The Earth's atmosphere is bombarded by radiation known as cosmic rays, the most powerful of which have energies in the petaelectronvolt (PeV) range – a million billion times higher than visible light. Until recently it was assumed these come from supernova remnants, but theoretical modeling has challenged the idea this is possible, leaving astronomers chasing a different source. A new proposal suggests that what one star can't manage, many working together can.
Only a tiny proportion of cosmic rays reach PeV energies, which is fortunate because otherwise we all might be irradiated to death. Nevertheless, we have evidence they do exist, and therefore they require explaining. The extreme conditions left behind by supernovas were the obvious pick, but NASA's Dr Henrike Fleischhack said in a statement that "They do accelerate cosmic rays, but they are not able to get to highest energies.”
To achieve such extreme energies, particles need to be accelerated over a substantial period of time, and supernova remnants are now thought only to provide swift, short shocks.
A paper in Nature Astronomy has reported signs of gamma rays approaching PeV energy coming from the Cygnus OB2 cluster, 5,000 light-years away. A large team of authors argue that the intense magnetic fields within clusters like this, produced by multiple stars, accelerate protons over long periods of time until they approach the speed of light. When these relativistic protons collide with gas, they emit energies up to PeV. One might say it takes a stellar village to raise a PeV ray.
Most stars are formed in star clusters when large clouds of gas collapse. You can observe the process with a backyard telescope pointed at the Orion Nebula – although since it takes hundreds of thousands of years, don't expect any major developments in your life span. Under the right conditions, many of the stars formed in these clusters are exceptionally hot, bright, and short-lived, and Cygnus OB2 is one of the most extreme examples we know of. A dust cloud known as the Cygnus rift prevents us from seeing it clearly except with X-Ray telescopes, but Cygnus OB2 includes enough O and B-type stars to put more famous star-forming nebulae to shame.
"Spectral type O stars are the most massive," said co-author Binita Hona. "When their winds interact with each other, shock waves form, which is where acceleration happens."
Detecting PeV rays is difficult. However, the High-Altitude Water Cherenkov (HAWC) observatory can find the origin of rays with energies up to 200 TeV (0.2 PeV), although even then it has to do it indirectly. When such powerful rays hit the Earth's atmosphere they produce showers of secondary rays. "We use the particle charge and time information to reconstruct information from the primary gamma." doctoral student Dezhi Huang said. That includes determining its path and therefore which part of the sky it comes from.
The massive stars that produce supernovas (and their remnants) have short lives, often exploding while still in clusters that contain other O-type stars. When we detect cosmic rays coming from clusters like this, we can't tell if the remnant or the surviving stars are responsible. However, Cygnus OB2 is just a few million years old – too young for even the shortest-lived star to have ended its life – narrowing down the possibilities.
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