Irish scientists are using large telescopes in Africa in an attempt to discover where cosmic rays come from, writes John Moore.
Telescopes based in Africa are helping a Dublin scientist unravel one of the most confounding puzzles in astrophysics today - the origin of very high-energy cosmic rays.
Cosmic rays bombard us from space every moment of the day. The trick is being able to track them back to their source to better understand what is causing them, explains Luke Drury of the Dublin Institute for Advanced Studies.
These highly charged protons and electrons get deflected by magnetic fields as they travel through our galaxy. This makes it difficult to determine where initially they come from.
However, recent studies of eight very high energy gamma-ray sources in our Milky Way galaxy made by the High Energy Stereoscopic System (Hess) array of telescopes in Africa may provide an answer to the long-standing puzzle. Drury uses Hess to study the origin of cosmic rays and his research suggests that the sources could be objects called supernova remnants (SNRs).
"A SNR is the exploded debris blown off a once massive star," says Drury. "As the shell of material expands outwards and collides with gas and dust in space, shockwaves develop and accelerate the cosmic rays to very high energies.
"These energetic cosmic rays are producing the very high-energy gamma rays," he says. "As these rays are also photons, they travel in straight lines and this allows us to trace back accurately with Hess their true source of origin, which possibly might be the SNRs."
Just over 10 years ago Drury and colleagues theorised that SNRs should be detectable sources for very high-energy gamma rays. However, when it came to actually finding them, telescopes at the time couldn't see any evidence near to the source he was suggesting.
"We actually got quite a piece of stick for wasting people's time," says Drury. "However, 10 years on, we've found several examples including two shell-type SNRs from using the Hess."
Hess currently ranks as the most sensitive array of gamma-ray telescopes in the world. Hess consists of four 13-metre-diameter telescopes that can stereoscopically (three-dimensionally) image where the gamma rays originated, using a technique known as the atmospheric Cerenkov effect.
This effect is seen as faint blue flashes of light when the gamma rays strike our atmosphere and kick off charged particles. The flashes don't last very long given that they travel faster than the speed of light, but their paths point back towards their source of origin. If all four telescopes observe a flash event simultaneously, a stereoscopic image is produced. Counting the number of particles that arise reflects the energy of the incoming cosmic ray.
"Hess gives us several views of the shower development as well as discriminating between similar background showers," says Drury. "It also allows us to estimate quite well the energies of the incident gamma rays which are important for understanding the acceleration process."
The accelerated charged cosmic ray particles can interact with the source to produce gamma-rays, explains Drury, so what we actually see with Hess is these secondary gamma rays.
They are produced either by accelerated protons, called the hadronic route, or by electrons, called the Inverse Compton mechanism, he says, but distinguishing between these cases is proving difficult.
"In either picture, however, the SNR is certainly accelerating the charged particles to very high energies as required if they are to be the sources of the bulk of the Galactic cosmic rays," says Drury.
If that isn't enough for an astrophysicist to contend with already, the Hess results - the most sensitive ever undertaken to date at such short wavelengths - showed two of the eight gamma-ray sources to be a new class of "dark" accelerator never seen before.
"We have no idea of what they might be and we need more information," says Drury. "Whenever you open up a new field of astronomy like this you tend to find things that you haven't dreamt of before."