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Nasa's eye on the 'violent cosmos'

10 June 08 21:01 GMT

By Paul Rincon
Science reporter, BBC News

A US space agency (Nasa) mission is about to go one better than Superman.

While the comic-book Kryptonian had to make do with X-ray vision, Nasa is set to launch a space telescope called Glast which will enable astronomers to view the Universe with "gamma-ray glasses".

Gamma rays are the highest-energy form of light, vastly more energetic than the light we see with our eyes, or even X-rays.

The upper end of this energy range is almost unexplored in astronomy. So the Gamma-ray Large Area Space Telescope will open up a high-energy frontier where important discoveries are almost guaranteed.

Glast will study some of the most extreme and exotic phenomena in the Universe.

These include massive explosions that release as much energy in a second as the Sun will release over its 10-billion-year lifetime and supermassive black holes that hurl matter vast distances across space at close to the speed of light.

According to Dr Steven Ritz, project scientist for the Glast mission, one of the most interesting things about the gamma-ray sky is that it is always changing.

"If you look up at the night sky, once a decade or so you might see a comet. You might notice that things are moving incredibly slowly. But it looks fairly placid and unchanging," Dr Ritz told BBC News.

"If you had gamma-ray glasses, it would look completely different. Once a day there's an explosion somewhere in the Universe where huge amounts of power are released - so-called gamma-ray bursts.

"Supermassive black hole systems are flaring brightly, changing their brightness very quickly. You would see objects pulsating - what we call pulsars."

Dr Dave Thompson, one of the deputy project scientists on Glast, told BBC News: "Our Sun, except when it has a big solar flare, is pretty dim in gamma-rays - almost invisible. So we don't see objects like that. What we do see are things with lots of 'oomph' - lot's of energetic activity."

Historically, Glast follows in the footsteps of another American satellite, the Compton Gamma-Ray Observatory (CGRO). But Glast represents a major step up in capability, covering an incredible range of energy.

"If Glast were a piano, it would have about 23 octaves," explains Dr Ritz.

Dr Thompson adds: "One of the big mysteries left over from the CGRO mission is that half the sources were unidentified - we don't know what they are. That's something we'll be out to solve."

Keeping watch

Gamma-rays are far too energetic to capture in the conventional way. The main scientific instrument on Glast is a telescope without lenses or mirrors: the Large Area Telescope, or Lat. It uses technology adapted from ground-based particle accelerators.

It has 16 so-called tower modules assembled in a four-by-four array. Each tower contains layers of silicon detectors interleaved by thin sheets of tungsten foil.

The gamma-rays are so energetic that when they hit the foil, they are converted into matter, namely an electron and its anti-matter partner the positron. The subsequent paths taken by these particles are tracked by the silicon detectors to reveal where in the sky the gamma-ray came from.

The electron and positron travel down to a calorimeter which measures their energies - and therefore the energy of the original gamma-ray.

Lat's field of view is comparable to the human eye - seeing about 20% of the sky at a time.

"We are sweeping that field of view across the entire sky every three hours. Each region will be exposed for something like 30 minutes," Dr Ritz explains.

"We're keeping watch over the entire sky all the time. And over time, we are able to see dimmer and dimmer things. We're very excited because there are lots of questions as to how variable things are, and the variability is the key to how some of these incredibly powerful engines work."

The other instrument on Glast is the Glast Burst Monitor (GBM), designed specifically to shed light on gamma-ray bursts (GRBs).

Despite lasting only a few milliseconds to several minutes, these are the brightest gamma-ray phenomena known to science. Missions such as Nasa's Swift space telescope have greatly extended our knowledge.

GRBs lasting two seconds or longer are thought to be associated with the explosive deaths of massive stars. Those lasting less than two seconds may arise through a variety of events, such as the merger of two neutron stars, or the merger of a black hole and a neutron star.

But crucial questions remained unanswered.

Bread and butter

"Some bursts have a mysterious delayed emission of high energy gamma-rays. That's something we're going to learn about on this mission," says Charles "Chip" Meegan of Nasa's Marshall Space Flight Center in Huntsville, Alabama, and the chief scientist on the GBM.

The GBM will detect about 200 of these fleeting events each year, providing real-time locations for follow-up by ground-based and other space-based telescopes.

But Glast's "bread and butter" will be studying active galactic nuclei, or AGN for short. These are galaxies with extremely luminous cores powered by monster black holes.

"Just to give you a sense of the brightness, the power output in gamma-rays of one of these objects is equivalent to all the power of all the stars in an ordinary galaxy shining over all their wavelengths," Dr Ritz explains.

Huge amounts of energy are liberated as the gas surrounding these "supermassive" black holes falls in like water spiralling down a plughole. These galactic cores squirt out enormous jets of very high energy particles that move at near light-speed and can travel far beyond their galaxy of origin.

When these are pointed almost directly at Earth, they are known as "blazars": "In a sense, we're looking down the barrel of a gun," says Dave Thompson.

Despite the fantastic scale and speed of the jets, astronomers have not been able to explain how black holes interact with their environment to accelerate matter to more than 99% the speed of light.

Glast should also shed new light on pulsars, a neutron star emitting powerful beams of radiation that sweep across the Earth's line of sight like lighthouse beacons.

They are formed when the core of a massive star collapses and matter is squeezed so tightly that an amount of material the size of a sugar cube would weigh more than one billion tonnes - about the same as Mount Everest.

"We want to know how particles are accelerated in pulsars. They are some of the most exotic laboratories in the Universe: extreme gravity, extreme magnetic fields, extreme electric fields, high speeds - all of these things are found in pulsars," says Dave Thompson.

"So they are like cosmic labs for things we can't come close to producing on Earth."

The spacecraft will probe the origin of cosmic rays, particles from deep space that bombard the Earth's atmosphere. Glast will test the theory that these particles are created when massive stars explode in supernovas.

Searching for the identity of dark matter, one of the most persistent problems in physics, is also on Glast's to-do list. Despite accounting for 22% of everything in the Universe, the fundamental make-up of this dark "stuff" continues to elude physicists.

Unknown unknowns

A leading candidate is the weakly interacting massive particle, or Wimp. According to one model, when two Wimps collide, they destroy - or "annihilate" - one another. This should generate two gamma-ray photons with a combined energy equal to the mass of the original dark particles.

If astronomers see too many gamma-rays in a particular high-energy range, they will know that dark matter is involved.

This observation could be very difficult to make. Most of the radiation released by Wimp collisions should emerge over a broad range of energy. So this signal will be swamped by the radiation produced by all other gamma-ray sources in the Universe.

But gamma-rays from dark matter annihilations should have a distinctive spectrum and distribution. They should clump together near the centres of galaxies, and this bias may aid their detection.

The most exciting discoveries, however, might be those that no one expects.

"Are there new classes of objects producing gamma-rays in ways we didn't know about? Are there types of things we know about but which we didn't know produced gamma-rays? These are some of the big questions," Dave Thompson tells me.

The mission's scientific promise has attracted researchers from six countries and a variety of scientific backgrounds.

"There are particle physicists and astrophysicists and astronomers all working together to design, build, operate and use the data from Glast," Dr Ritz explains.

"That's exciting, because it forms a nexus of these areas and great things come out of that."

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