2. X-ray and H I data
The X-ray data were obtained from the ROSAT all-sky survey (Snowden & Schmitt 1990; Voges 1992; Snowden et al. 1997). Photon events detected by the Position Sensitive Proportional Counter (PSPC: Pfeffermann et al. 1986) were binned into seven pulse-height channels (R1-R7: Snowden et al. 1994a) covering the entire ROSAT PSPC energy window. The SXRB radiation between keV was measured in the R1 and R2 bands. Combining the R1 and R2 bands to produce the ROSAT keV data offers the highest statistical significance of soft X-ray material available. The keV energy range is the most sensitive of the ROSAT PSPC bands to photoelectric absorption by the interstellar medium. In this band the interstellar absorption cross section is about . In consequence, the product of soft X-ray absorption cross section and the H I column density, , is close to or greater than unity across the sky, with the exception of a few lines of sight. The data are corrected for scattered solar X-rays (Snowden & Freyberg 1993), as well as for particle background (Plucinsky et al. 1993) and long-term X-ray enhancements (Snowden et al. 1995). The full intrinsic angular resolution of the PSPC has been used, yielding maps with resolution; point sources have been removed to a minimum count rate of (Snowden et al. 1997).
The H I data are those of the Leiden/Dwingeloo survey of Hartmann & Burton (1997), who used the Dwingeloo 25-m telescope to observe the sky at with a true-angle grid spacing of in both l and b. The velocity resolution is set by the interval of 1.03 km s-1 between each of the 1024 channels of the spectrometer; the material covers LSR velocities between -450 km s-1 and km s-1, and thus encompasses essentially all HVC emission. The rms limit on the measured brightness-temperature fluctuations is K. The correction for stray radiation is described by Hartmann et al. (1996). The H I data are published as FITS files on a CD-ROM by Hartmann & Burton (1997), together with an atlas of maps.
Table 1 summarizes the main parameters of the regions studied as well as their typical X-ray intensities and H I column densities. We projected the distribution, regridded to an angular resolution of , onto the polar-grid projection of the ROSAT survey. The choice of angular resolution aimed at enhancing the statistical significance of the X-ray data and allowing differentiation between systematic uncertainties introduced by X-ray raw-data processing (e.g. residual point source contributions and scanning stripes) and modelling of the X-ray intensity distribution. The statistical significance, (corresponding to the uncertainty within a area), of soft X-ray enhancements and depressions was evaluated using the ROSAT uncertainty maps, which account only for the number of photon events: they do not include any systematic uncertainties introduced by non-cosmic X-ray backgrounds.
Table 1. Location of the HVC fields selected, and the and X-ray count rate ranges encountered in each field. The mean ROSAT integration times, , are also given, with minimum and maximum times noted in parentheses.
© European Southern Observatory (ESO) 1999
Online publication: December 22, 1998