we could clearly see the bubbles."

From the vantage point of most Earth-bound telescopes, all but the highest-energy gamma rays are completely screened out by our atmosphere.

It wasn't until the era of orbiting gamma-ray observatories like Fermi that scientists discovered how common extra-terrestrial gamma rays really are.

Pulsars, supermassive black holes in other galaxies and supernovae are all gamma rays point sources, like distant stars are point sources of visible light, and all those gamma rays had to be scrubbed from the Fermi data.

Hardest to remove were the galactic diffuse emissions, a gamma ray fog that fills the galaxy from cosmic rays interacting with interstellar particles.

"Subtracting all those contributions didn't subtract the bubbles," Franckowiak said. "The bubbles do exist and their properties are robust."

In other words, the bubbles don't disappear when other gamma-ray sources are pulled out of the Fermi data, in fact, they stand out quite clearly.

Franckowiak says more data is necessary before they can narrow down the origin of the bubbles any further.

"What would be very interesting would be to get a better view of them closer to the galactic center," she said, "but the galactic gamma ray emissions are so bright we'd need to get a lot better at being able to subtract them."

Fermi is continuing to gather the data Franckowiak wants, but for now, both researchers said, there are a lot of open questions.

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