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Astron. Astrophys. 342, 213-232 (1999)

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A search for soft X-ray emission associated with prominent high-velocity-cloud complexes

J. Kerp 1,2, W.B. Burton 3, R. Egger 1, M.J. Freyberg 1, Dap Hartmann 4,2, P.M.W. Kalberla 2, U. Mebold 2 and J. Pietz 2

1 Max-Planck-Institut für Extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany
2 Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
3 Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
4 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received 23 September 1996 / Accepted 7 October 1998


We correlate the ROSAT [FORMULA] keV all-sky survey with the Leiden/Dwingeloo H I survey, looking for soft X-ray signatures of prominent high-velocity-cloud (HVC) complexes. We study the transfer of [FORMULA] keV photons through the interstellar medium in order to distinguish variations in the soft X-ray background (SXRB) intensity caused by photoelectric absorption effects from those due to excess X-ray emission. The X-ray data are modelled as a combination of emission from the Local Hot Bubble (LHB) and emission from a distant plasma in the galactic halo and extragalactic sources. The X-ray radiation intensity of the galactic halo and extragalactic X-ray background is modulated by the photoelectric absorption of the intervening galactic interstellar matter. We show that large- and small-scale intensity variations of the [FORMULA] keV SXRB are caused by photoelectric absorption which is predominantly traced by the total [FORMULA] distribution. The extensive coverage of the two surveys supports evidence for a hot, X-ray emitting corona. We show that this leads to a good representation of the SXRB observations. For four large areas on the sky, we search for regions where the modelled and observed X-ray emission differ. We find that there is excess X-ray emission towards regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray emission is positionally correlated with the high-velocity clouds. Some lines of sight towards HVCs also pass through significant amounts of intermediate-velocity gas, so we cannot constrain the possible role played by IVC gas in these directions of HVC and IVC overlap, in determining the X-ray excesses.

Key words: ISM: clouds – Galaxy: halo – Galaxy: kinematics and dynamics – X-rays: ISM

Send offprint requests to: J. Kerp, Bonn address

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998