Comets are cold, dusty snowballs with a cold cloud of gas around them. X-rays are typical for objects as hot as the Sun, milliions of degrees. Why then are comets emitting X-rays?

Highly charged ions from the Sun -the solar wind- fly unhindered through interplanetary space until they collide with the gas around comets. There, they are neutralized through charge exchange reactions. The electron is captured into a highly excited state of the ion, from which it decays by X-ray and UV emission.

We studied these collisions in the lab and compared these experiments with observations by X-ray telescopes such as Chandra, XMM-Newton, and Swift. We have observed about 12 comets and developed an emission model to analyze solar wind charge exchange spectra. Using this method we found that cometary X-ray emission is very sensitive to the type of solar wind it encounters.


  1. ‘Cometary Charge Exchange Diagnostics’, Bodewits et al. 2012, Astronomische Nachrichten [ArXiv]

  2. Simultaneous UV and X-ray observations of Comet Lulin’, Carter et al. 2012, A&A [A&A]

  3. ‘Spectral Analysis of the Chandra Comet Survey’, Bodewits et al. 2007, A&A 469 p1183 [ADS]

LEFT: Simultaneous X-ray (red) and UV (blue) observations C/2007 N3 (Lulin) obtained by two instruments on board Swift. In ultraviolet light we see the emission of OH gas, formed by the destruction of water produced by the comet (see also ‘Swift & Comets’). This process results in a spherical gas coma of 100,000 km in radius. The X-rays are produced by solar wind ions capturing electrons from this gas cloud, and for an umbrella-shape emission towards the Sun (towards lower left corner).