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In an effort to understand the newly discovered phenomenon of cometary X-ray emission, we have obtained observations of the short period (3.3 years), well-studied comet 2P/Encke (Encke) during its July 1997 close approach to Earth. Extended, variable emission on the sunward side of the nucleus was found in the Rontgen X-ray satellite High Resolution Imager (HRI) at 0.090–0.75 keV and in the Extreme Ultraviolet Explorer (EUVE) scanner telescopes' Lexan B 0.090- to 0.28-keV and Al/Ti/C 0.050- to 0.16-keV bandpasses; useful upper limits were found in the Ti/Sb/Al 0.020- to 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 toward 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 h and an amplitude of ∼3 times the baseline. A count rate of ∼0.17 counts s−1 in the HRI was measured for the slowly varying emission, corresponding to a total luminosity Lx of 4×1014 erg s−1. Unlike Hyakutake, however, the bulk of the emission clearly originates outside the comet's bowshock, arguing against the magnetic reconnection and plasma dust emission models. The comet's lightcurve does not correlate with the solar X-ray lightcurve, ruling out scattering of solar X-rays as the emission mechanism. The multiwavelength HRI/EUVE photometry is inconsistent with the Haberli et al. (1997, Science 276, 939–942) charge transfer, plasma dust, and attogram dust models of cometary X-ray emission and is consistent with the Wegmann et al. (1988, Planet. Space Sci. 46, 603–612) charge exchange, 0.15- to 0.45-keV thermal bremsstrahlung, and photon index 1.6–2.0 power law models. While the impulsive event 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, suggesting that new models of the current sheet are necessary. |
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