2 Selection of the AGN and X-ray binary sample

The sample of spectrally hard sources has been taken from the catalog of HP99. It comprises the sources which have already been identified by HP99 with AGN, quasars and galaxies. This sample is largely the sample which has been used in Paper I for X-ray spectral fitting. A few sources have less than $\sim$100 counts and no X-ray spectra have been fitted for these sources. In addition, sources classified as [hard] by HP99 have been investigated. Source and background spectra have been created and from these data hardness ratios have been determined. These hardness ratios are for the ROSAT PSPC defined as

$$H\!R1 = \frac{(Hard-Soft)}{(Hard+Soft)}$$ (1)

$$H\!R2 = \frac{(Hard2-Hard1)}{(Hard1+Hard2)}\cdot$$ (2)

They are determined from the counts in the standard ROSAT PSPC bands Soft= (channel 11-41, 0.1-0.4 keV), Hard= (channel 52-201, 0.5-2.0 keV), Hard1= (channel 52-90, 0.5-0.9 keV) and Hard2= (channel 91-201, 0.9-2.0 keV). The location of a source in the $H\!R1$ - $H\!R2$ plane has been used for a source classification: In case the source was located in the AGN band $-\Gamma = (2.0{-}3.0)$ it was classified as an AGN. The source has been classified as an X-ray binary in case it was "above'' the AGN band (in the regime of spectra with less steep powerlaw indices). Not all sources classified as [hard] by HP99 are consistent with AGN or X-ray binaries (some sources are extended).

This sample has been extended by taking sources from the HP99 catalog into account which have been selected using criteria on the hardness ratios $H\!R1$ and $H\!R2$ and in addition on the extent likelihood ratio $ML_{\rm ext}$ (the extent likelihood ratio equals $ML_{\rm ext} = -\ln(P)$, with P the probability that the measured photon distribution deviates from the instrument point-spread function). These criteria are: $H\!R1>0.5$, $H\!R2>0.1$, $H\!R2 - \delta H\!R2<0.4$ and $ML_{\rm ext}<30$ (for AGN) and $H\!R1>0.5$, $H\!R2 - \delta H\!R2>0.4$ and $ML_{\rm ext}<30$ (for X-ray binaries). Only sources which have been observed in the inner 20$\arcmin$ of the detector have been used for the analysis. This selection may not be complete as AGN with very low absorbing columns (e.g. located in the outskirts of the LMC) are not necessarily considered. Therefore additional ROSAT PSPC sources have been investigated which were inside the 20$\arcmin$ radius of the PSPC detector and which had at least 100 counts in the broad spectral band. For these sources a source and a background spectrum have been created and from these data hardness ratios have been calculated. Taking the value of the galactic absorbing column and the low LMC absorbing column into account the location of these sources in the $H\!R1$ - $H\!R2$ plane has been used to classify these sources. There were 141 sources which were classified as AGN based on this selection. The X-ray binary sample comprises 30 sources and is discussed in more detail in Sect. 6.