X-rays are normally associated with hot sources such as the sun, and comets are among the coldest objects in the solar system. Yet why do comets radiate X-rays. This long-standing mystery has finally been solved by a team of scientists.
When comets travel through the solar system, they interact with solar radiation, wind and magnetic field. This produces a visible atmosphere which is known as a coma, the famous cometary tail, and in some cases, X-rays. The X-rays are generated on the sunward side of the comet, where the solar wind impacts the cometary atmosphere to form a bow shock.
A team of scientists from 15 institutes probed the question by carrying out experiments at the LULI laser facility in Paris, in which they replicated the interaction of the solar wind with a comet. Their paper has been published in Nature Physics.
“These experimental results are important as they provide direct laboratory evidence that objects moving through magnetised plasmas can be sites of electron acceleration,” said Bob Bingham of the University of Strathclyde’s physics department who provided the theoretical support for the project, and had published a theoretical paper on X-rays from comets that formed the basis of the investigation. “This is a very general situation in astrophysics that takes place not only in comets, but also in planetary magnetospheres—such as our own Earth—or even in supernova remnants, where the ejected material moves across the interstellar gas. The experiments also confirm theoretical models developed by the team.”
Alexandra Rigby, a University of Oxford PhD student working on the project, said, “The laser beams are fired onto a plastic foil, which explodes, causing a stream of electrons and ions to be expelled, forming a high speed flow of ionized gas–plasma—like the solar wind. When this plasma flow impacts onto a solid sphere, the laboratory `comet’, placed nearly a centimetre away from the plastic foil, resembles what happens when a real comet passes through the solar system. It was found that electrons are heated to about a million degrees in the up-stream plasma by plasma turbulence. These hot electrons are responsible for emitting X-rays but only in the presence of a magnetic field”.
The research also shed light on a cosmic ray mystery known as the injection problem. It is widely recognised that strong shock waves accelerate particles to very high energies. However, they require a source of particles fast enough to cross the shock. This latest experiment clearly demonstrates that plasma turbulence can provide a source of fast particles which can overcome the injection problem.