Most of their time is spent frozen in the outer reaches of the solar system. But when these balls of ice and dust, which we know as comets, decide to make an appearance, the spectacle is often grandiose. This is mainly caused by their warming up as they approach the Sun. Astronomers then have a chance to investigate comets closely, including at X-ray wavelengths, as XMM-Newton did at the end of January 2001.

XMM-Newton image of comet McNaught-Hartley. Photo: ESA

The primary objective of the XMM-Newton mission is the study of the X-ray-hot, violent universe. However since the ROSAT observatory's discovery in 1996 that 'dirty snowballs' also shine quite unexpectedly in X-rays, it had been envisaged that XMM-Newton would be asked to contribute and help astronomers trying to understand this X-ray emission.

The comet McNaught-Hartley was revealed in October 1999. During most of last year it was visible in the Southern hemisphere steadily becoming brighter and by mid-December, now in view of the Northern hemisphere, it had reached 8th magnitude. The coma diameter was estimated to be between 3 and 4 arcminutes. Brightness is important because for most comets the X-ray luminosity tends to follow the optical luminosity.

McNaught-Hartley was the opportunity several X-ray astronomers had been waiting for. At the Max Planck Institute for Extraterrestrial Physics in Garching, Konrad Dennerl - who had already been involved in the initial ROSAT discovery - and XMM-Newton Telescope scientist Bernd Aschenbach proposed to carry out an observation.

"XMM-Newton's field of view is very small for these type of observations, only 30 arcminutes and catching a comet is not easy," explains Bernd Aschenbach. "The satellite's normal operating mode is to have a fixed attitude. The observatory currently has no scan mode and can not continuously track a moving object. So once we had Project Scientist Fred Jansen's green light to make the attempt, as part of my guaranteed time, we had to find the right strategy."

Early in December, Konrad Dennerl and Pedro Rodriguez, a mission planner from XMM-Newton's Science Operations Centre at VILSPA, started calculating whether the speed at which McNaught-Hartley was travelling was slow enough to get enough exposure as it passed over the telescope's field of view.

They were lucky! In three separate 10 thousand second duration observations on 29 and 30 January, XMM-Newton was able to catch its first comet. McNaught-Hartley, which is a dynamically new comet coming perpendicularly to the ecliptic plane, was then at a distance of some 192 million kilometres (1,29 AU).

Comets may be bright at optical wavelengths, but their X-ray flux is generally some ten thousand times weaker. With its large collecting area for soft X-rays, where comets are expected to radiate predominantly, XMM-Newton is an excellent tool. The data obtained by the EPIC cameras is still being analysed but already Konrad Dennerl and Bernd Aschenbach are very excited.

The study of X-ray emission in comets is still very much virgin territory and a field where there is intense competition. Five theories, from most to least probable according to current thinking, have been advanced to explain the phenomenon:

• interaction of highly ionised elements in the solar wind (such as carbon, nitrogen and oxygen) with the gas being released by the comet. This would be a 'charge exchange' process, where for instance, oxygen atoms with six or seven lost electrons, would be recharged by the cometary gas.
  
  
• a 'Bremsstrahlung' effect when fast moving electrons in the solar wind hit, and are slowed down, by the nuclei of cometary atoms.
  
  
• X-rays from the Sun itself may be scattered, and re-emitted by dust particles of the comet.
  
  
• Interaction between the ionised plasma in the solar wind and the plasma in the vicinity of the comet, with a heating process in both.
  
  
• Collision and 'sputtering' of solar system dust particles and cometary dust particles at relative velocities high enough to produce X-rays.
  
  

XMM-Newton obtained sufficient X-ray counts with the EPIC-pn camera. EPIC-MOS camera data will also be examined because of their high spectral resolution at low energies. In addition, the investigation will benefit from the image obtained simultaneously in the ultra-violet by the Optical Monitor.

Ground-based image of comet McNaught-Hartley. Photo: Juan Rodriguez/the Observatori Astronomic de Mallorca

"The EPIC-pn camera image I have produced, at a very early state of analysis, is proof that XMM-Newton is able to observe cometary X-ray emission, which is a challenge in itself," says Konrad Dennerl. "The image looks quite similar to what ROSAT observations showed, with an X-ray halo which is elongated perpendicularly to the solar direction."

"But the really exciting part is the spectroscopy, as a first glimpse of the data shows. With possible contamination of the soft X-ray signal by optical light, it is one of the most challenging tasks of data analysis and this will certainly take some time."

Konrad Dennerl and Bernd Aschenbach are sure that the scientific clues lie in the excellent spectra they have obtained from McNaught-Hartley, and that from the proposed theories, they can probably settle for just one. It could be the fullest explanation ever of cometary X-ray emission.

Acknowledgements to Konrad Dennerl and Bernd Aschenbach of the Max Planck Institute for Extraterrestrial Physics, Garching, Germany.