A vast bubble of hot, rarefied gas has been revealed as a source of cosmic rays – the mysterious particles that batter the Earth continuously. The observation of the so-called superbubble, measuring more than 100 light-years across, was made using gamma rays collected by NASA's Fermi satellite and sheds light on the origin of cosmic rays in regions of massive-star formation.

Cosmic rays are highly energetic protons, nuclei and electrons that arrive at the Earth from space. Ever since they were discovered in 1912 by the Austrian physicist Victor Hess, scientists have debated where they come from and how they are accelerated. Travelling at close to the speed of light, they can have energies many orders of magnitude higher than can be achieved in the most powerful accelerators on Earth.

Many scientists believe that cosmic rays with an energy up to about 1015 eV are accelerated by the shock waves produced when a supernova (an exploding high-mass star) ejects material into space at very high speeds. Data analysed from NASA's Advanced Composition Explorer spacecraft in 2003 provided indirect evidence that at least some cosmic rays are accelerated within regions of massive-star formation, where about 80% of supernova remnants reside. The satellite measured the relative abundance of different isotopes within samples of cosmic rays reaching the Earth, and found that while four-fifths of this material resembles that found in our own solar system, about a fifth corresponds to material ejected by heavy stars.

More direct support

The latest research provides more direct support for this hypothesis. An international team of astrophysicists has analysed gamma-ray data recorded by the Large Area Telescope onboard the Fermi satellite, and has found an extended source of gamma rays emitted from within the region of the Cygnus constellation. The gamma-ray emission extends along a line measuring about 160 light-years, between two clusters of massive stars, one containing more than 500 massive stars and the other about 75.

Massive stars are formed inside dense clouds of gas and as they grow they eject matter in the form of stellar winds and when they explode as supernovae. The pressure of these ejections pushes gas away from the stars, creating cavities, or bubbles, around them. These bubbles can grow until they merge with bubbles from neighbouring stars, so producing superbubbles.

The Fermi researchers believe that the gamma rays they have observed are the result of cosmic rays being produced inside a superbubble that then interact with the gas and light contained within the bubble. Astrophysicists use such gamma rays to observe the behaviour of cosmic rays because, unlike cosmic rays, gamma rays are not deflected by the magnetic fields that permeate space, and therefore their origins are easier to pinpoint.

The idea that the Fermi satellite is seeing cosmic rays produced inside the Cygnus superbubble is given added support by the relatively large number of higher-energy photons it detected. This "hard" gamma-ray spectrum suggests that the cosmic rays were accelerated close to where they produced the gamma rays, that is to say inside the superbubble. The gamma-ray emission produced by cosmic rays in the neighbourhood of the Earth, in contrast, has a "soft" spectrum because the cosmic rays have travelled further from their sources and lost energy in the process.

First "firm proof"

"This is the first time we have firm proof of cosmic-ray sources inside massive-star-forming regions," says group member Luigi Tibaldo of the University of Padua in Italy. "This is an important step forward in the quest to understand the mystery of cosmic rays."

The next step is to work out exactly what is doing the accelerating. As Tibaldo explains, the culprit could be isolated shock waves generated by single supernova remnants, or else the collective action of many different shock waves. Shedding light on this question will involve higher-resolution observations of the Cygnus superbubble, as well as more refined models of superbubble acceleration and more data from other massive-star-forming regions, both within and beyond our galaxy, says Tibaldo.

Tibaldo does point out, however, that such superbubble acceleration could not solve the cosmic-ray mystery single-handedly. That is because the shockwaves produced by supernova remnants or clusters of massive stars do not pack enough punch to accelerate cosmic rays to the very highest energies at which they have been observed – 1020 eV and beyond.

Alan Watson of Leeds University, who is not part of the Fermi team, believes that the latest results are "an important discovery" in cosmic-ray physics because, he says, "it is clear the researchers have established that there are freshly accelerated particles in the region of the superbubble", but argues that "questions remain", such as whether the particles being accelerated are protons or electrons. "Unfortunately, it does not seem that the problem of the origin of cosmic rays is going to be completely solved in time for the centenary in August 2012," he adds.

The research is published in Science 334 1103.