Mystery of Milky Way's gamma rays solved

A mysterious gamma-ray glow that suffuses our galaxy has been tracked down to a set of distinct sources - many of them black holes.

The new work settles a 30-year-old debate about where "soft", or relatively low energy, gamma rays come from and lays to rest convoluted theories devised to account for the radiation.

"There was a profound mystery in gamma-ray astronomy about what was seen from our own galaxy ... now the mystery is solved," says Neil Gehrels, at NASA's Goddard Space Flight Center.

Gamma rays are absorbed by the Earth's atmosphere, so the first measurements were not made until high-altitude balloon flights in the late 1960s. These showed the Milky Way was awash with gamma-rays, but their origin could not be pinned down accurately.

In the early 1990s, a French space telescope named SIGMA found about 10 discrete sources that accounted for nearly half the soft gamma-ray glow. These sources were binaries, comprised of a dense, compact object - a black hole or neutron star - that pulled matter from another star, generating high-energy radiation in the process.

The other half of the glow was attributed "by default" to diffuse processes originating in the space between stars, says astronomer Francois Lebrun, at the French Atomic Energy Commission and leader of the new research.

Unreasonably energetic

In these scenarios, cosmic-ray electrons, probably accelerated by supernova remnants, interact with either photons or gas in the interstellar medium to produce gamma rays. But the explanations led to new quandaries: They should have produced detectable radio waves, which were not seen, or have been unreasonably energetic.

"Solving one problem, we faced another one," Lebrun told New Scientist. His team used the European Space Agency's INTEGRAL space telescope, launched in 2002, to observe a region 20,000 light-years across at the centre of the Milky Way.

Four months of observations in 2003 revealed 91 sources for the soft gamma-ray glow. Together these now account for 90 per cent of the galaxy's soft gamma rays.

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The team checked the sources and 65 were previously catalogued objects, mostly black hole or neutron star binaries. These are likely to emit gamma rays more or less continuously for thousands of years - unlike the higher energy gamma-ray bursts, which last for seconds or minutes and are thought to arise from the deaths of massive stars.

Two of the sources are of particular interest to Lebrun, These binaries are cocooned in dense gas and dust that absorb all but the highest-energy radiation. "Probably the cocoon is linked to the binary system - a lot of observation is going on to understand what those objects are," he says.

"This discovery points to a future in which soft gamma-rays will become a powerful tool for finding black holes in otherwise obscured regions," believes Nicholas White, an astronomer at Goddard Space Flight Center, writing in an accompanying article to Lebrun's paper in Nature.

Journal reference: Nature (vol 428, p 293)

Maggie McKee