EUVE Search for X-rays from Comets Encke, Mueller (C/1993 A1), Borrelly, and Postperihelion Hale–Bopp

Abstract

Our observation of postperihelion Hale–Bopp on November 17–23, 1997, revealed the comet soft X-ray luminosity of (3.7±1.2)×10 24 photons s −1 in an aperture of 6×10 5 km and in the EUVE range of 97–165 eV. We also analyzed observations of three comets from the EUVE archive. The X-ray luminosity of Comet Borrelly on November 20–22, 1994, was (7.5±0.6)×10 23 photons s −1 for the aperture of ρ=7×10 4 km. Upper limits of 2.7×10 23 and 1.4×10 24 photons s −1 were obtained for Comets Encke and Mueller (C/1993 A1) on November 30, 1993, and June 9–11, 1994, for ρ=1.7×10 5 and 4.1×10 5 km, respectively. This work has doubled the number of comet observations in our EUVE database. Soft X-ray emissions have been detected in five of the eight observations. The measured X-ray luminocities are consistent with a r 3/2 Q gas dependence having the efficiency of (6.4±0.9)×10 −5 AU 3/2 and therefore favors a gas-related mechanism. The only viable candidate is the charge transfer mechanism. Using our X-ray luminosities as functions of aperture and assuming the presentation of the charge transfer spectrum by thermal bremsstrahlung or the power law, it is possible to make careful comparison of X-ray observations made with different instruments. While both our pre- and postperihelion observations of Hale–Bopp demonstrate a regular behavior of X-rays from that comet, the outburst detected with the BeppoSAX and the nondetection with the ROSAT look puzzling. We suggest that the long-term EUVE observations reflect a mean X-ray emission while the comparatively short BeppoSAX and ROSAT observations of Comet Hale–Bopp could coincide with a maximum and a minimum in the heavy ion flux, respectively.