2. Data reduction
We used all-sky survey (RASS) as well as archival and pointed serendipitous ROSAT (Trümper 1983) PSPC (Pfeffermann et al. 1987, Briel et al. 1994) observations of the galaxies. The observations are summarized in Table 1.
Table 1. Log of observations. gives the exposure time in ksec, CR the countrate in cts/s. The data for NGC 4051 are separately listed in Table 3. `S' refers to RASS data, `P' to PSPC observations in the pointed mode.
For further analysis, the source photons were extracted within a circular cell centered on the target source. The background was determined in a source-free ring around the target source and subtracted. The data were corrected for vignetting and dead-time, using the EXSAS software package (Zimmermann et al. 1994). In case of RASS data the background was determined in a source-free region along the scanning direction of the telescope.
To carry out the spectral analysis source photons in the amplitude channels 11-240 were binned according to a constant signal/noise ratio of at least 3, and higher for brighter sources.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000