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

Our observation of postperihelion Hale-Bopp on November 17-23, 1997, reveal ed the comet soft X-ray luminosity of (3.7 +/- 1.2) x 10(24) photons s(-1) in an aperture of 6 x 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 lumi nosity of Comet Borrelly on November 20-22, 1994, was (7.5 +/- 0.6) x 10(24 ) photons s(-1) for the aperture of rho = 7 x 10(4) km. Upper limits of 2.7 x 10(23) and 1.4 x 10(24) photons s(-1) were obtained for Comets Encke and Mueller (C/1993 Al) on November 30, 1993, and June 9-11, 1994, for rho = 1 .7 x 105 and 4.1 x 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 luminocitie s are consistent with a r(3/2)Q(gas) dependence having the efficiency of (6 .4 +/- 0.9) x 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 differ ent instruments. While both our pre- and postperihelion observations of Hal e-Bopp demonstrate a regular behavior of X-rays from that comet, the outbur st detected with the BeppoSAX and the nondetection with the ROSAT look puzz ling. We suggest that the longterm EUVE observations reflect a mean X-ray e mission while the comparatively short BeppoSAX and ROSAT observations of Co met Hale-Bopp could coincide with a maximum and a minimum in the heavy ion flux, respectively. (C) 2000 Aacademic Press.