4. Classification of the X-ray sources
The X-ray colors (hardness ratios and ) as given in Table 6 can be used for a source classification. Kahabka et al. (1999, hereafter KPFH99) have made a classification of the ROSAT PSPC X-ray sources in the field of the Small Magellanic Cloud (SMC). X-ray binaries have on average harder X-ray spectra than supernova remnants and background AGN. The range of hardness ratios and is then different. In addition absorption due to intervening cold gas affects the X-ray colors. KPFH99 have derived the dependence of and on the absorbing column density for a low metallicity galaxy like the SMC. The hydrogen column density in the direction of the X-ray source has been derived from the high-resolution 21-cm map of Stanimirovic et al. (1999). Then from the measured X-ray color a classification could be obtained. The strength of this method is that X-ray binaries and AGN cover different parts of the diagram for intervening hydrogen columns . AGN are located in a band in this diagram with radio loud AGN populating the upper regime of the band while radio quiet AGN occupy the lower part of the band (cf. Laor et al. 1997; Brinkmann et al. 1997).
For radio loud AGN spectra (powerlaw photon index -2.0) the dependence of the X-ray colors and on the hydrogen column density () for abundances 0.2 solar (SMC abundances) has been derived from simulations as
and for radio quiet AGN spectra (powerlaw photon index -2.6)
For X-ray binaries with powerlaw photon index -0.8 spectra the dependence of the X-ray colors and on the hydrogen column density is
Eqs. 3 to 8 can be solved for the hydrogen column density . In combining the equations for and one can derive analytical solutions of X-ray binary and AGN tracks in the - plane.
For radio loud AGN (powerlaw photon index -2.0) one derives the track
and for radio quiet AGN (powerlaw photon index -2.6) the track
For X-ray binaries (powerlaw photon index -0.8) we derive the track
These tracks allow a classification of a ROSAT source as an AGN (or an X-ray binary) without the requirement that the intervening hydrogen column density is known. It even is possible to constrain in the - plane the intervening hydrogen column density for the source. We note that X-ray binaries are not necessarily seen through the total gas column of the galaxy disk. With a galactic foreground column of towards NGC 3109 we derive with Eq. 11 for X-ray binaries a lower bound on of .
The tracks given above have been derived for a metallicity 0.2 solar which is consistent with the metallicity derived for HII regions in NGC 3109 (cf. Minniti et al. 1999). Higher metallicity tracks deviate somewhat from these tracks especially if one approaches the high column density regime .
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000