3 Determination of the basic source parameters

The basic source parameters are the coordinates, the count rate in the spectrally broad and hard band, the spectral colors (hardness ratios $H\!R1$ and $H\!R2$) and the amplitude of time variability. The hardness ratios will be used in a later section for a source classification. The count rate is used to construct the $\log N - \log S$ relation. As the standard $\log N - \log S$ uses the count rate in the hard band it is of importance that this count rate is made available. I note that usually only the count rate in the broad band is given in catalogs. Conversion from one to another is in principle possible (from simulations) but then one has to assume the proper spectral index which is a priori not known. I recalculated the count rates and the hardness ratios for a subsample using the merged data. The values which are given are the mean values integrated over all observations. I applied the EXSAS spectral fitting task (cf. Zimmermann et al. 1994) for binning and instrument correction (vignetting and dead time). I have individually chosen a circular region for the background subtraction which was in general close to the source and had the same size as the source circle. This procedure allowed precise values to be derived and the statistical errors to be minimized. It has to be noted that values for the hardness ratios $H\!R1$ and $H\!R2$ which are larger than 1.0 are possible in case the source has a negligible flux in the 0.1-0.5 keV band and background subtraction results in a negative count rate for this band. Still consideration of the errors in the hardness ratios for such cases allows values for the hardness ratios which are consistent with 1.0 (in Figs. 1, 3-5 we present or consider sources with hardness ratios $H\!R1 > 1.0$ and $H\!R2 > 1.0$ in the following way: We set $H\!R1 = 1.0$ and $H\!R2 = 1.0$ and we show the error bar towards the minimum value).

In general the source coordinates have not been recalculated and it will be referred to the coordinates given in HP99.

Figure 1: Upper panel: broad band (0.1-2.4 keV) PSPC count rates as derived in this work (K01, Table 1) in comparison with the PSPC count rates derived by HP99. Middle panel: soft hardness ratio $H\!R1$ as derived in this work (K01) in comparison with $H\!R1$ derived by HP99 (only sources with $\delta H\!R1<0.25$ have been plotted). Lower panel: hard hardness ratio $H\!R2$ as derived in this work (K01, Table 1) in comparison with $H\!R2$ derived by HP99 (only sources with $\delta H\!R2<0.25$ have been plotted). For sources with $H\!R1 > 1.0$ and $H\!R2 > 1.0$ a value of 1.0 is given and the error bar towards the minimum allowed value.

3.1 Comparison with the source parameters given in HP99

A different approach has been chosen than has been used by HP99 to derive source parameters (count rates and hardness ratios). I have derived mean parameters from the merged data of all available observations with integration times of at least 1000 s. This, in general, reduced the statistical errors. In addition spectra (source and source plus background) have been accumulated for each individual source and the standard instrument corrections have been applied. For time variable sources average values have been derived in the analysis. It is expected to derive more precise values for the source count rate and the X-ray colors (hardness ratios $H\!R1$ and $H\!R2$) than given in HP99. In Fig. 1 (upper panel) the broad band (0.1-2.4 keV) ROSAT PSPC count rates as derived by HP99 and in this analysis are compared. The count rate derived by these two methods follows the same trend but individual rates scatter. In addition it is found that the count rates derived by HP99 are systematically larger (by about a factor of 1.5) than the count rates derived by K01. Such a systematic difference may be explained by the different method which has been applied to determine the source counts. In addition HP99 determined the count rate of a source for the observation (in case multiple observations exist) in which the derived position was most accurate. In case of time variable sources such an approach can bias the count rate to values which are systematically larger than the mean value averaged over many observations. The importance of this effect was checked for two bright AGN in the field of the LMC (RX J0524.0-7011 and RX J0503.1-6634) which were found to be variable with a timescale of a few hundred days to few years. It was found that for these two AGN the count rate averaged over observations spread over a few years was a factor of 3 and 1.6 respectively smaller than the count rate given in the catalog of HP99. In addition it is found that the count rates of 17 sources deviate more than $5\sigma$ and of 9 sources more than $10\sigma$. 7 of the 9 sources are X-ray binaries which show large variability in the count rate with time. In the catalog presented here count rates averaged over all observations are given while in HP99 the count rate for the observation is given where the source position has been determined (the largest deviation with a few hundred sigma is found for LMC X-4, for this source a low count rate is given in the catalog of HP99). The remaining two sources are the AGN RX J0524.0-7011 and RX J0503.1-6634 which are variable in time (see discussion above).

In this work mainly spectrally hard sources (candidates for AGN and X-ray binaries) have been selected which have hardness ratios $H\!R1$ and $H\!R2$ >0.0. In Fig. 1 (middle and lower panel) the comparison between the hardness ratios $H\!R1$ and $H\!R2$ is shown as derived by both methods for sources with precise hardness ratios ( $\delta H\!R1<0.25$). There is some scatter in these values which may be explained by the different methods which have been applied. A few sources were found in this analysis to be softer than given in the catalog of HP99 (cf. the sources in the lower panel of Fig. 1 which are found at $H\!R2$ values <0). These sources were not classified as AGN, cf. Table 1). In the further analysis the values for the count rate and hardness ratios derived in this work have been used.