CXO X-ray spectroscopy of comets and abundances of heavy ions in the solar wind

Abstract

X-rays from comets originate in charge exchange between heavy ions of the solar wind and cometary species. Spectra of nine comets observed by the Chandra X-ray Observatory (CXO) are analyzed using the time-dependent instrument sensitivity and the energy-dependent spectral resolution. X-ray emissions are extracted from the spectra in the range of 150–1100eV using the χ2-fitting. Production of X-rays varies in the observed comets by a factor of 500 from 4.4×1013ergs−1 in Comet 73P to 2.2×1016ergs−1 in Comet Ikeya–Zhang. The measured solar wind flow varies within a factor of 20, being the weakest in Comet 73P and the strongest in 9P/Tempel 1. The retrieved X-ray line intensities vary within a factor of 5×104. These lines above 300eV are attributed to emissions of the H- and He-like ions, and laboratory data on the excitation cross sections for these emissions (Greenwood et al. [2000]. Astrophys. J. 533, L175–L178) are used to convert the observed emissions into abundances of heavy ions in the solar wind. Continuity equations for charge exchange in comets are solved analytically and result in relationships between the X-ray emissions and the ion fluxes. The flux of O7+ scaled to 1AU varies within a factor of 35 with a mean value of 1.6×104cm−2s−1. The retrieved ratios of O8+/O7+, C6+/C5+, Ne10+/Ne9+, C6+/O7+, N6+/O7+, and Ne9+/O7+ demonstrate significant variations, while their mean values for O, C, and N agree with those recommended by Schwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566) for the slow and fast solar wind. (Data on Ne9+ and Ne10+ are lacking in Scwadron and Cravens (Schwadron and Cravens [2000]. Astrophys. J. 544, 558–566).) The results are compared with the ion ratios from Bodewits et al. (Bodewits et al. [2007]. Astron. Astrophys. 469, 1183–1195) that were obtained from the same CXO spectra of comets, and some significant differences are briefly discussed. CXO X-ray spectroscopy of comets is a diagnostic tool to study the composition of the solar wind and its variations.