This false-colour image of Comet Hyakutake at extreme-ultraviolet (EUV) wavelengths (at around 10 nm, equivalent to a photon energy of about 120 eV) was obtained by the U.K.'s Wide Field Camera (WFC) on-board Germany's ROSAT
satellite. The observation was made on 27 March 1996, simultaneously with the X-ray measurements from the U.S.-provided High Resolution Imager (ROSAT HRI), as reported by Lisse et al. on 4 April. This observation
was part of a sequence of ROSAT measurements of the comet over the
period 26-28 March, which resulted in the discovery of very bright
X-ray and extreme-ultraviolet emission from the comet --- the first
time any comet has ever been seen at these wavelengths. This important
discovery shows that there must be previously unsuspected
``high-energy'' processes taking place in the comet, probably due to
the influence of the Sun's radiation and/or the solar wind. Bright, diffuse emission is seen, roughly co-incident with the X-rays and sunward of the nucleus, and strongly time-variable in a similar manner. The WFC images also show fainter diffuse emission extending beyond the edge of the HRI field of view, i.e. at about 0.25-0.7 degrees from the centre of the comet nucleus, forming an arc around the bright central region.
The combination of simultaneous X-ray and EUV measurements provides a powerful means of investigating the cause of the comet's unexpected brightness at these short wavelengths.
The ROSAT observation was proposed as a `Target of Opportunity' by planetary scientists Drs. Carey Lisse and Mike Mumma of NASA's Goddard Space Flight Center (GSFC). The research is being conducted in collaboration with Drs. Konrad Dennerl, Jakob Englhauser and Jürgen Schmitt of the Max-Planck-Institut für extraterrestrische Physik, Garching, Germany; Dr. Richard G. West of Leicester University, U.K.; Mr. Mike Harden and Dr. Martin Ricketts of the Rutherford Appleton Laboratory, U.K.; and the German, U.S., and U.K. Project Scientists, Professor Joachim Trümper (MPE), Dr. Robert Petre (GSFC) and Dr. John Pye (Leicester University), respectively.