Abstract. Diffuse excess 1/4 keV soft X-ray emission was found to be positionally correlated with the column density distribution of the high velocity cloud (HVC) complex C (Kerp et al. 1996). Here we point out that the detected diffuse X-ray emission is indeed associated with the HVC phenomenon. For this purpose we study the 1/4 keV radiation transfer as well as the Hi column density distribution of HVCs and intermediate velocity clouds (IVCs) towards HVC complex C in detail. We present evidence that on arcmin scales the 3/4 keV soft X-ray emission is positionally anticorrelated with the HVC column density distribution of an individual HVC filament of complex C.
IntroductionUp to now HVCs have been detected, in emission, only in the line radiation of the neutral atomic hydrogen (for a comprehensive review see Wakker & van Woerden 1997). Recent analyses of ROSAT data, suggest that HVCs also emit soft X-rays (Herbstmeier et al. 1995and Kerp et al. 1994, 1995, 1996. Here we focus on HVC complex C towards high galactic latitudes. We present a comparative analysis of X-ray and HI 21 cm line data available in this region (73° < I < 118°, 38° < b < 63°). In the l/4keV energy regime one has to disentangle the X-ray emission of the individual soft X-ray sources which contribute to the observed X-ray background in order to reveal the excess emission associated with the HVCs. Especially the intermediate-and low-velocity gas determines the intensity distribution of the soft X-ray sky, by photoelectric absorption. Because of the much smaller absorption cross section in the 3/4 keV band such a decomposition is not necessary. We will demonstrate that it is feasible to separate the emission of the "normal" soft X-ray background (SXRB) from that of the excess emission associated with HVCs. In consequence we have to solve the radiation transport of soft X-ray photons through the galactic interstellar medium (ISM) to identify sky areas which reveal deviations from this "normal" SXRB intensity distribution. Here we focus on two aspects which are important to understanding the recent results on X-ray emission of HVCs. First, we will show that the ROSAT 1/4 keV all-sky survey data reveal soft X-ray emission which is positionally correlated with the HVCs of complex C. We show new H121 cm line maps of the total and IVC column density distribution and discuss the relation of the individual cloud populations to the appearance of the soft X-ray intensity distribution on the sky. Second, we will discuss in Sect. 3 the small scale positional association of an individual HVC filament with excess soft X-ray emission in the ROSAT 3/4keV energy band. This is of importance because the 3/4 keV radiation can penetrate the galactic ISM without significant attenuation towards high galactic latitudes.
T h e Soft X-ray BackgroundIt is a matter of discussion whether the SXRB has a patchy source intensity distribution or not (see also Pietz et al., Snowden, and Wang; this volume). For our purpose knowledge of the soft X-ray intensity distribution on ...