Constraints on models of Cometary x-ray emission with EUV and soft X-ray spectrophotometry

We report on our Extreme Ultraviolet Explorer (EUVE) and ROSAT HRI photometric studies of close approaching comets, in particular Comets 2P/Encke and 55P/Tempel-Tuttle. EUVE observations of Comet 2P/Encke were obtained in July 1997 with the scanning telescopes and ROSAT HRI. Similar observations were obtained for Comet 55P/Tempel-Tuttle in January 1998. Extended, variable emission on the sunward side of the nucleus was found for both comets in the ROSAT HRI at 0.090 - 0.75 keV and in the EUVE scanner telescopes' Lexan B 0.090 - 0.28 keV and Al/Ti/C 0.050 - 0.16 keV bandpasses; useful upper limits were found in the Dagwood 0.020 - 0.040 eV bandpass. Similar to results obtained for other ROSAT detected comets, the emission morphology was roughly symmetric with respect to a vector from the comet's nucleus towards the Sun. The light curves from both comets showed a slowly varying baseline emission and large impulsive events. From the observed HRI count rates, we derive luminosities of 4x10¹⁴ erg sec^-1 and 2x10¹⁵ erg sec^-1 for Encke and Tempel-Tuttle, respectively. Using the results of our EUVE and HRI broad band spectrophotometry, we have found that the Haeberli et al. (1997) charge transfer, plasma-dust, and attogram dust models of cometary x-ray emission are inconsistent with the data, while the Wegmann et al. (1998) charge exchange, 0.15-0.45 keV thermal bremsstrahlung, and photon index 1.6 - 2.0 power law models are most consistent with our observations.