Galaxys most energetic gamma rays

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Galaxys most energetic gamma rays

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Milky Way

Physicists have measured the highest energy photons ever seen emerging from our galaxy, the Milky Way.

Observatory spots galaxy's most energetic gamma rays

Physicists have measured the highest energy photons ever seen emerging from our galaxy, the Milky Way.

Previous observations with the orbiting Compton Gamma Ray Observatory had found very high energy (VHE) gamma rays emanating from the equator of the galactic disc at up to 30 billion electronvolts.

Now, using three years' worth of data from the Milagro Gamma Ray Observatory in New Mexico, US, physicists from nine institutions have found gamma rays with an average energy of 3.5 trillion electronvolts.

“Several major experiments have tried to make this measurement and the Milagro group have had the persistence and sensitivity to pull it off,” says Trevor Weekes, an astrophysicist with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who was not involved in the study.

The work is difficult because as the energy of the rays rises, they become rarer. Attempts to measure VHE gamma rays had focused on the Milky Way's equator because it generates the highest number of gamma rays at lower energies.

Extra rays
VHE gamma rays can be spawned when cosmic rays - high energy charged particles - collide with particles in the interstellar medium. Previous observations of lower energy gamma rays suggested there would be more VHE gamma rays than predicted by theory. This might mean some unknown mechanism was generating the extra rays.

But the new observations of the Milky Way confirm that another mechanism is not needed, as the flux of gamma rays in our solar system is similar to that near the equator of our galaxy.

“What we observed was consistent with what you expect from theory,” says Allen Mincer, a member of the team and a physicist at New York University, US.

Previous research in 2005, conducted with the High Energy Stereoscopic System (HESS) in Namibia, identified the location of eight new sources of VHE gamma rays in the Milky Way. The new work was not designed to pinpoint sources - it can only show that a particular photon came from a region in the sky about the size of the Sun.

Mincer told New Scientist that the detector used by his team scans the whole sky, while HESS focuses on specific regions. “We complement them.”

Cosmic shower
As VHE gamma rays slam into Earth's atmosphere, they create a shower of electrons, positrons and lower-energy gamma rays. Because they are traveling faster than the speed of light in air, the secondary particles emit a blue light, known as Cherenkov radiation.

The Milagro detector can distinguish this faint light from the slightly different Cherenkov radiation produced by cosmic rays. Gamma rays tend to induce relatively uniform Cherenkov light distribution, while cosmic rays result in patchy illumination. The measurements give the direction and energy of the original rays.

Space-based observatories, like Compton, are not as effective at finding VHE gamma rays because it is the interaction with the atmosphere that produces bright flashes of Cherenkov radiation. To have the same sensitivity, a satellite would need a very large detector - perhaps covering a square kilometre.

The work will be published in an upcoming issue of Physical Review Letters.

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