GRB Explosion illuminates invisible galaxy in the dark ages

Before light from the gamma-ray burst arrives at the Earth for astronomers to study, it passes through interstellar gas in its host galaxy (close-up view, left), and intergalactic gas between the distant galaxy and us (wide view, right).

This gas filters the light by absorbing some colors and leaves a signature on the light that can be seen in its spectrum.

This "signature" allows scientists to characterize the gamma-ray burst, its environment, and the material between us and the distant galaxy.

Credit: Gemini Observatory/AURA, artwork by Lynette Cook

More than 12 billion years ago a star exploded, ripping itself apart and blasting its remains outward in twin jets at nearly the speed of light. At its death it glowed so brightly that it outshone its entire galaxy by a million times.

This brilliant flash traveled across space for 12.7 billion years to a planet that hadn't even existed at the time of the explosion - our Earth.

By analyzing this light, astronomers learned about a galaxy that was otherwise too small, faint and far away for even the Hubble Space Telescope to see.

Ryan Chornock

"This star lived at a very interesting time, the so-called dark ages just a billion years after the Big Bang," says lead author Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA).

"In a sense, we're forensic scientists investigating the death of a star and the life of a galaxy in the earliest phases of cosmic time," he adds.

The star announced its death with a flash of gamma rays, an event known as a gamma-ray burst (GRB).

GRB 130606A was classified as a long GRB since the burst lasted for more than four minutes.

It was detected by NASA's Swift spacecraft on June 6th. Chornock and his team quickly organized follow-up observations by the MMT Telescope in Arizona and the Gemini North telescope in Hawaii.

"We were able to get right on target in a matter of hours," Chornock says. "That speed was crucial in detecting and studying the afterglow."

A GRB afterglow occurs when jets from the burst slam into surrounding gas, sweeping that material up like a snowplow, heating it, and causing it to glow.

As the afterglow's light travels through the dead star's host galaxy, it passes through clouds of interstellar gas.

Chemical elements within those clouds absorb light at certain wavelengths, leaving "fingerprints." By splitting the light into a rainbow spectrum, astronomers can study those fingerprints and learn what gases the distant galaxy contained.

All chemical elements heavier than hydrogen, helium, and lithium had to be created by stars.

As a result those heavy elements, which astronomers collectively call "metals," took time to accumulate. Life could not have existed in the early universe because the elements of life, including carbon and oxygen, did not exist.

Chornock and his colleagues found that the GRB galaxy contained only about one-tenth of the metals in our solar system. Theory suggests that although rocky planets might have been able to form, life probably could not thrive yet.

"At the time this star died, the universe was still getting ready for life. It didn't have life yet, but was building the required elements," says Chornock.

At a redshift of 5.9, or a distance of 12.7 billion light-years, GRB 130606A is one of the most distant gamma-ray bursts ever found.

"In the future we will be able to find and exploit even more distant GRBs with the planned Giant Magellan Telescope," says Edo Berger of the CfA, a co-author on the publication.

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Invisibility cloak: Researchers create terahertz version

Researchers at Northwestern University have created a new kind of cloaking material that can render objects invisible in the terahertz range. Though this design can't translate into an invisibility cloak for the visible spectrum, it could have implications in diagnostics, security, and communication.

The cloak, designed by Cheng Sun, assistant professor of mechanical engineering at Northwestern's McCormick School of Engineering and Applied Science, uses microfabricated gradient-index materials to manipulate the reflection and refraction of light. Sun's results will be presented May 4 at CLEO: 2011, the annual Conference on Lasers and Electro-Optics.

Humans generally recognise objects through two features: their shape and colour. To render an object invisible, one must be able to manipulate light so that it will neither scatter at an object's surface nor be absorbed or reflected by it (the process which gives objects colour).

In order to manipulate light in the terahertz frequency, which lies between infrared and microwaves, Sun and his group developed metamaterials: materials that are designed at the atomic level.

Sun's tiny, prism-shaped cloaking structure, less than 10 millimeters long, was created using a technique called electronic transfer microstereolithography, where researchers use a data projector to project an image on a liquid polymer, then use light to transform the liquid layer into a thin solid layer.

Each of the prism's 220 layers has tiny holes that are much smaller than terahertz wavelengths, which means they can vary the refraction index of the light and render invisible anything located beneath a bump on the prism's bottom surface; the light then appears to be reflected by a flat surface.

Sun says the purpose of the cloak is not to hide items but to get a better understanding of how to design materials that can manipulate light propagation.

"This demonstrates that we have the freedom to design materials that can change the refraction index," Sun said. "By doing this we can manipulate light propagation much more effectively."

The terahertz range has been historically ignored because the frequency is too high for electronics. But many organic compounds have a resonant frequency at the terahertz level, which means they could potentially be identified using a terahertz scanner.

Sun's research into terahertz optics could have implications in biomedical research (safer detection of certain kinds of cancers) and security (using terahertz scanners at airports).

Next Sun hopes to use what he's learned through the cloak to create its opposite: a terahertz lens. He has no immediate plans to extend his invisibility cloak to visible frequencies.

"That is still far away," he said. "We're focusing on one frequency range, and such a cloak would have to work across the entire spectrum."

The Cloak of Invisibility

When J. K. Rowling described Harry Potter's invisibility cloak as "fluid and silvery", she probably wasn't thinking specifically about silver-plated nanoparticles suspended in water. But a team of theorists believe that using such a set-up would make the first soft, tunable metamaterial – the "active ingredient" in an invisibility device.

The fluid proposed by Ji-Ping Huang of Fudan University in Shanghai, China, and colleagues, contains magnetite balls 10 nanometres in diameter, coated with a 5-nanometre-thick layer of silver, possibly with polymer chains attached to keep them from clumping.

In the absence of a magnetic field, such nanoparticles would simply float around in the water, but if a field were introduced, the particles wouldself-assemble into chains whose lengths depend on the strength of the field, and which can also attract one another to form thicker columns.

The chains and columns would lie along the direction of the magnetic field. If they were oriented vertically in a pool of water, light striking the surface would refract negatively – bent in way that no natural material can manage.

This property could be exploited for invisibility devices, directing light around an object so that it appears as if nothing is there, or be put to use in lenses that could capture finer details than any optical microscope.

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Don’t believe everything you see. Metamaterials could make a cup look like a spoon (Image: Edgar Mueller/Rex Features)

AN ILLUSION device that makes one object look like another could one day be used to camouflage military planes or create "holes" in solid walls.

The idea builds on the optical properties of so-called metamaterials, which can bend light in almost any direction. In 2006, researchers used this idea to create an "invisibility cloak" that bent microwaves around a central cavity, like water flowing around a stone. Any object in this cavity is effectively invisible.

Now a group of researchers has gone a step further. "Invisibility is just an illusion of free space, of air," says Che Ting Chan, a physicist at the Hong Kong University of Science and Technology and a co-author of the study. "We are extending that concept. We can make it look like not just air but anything we want."

Instead of bending light around a central cavity, the team has worked out the mathematical rules for bending light in other ways. For example, a material could be designed to bend light in the same way as a spoon would. So the light hitting the material would be distorted to make it look as if a spoon were there.

It is also possible to design a complementary material that has the opposite effect - to exactly cancel out the effect that an object has on light. So light distorted by a spoon could be passed through a complementary material to eliminate these distortions.

The new illusion device uses these two ideas together. To make a cup look like a spoon, for example, light first strikes the cup and is distorted. It then passes through a complementary metamaterial which cancels out the distortions to make the cup seem invisible. The light then moves into a region of the metamaterial that creates a distortion as if a spoon were present. The result is that an observer looking at the cup through the metamaterial would see a spoon (Physical Review Letters, vol 102, p 253902).