[/caption]Things appeared to get a little strange in the field of X-ray astronomy when the NASA/ESA ROSAT observatory started seeing emissions from a series of comets. This discovery in 1996 was a conundrum; how could X-rays, more commonly associated with hot plasmas, be produced by some of the coldest bodies in the Solar System? In 2005, NASA’s Swift observatory was launched to look out for some of the most energetic events in the observable Universe: gamma-ray bursts (GRBs) and supernovae. But in the last three years, Swift has also proven itself to be an expert comet hunter.
If X-rays are usually emitted by multi-million Kelvin plasmas, how can X-rays possibly be generated by comets composed of ice and dust? It turns out there is an interesting quirk as comets interact with the solar wind within 3AU from the solar surface, allowing instrumentation designed to observe the most violent explosions in the Universe to also study the most elegant objects closer to home…
“It was a big surprise in 1996 when the NASA-European ROSAT mission showed that comet Hyakutake was emitting X-rays,” said Dennis Bodewits, NASA Postdoctural Fellow at the Goddard Space Flight Centre. “After that discovery, astronomers searched through ROSAT archives. It turns out that most comets emit X-rays when they come within about three times Earth’s distance from the sun.” And it must have been a very big surprise for researchers who assumed ROSAT could only be used to glimpse the transient flash of a GRB or supernova, possibly spawning the birth of black holes. Comets simply did not feature in the design of this mission.
However, since the launch of another GRB hunter in 2005, NASA’s Swift Gamma-ray Explorer has spotted 380 GRBs, 80 supernovae and… 6 comets. So how can a comet possibly be studied by equipment intended for something so radically different?
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As comets begin their death-defying sunward orbit, they heat up. Their frozen surfaces begin to blast gas and dust into space. Solar wind pressure causes the coma (the comet’s temporary atmosphere) to eject gas and dust behind the comet, away from the Sun. Neutral particles will be carried away by solar wind pressure, whereas charged particles will follow the interplanetary magnetic field (IMF) as an “ion tail”. Comets therefore can often be seen with two tails, a neutral tail and an ion tail.
This interaction between the solar wind and comet has another effect: charge exchange.
Energetic solar wind ions impact the coma, capturing electrons from neutral atoms. As the electrons become attached to their new parent nuclei (the solar wind ion), energy is released in the form of X-rays. As the coma can measure several thousand miles in diameter, the comet atmosphere has a huge cross section, allowing a vast number of these charge exchange events to occur. Comets suddenly become significant X-ray generators as they get blasted by solar wind ions. The total power output from the coma can top a billion Watts.
Charge exchange can occur in any system where a hot stream of ions interact with a cooler neutral gas. Using missions such as Swift to study the interaction of comets with the solar wind can provide a valuable laboratory for scientists to understand otherwise confusing X-ray emissions from other systems.