3. Data analysis: general considerations and assumptions
The following models were fit to the X-ray spectra of all objects: (i) a powerlaw of the form , and (ii) emission from a Raymond-Smith (1977; RS hereafter) plasma, abundances fixed to either the solar value (Anders & Grevesse 1989) or below, up to 1/100 solar. The amount of cold absorption was constrained not to underpredict the Galactic value (Dickey & Lockman 1990) along the line-of-sight.
If several PSPC observations were available, we used the one with the deepest exposure time (for the present sample, this also always happened to correspond to the pointing were the source was on-axis, if an on-axis pointing existed at all.)
To calculate X-ray fluxes and luminosities, we proceeded as follows: For sources bright enough to allow spectral fits we integrated over the SED in the (0.1-2.4 keV) band after correcting for cold absorption. For RASS sources too weak to perform spectral fits, and for HRI sources we assumed a powerlaw spectrum with =-1.9 and absorption of the Galactic value. Distances were calculated using a Hubble constant of =75 km/s/Mpc for the distant ( km/s) objects. For those nearby or even with blueshift we used Tully's (1988) catalog of nearby galaxies (if not stated otherwise) which is based on the virgocentric model of Tully & Shaya (1984). To check for the influence of the assumed distances, which can be important given that all galaxies are very nearby, we re-calculated all luminosities based on distances obtained with the flow field model of Mould et al. (2000; ApJ, in prep.). We find that our conclusions are unaltered and that luminosities of individual objects are changed by a factor 2. If not stated otherwise, X-ray luminosities given below refer to the energy interval (0.1-2.4) keV.
Some of the RASS sources were not significantly detected. Namely, formally only 1, 3, 0, 5, 1, 2 source photons above the background were registered in the (0.5-2) keV band for NGC 404, NGC 1167, NGC 4419, NGC 5675, NGC 5851, and IC 1481, respectively. In this case we conservatively assumed that 10 source photons would have escaped detection and the upper limits for countrates and luminosities listed in Tables 1, 2 were calculated correspondingly.
To derive blue luminosities, we used the observed blue magnitudes of de Vaucouleur et al. (1991; see also Huchra & Burg 1992). To carry out the extinction correction we adopted the same amount of Galactic absorption (Dickey & Lockman 1990) as we did in the X-ray analysis, assumed a standard gas/dust ratio, and utilized the relation of Bohlin et al. (1978; see also Predehl & Schmitt 1995).
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999