Comet McNaught-Hartley was observed in five 1-h exposures on
January 8-14 2001 using the advanced CCD imaging spectrometer on board the Chandra X-ray Observatory. The X-ray image of the comet does not show a crescent-like shape. The brightest region is offset from the nucleus between the sunward and comet velocity directions. The comet mean X-ray luminosity is equal to 7.8 ร 10 15 erg s -1 for photon energy E > 150 eV and aperture ฯ = 1.5 ร 10 5 km where the comet X-ray brightness exceeds 20% of the peak value. Gas production rate was 10 29 s -1 during the observations, and the efficiency of X-ray excitation was equal to 4 ร 10 -14 erg AU 3/2 . Dayto-day variations in X-rays reached a factor of 5. The strongest short-term variation was by a factor of 1.75 for 1600 s. This variation may be explained by a decline in the solar-wind flux by the same factor in โ800 s. The comet and Earth were seeing different faces of the Sun, and time delay in the solar-wind events on the Earth and the comet was long, equal to 6 days. The best correlation between the comet X-ray luminosity and the solar-wind proton density is for the time delay of 5.5 days and may be explained by the higher velocity of heavy ions.
Careful background subtraction made it possible to extract the comet spectrum from 150 to 1000 eV. No signal was detected at E > 1000 eV, and a 3ฯ upper limit to any emission with E > 1000 eV is 0.3% of the photon emission at 150-1000 eV. The best ฯ 2 -fit model to the spectrum consists of nine narrow emission features. The emission energies and intensities are in good agreement with a charge exchange spectrum calculated by us for the slow solar wind. Using this spectrum, we identify the observed emissions as (Ne 7+ + Mg 7+ + Mg 8+ ) at 195 eV, (Mg 8+ + Mg 9+ + Si 8+ ) at 250 eV, C 5+ at 370 and 460 eV, O 6+ at 560 eV, O 7+ at 650, 780, and 840 eV, and Ne 8+ at 940 eV. X-ray spectroscopy of comets may be used to diagnose the solar-wind composition and its interaction with comets.