Comparison of X-rays from Comets LINEAR (C/1999 S4) and McNaught-Hartley (C/1999 T1)

The Chandra X-ray Observatory (CXO) observations of Comets McNaught-Hartley (MH) and LINEAR S4 (S4) have been processed in the same way to compare X-rays from those comets. The X-ray isophotes are crescent-like in S4 and more circular in MH because of the different phase angles (98° and 44°, respectively). The peak X-ray brightness is greater in S4 than that in MH by a factor of 1.5 and smaller by a factor of 1.7 after the correction for heliocentric distance. The X-ray luminosities of MH and S4 are equal to 8.6 and 1.4×10¹⁵ erg s⁻¹ inside the apertures of ρ=1.5 and 0.5×10⁴ km, respectively. (Brightness is 20% of the peak value at these ρ.) Efficiencies of X-ray excitation corrected to the solar wind flow are similar and equal to 4.3×10⁻¹⁴ erg AU^3/2 in both comets. This confirms the solar wind excitation of X-rays in comets. Spectra of the comets were extracted with a special care of the background correction and using an energy-dependent spectral resolution code. The MH spectrum consists of ten emissions instead of nine emissions in the previously published spectrum. The new emission at 307 eV fills in a strong minimum in the previous spectrum and removes the major difference between that spectrum and the synthetic spectrum. This emission is assigned to the C⁺⁴ and Mg⁺⁹ lines. The positions of the other emissions and their identification are similar to those in the previous spectrum. The S4 spectrum consists of eight emissions, and four emissions are the same as in MH. The line identification is given. Ion ratios in the solar wind have been extracted from the spectra. O⁺⁸/O⁺⁷ is equal to 0.29±0.04 and 0.14±0.02 in MH and S4, and this difference correlates with the higher solar wind speed in S4. Ne⁺⁹/O⁺⁷ is (15±6)×10⁻³ and (19±7)×10⁻³, and these are the first data on Ne⁺⁹ in the solar wind. C⁺⁶/O⁺⁷ is 0.7±0.2 in both MH and S4. X-ray spectroscopy of comets may be used as a diagnostic tool to study the solar wind composition.