7 Conclusions

In the present paper, we discussed the properties of the natural photometric system of the Optical Monitor onboard the X-ray Multi-Mirror satellite. On the basis of numerical simulations, we investigated its transformability to the standard Johnson $U\!BV$system. We gave the main transformation equations both for individual filters and for colour indices. On the basis of the same technique of simulations, we showed that, for stars with effective temperatures higher than 9000 K observed with this system, it is possible to determine the temperature and the reddening (due to interstellar extinction) independently, as well as to detect non-standard reddening laws on given lines of sight. Finally, we made a detailed study of the possibilities to select quasar candidates on the basis of their location in the multicolour space definable from this particular photometric system. In particular, the interest of the use of the UV filters has been critically evaluated. The main conclusion of this analysis is that the use of the uvw1 and uvw2 filters allows to greatly enhance the non-degenerate stars - quasars discrimination in the [0.8;2.1] redshift range. For lower redshifts, the discrimination remains reasonably efficient. The advantage to make use of the XMM-OM photometric system is nevertheless less clear for what concerns the higher redshifts ($z \ge 2.3$) and the discrimination between the quasars and the degenerate stars. The quasar candidate selection could benefit from the addition of criterions based on the X/optical flux ratio, as these are usually larger for QSO/AGN than for stars (Stocke et al. 1991; Schmidt et al. 1998). The full treatment of the quasar candidate selection based on XMM observations is beyond the scope of the present paper, dedicated to the properties of the XMM-OM photometric system, and will be discussed elsewhere.

Acknowledgements

The authors are greatly indebted to D. Koester for the unpublished degenerate star models he kindly provided and to W. Zheng and P. Francis for a computer readable version of their composite quasar spectra. The authors also want to express their thanks to the Fonds National de la Recherche Scientifique (Belgium) for multiple supports. This research is supported in part by contract ARC94/99-178 "Action de recherche concertée de la Communauté Française'' (Belgium). Partial support through the PRODEX XMM-OM Project is also gratefully acknowledged.