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

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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 [FORMULA] keV was measured in the R1 and R2 bands. Combining the R1 and R2 bands to produce the ROSAT [FORMULA] keV data offers the highest statistical significance of soft X-ray material available. The [FORMULA] 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 [FORMULA]. In consequence, the product of soft X-ray absorption cross section and the H I column density, [FORMULA], 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 [FORMULA] resolution; point sources have been removed to a minimum count rate of [FORMULA] (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 [FORMULA] with a true-angle grid spacing of [FORMULA] 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 [FORMULA] km s-1, and thus encompasses essentially all HVC emission. The rms limit on the measured brightness-temperature fluctuations is [FORMULA] 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 [FORMULA] distribution, regridded to an angular resolution of [FORMULA], 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, [FORMULA] (corresponding to the uncertainty within a [FORMULA] 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 [FORMULA] and X-ray count rate ranges encountered in each field. The mean ROSAT integration times, [FORMULA], are also given, with minimum and maximum times noted in parentheses.

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998