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C. M. Lisse (U. Maryland), D. Christian (CEUA), K. Dennerl, J. Englhauser, J. Truemper (MPE), M. Desch, F.E. Marshall, R. Petre, S. Snowden (NASA/GSFC)
We report on x-ray observations of the short period, well studied comet 2P/Encke during its July 1997 close approach to Earth. Extended, variable emission on the sunward side of the nucleus was found 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 our results for C/Hyakutake, the emission morphology was roughly symmetric with respect to a vector from the comet's nucleus towards the Sun, with a light curve consisting of a slowly varying baseline emission and a large impulsive event on 7 July 1997 with a time scale of ~3 hours and amplitude of ~3 times the baseline. Unlike Hyakutake, however, the bulk of the emission clearly originates outside the comet's bowshock. A count rate of ~ 0.17 cps in the HRI was measured for the slowly varying emission, corresponding to a total luminosity Lx of 4\times 1014 erg sec-1. The multi-wavelength HRI/EUVE photometry is inconsistent with the Haeberli et al. (1997) charge transfer, plasma-dust, and attogram dust models of cometary x-ray emission, and is consistent with the Wegmann et al. (1998) charge exchange, 0.15-0.45 keV thermal bremsstrahlung, and photon index 1.6 - 2.0 power law models. The impulsive event does not correlate with increases in solar x-ray emission; while it correlates very well with the passage of a solar magnetic field boundary at the Earth and an increase in the solar wind particle flux, it is not coincident, according to current models of the solar wind magnetic current sheet, with the passage of the sector boundary by the comet.
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