Astronomers have discovered the largest and most distinctive loop-like structure yet observed in our galaxy. The immense loop stretches 20 light-years across and could be producing subatomic particles a thousand times more energetic than those manufactured in man-made particle accelerators.

A team of researchers led by Masaaki Sakano of the University of Leicester observed the new formation using the European x-ray satellite, XMM-Newton. (X-rays are preferable for observing the galactic center because the large amounts of dust present there obscure instruments at optical wavelengths.) Sakano classifies the loop's spectrum as extraordinary. "Most diffuse x-ray sources in the universe have a characteristic temperature because they are the residual radiation from an event, such as a supernova explosion," he remarks. "However, in this case the loop is nonthermal and this means that whatever the origin of the structure is, it is not stationary but rather the result of some ongoing process." He presented his findings last week at the meeting of the Royal Astronomical Society in Birmingham, England.

The team saw the x-ray emitting loop while observing the Arches Cluster, a star-forming region near the center of the Milky Way. Whether the loop is linked to the cluster is unclear, the astronomers report, because it could simply lie along the same line of sight. Future observations will help to resolve whether the high-energy particle acceleration is associated with the star-forming cluster, which would be the first time such an association has been observed in our galaxy.