Physicists Solve Mystery of Why Comets Emit X-Rays

The long-standing mystery of why comets give off X-ray emission has been solved by a group of experimental physicists led by the University of Oxford, UK.

When comets travel through the Solar System they interact with the solar radiation, the solar wind and the solar magnetic field. This interaction produces a visible atmosphere around the comet and the observed cometary tail, and in some cases produces X-rays.

These X-rays are generated on the sunward side of the comet where the solar wind impacts the cometary atmosphere forming a bow shock.

To investigate how X-rays can be emitted from a comet, Oxford physicist Alexandra Rigby and colleagues performed experiments at the LULI laser facility at École Polytechnique in Paris, France, where they replicated the interaction of the solar wind with a comet.

“Our experimental results are important as they provide direct laboratory evidence that objects moving through magnetized plasmas can be sites of electron acceleration — 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,” said Professor Bob Bingham, from the Rutherford Appleton Laboratory and the University of Strathclyde.

“As a theorist I find it amazing that it is possible to sensibly replicate astrophysical phenomena in the laboratory, to test our physical understanding of what Nature gets up to,” said Dr. Raoul Trines, from the Rutherford Appleton Laboratory.

For the experiments, the physicists fired laser beams onto a plastic foil, which exploded, causing a stream of electrons and ions to be expelled, forming a high speed flow of ionized gas (plasma) like the solar wind.

This ‘plasma flow’ then impacted onto a solid sphere, the so-called laboratory ‘comet,’ placed nearly a centimeter 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.

This research also sheds light on a cosmic ray mystery known as the injection problem.

“It is widely recognized that strong shock waves are expected to accelerate particles to very high energies, however, they require a source of particles fast enough to cross the shock, the injection problem. Each time the particles cross the shock they gain energy,” the scientists said.

“Our experiments clearly demonstrate that plasma turbulence can provide a source of fast particles overcoming the injection problem.”

The results appear in the journal Nature Physics.


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