2.1. Soft X-ray data
A detailed description of the RASS data and their reduction has been given by Snowden et al. (1995). The public RASS XRB maps are binned into three energy bands: the keV band (pulse-height invariant energy, PI, channels 11-41), the keV band (PI channels 52-91), and the 1.5 keV band (PI channels 92-201). The angular resolution is about . The integration time varies appreciably: from several thousands of seconds for directions near the ecliptic poles, down to several tens of seconds near the ecliptic equator.
The PSPC pointed observations have been corrected for various non-cosmic background components (Snowden et al. 1994). The short-term enhancements were excluded by selecting nighttime periods of the ROSAT orbits in the keV band. The particle background was subtracted using the method described by Plucinsky et al. (1993). Possible long-term enhancement contaminations were identified by comparing the pointed-observation count rates with the RASS count rates, and by comparing the count rates in positional overlapping areas of different pointings.
2.2. H I data
A survey of the neutral hydrogen in our Galaxy at was recently completed using the 25-m radio telescope in Dwingeloo. The Leiden/Dwingeloo survey (Hartmann 1994, Hartmann & Burton 1997) has a velocity coverage ranging from to , an angular sampling of , and an rms intensity limit of ; the survey has been corrected for stray radiation (Hartmann et al. 1996). This survey provides the most precise H I column densities of the northern sky available.
For analysis of the H I column density distribution in certain limited fields at higher angular resolution we obtained H I 21-cm spectra with the 100-m radio telescope in Effelsberg. The principles of the reduction of Effelsberg H I spectra have been described by Herbstmeier et al. (1996). The spectra in the selected fields have an angular resolution of , a velocity resolution of , and an rms level of .
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998