1 Introduction

Thanks to its unprecedented large X-ray collecting power, the XMM satellite (for X-ray Multi-Mirror) is expected to discover a wealth of new X-ray sources. In order to allow a quick and reliable identification of the sources, as well as to enable multiwavelength monitoring of their variability, the satellite is equipped with an optical complement known as the optical monitor, XMM-OM. This particular element is co-aligned with the three X-ray telescopes and consists of a 30 cm Ritchey-Chrétien telescope coupled with a photon-counting detector. The latter consists of a photocathode followed by three micro-channel plates as intensifier and by a tapered fiber-optic bundle connected to a fast scanning CCD. The usable area is made of 256 by 256 physical pixels but a centroiding process locates the events to 1/8 of a pixel thus mimicking a 2048 by 2048 device. The field of view is 17 arcmin by 17 arcmin with a centroided pixel size of 0.5 arcsec. At the end of the OM exposure, the cumulated image is downloaded to the ground. An engineering mode allows to transmit the whole image but the routine science mode necessitates predefined windowing and/or binning. The telescope was designed so that the limiting magnitude would be no less than 24 when working in unfiltered light. More details can be found in Fordham et al. (1992) and Mason et al. (1996).

Figure 1: a) XMM-OM's photometric system. The upper curve is the instrumental response in the absence of filter. It results from the product of the response curves of the detector, of the three mirrors and of the detector window. b) Comparison of the XMM-OM optical and the Johnson U, B and V filters. The pairs of filters are, from left to right: u and U; b and B; v and V. The dashed curves are the Johnson filters

XMM-OM holds a 6-filter, UV and optical, photometric system. Three bands of the system were designed to match the Johnson's $U\!BV$ system (Johnson 1955; Bessel 1990). The rectangular profiles of the latter filters render the colour transformations between the XMM-OM and the Johnson systems quite complicated (see Royer & Manfroid 1996 for a discussion on the untransformability of rectangularly shaped filters), or even not always possible. Such colour transformations will nevertheless remain necessary for those who will have to compare XMM-OM observations with ground-based ones. In this paper, we provide the reader with theoretical estimations of these colour transformations.

The XMM-OM filter set comprises three non-standard filters, exploring a wavelength domain (UV) where no extensive observations have been performed to date. Combined with the poor match between the XMM-OM optical filters and the classical ground-based standard photometric systems, this fact enhances the importance of testing our ability to determine some physical properties of the observed stars directly in the natural XMM-OM colour system. In the present paper, we will show that it is possible to estimate both the temperature and the amount of interstellar absorption (or "reddening'') for the hot stars that will be observed with XMM-OM.

   

Table 1: Main characteristics of the XMM-OM filters. The wavelengths are expressed in Å. $\lambda _0$ is the effective wavelength and $\lambda _{\rm max}$ is the position of the maximal transmission

filter	$\lambda _0$	$\lambda _{\rm max}$	FWHM
uvw2	2070	2000	$\sim\,500$
uvm2	2298	2210	439
uvw1	2905	2680	620
u	3472	3270	810
b	4334	3980	976
v	5407	5230	684

The natural XMM-OM colour system can also be used for other investigations. For example, we have explored the possibilities to discriminate quasars from stars in multidimensional colour spaces based on the XMM-OM photometry. Nevertheless, the goal of this paper is not so much to provide the reader with exact analytical relations for these matters as to give qualitative results that will tell him how to observe and treat the data in order to get the best outcome, and what kind of results are to be expected.

In Sect. 2, we present the tools we used for the synthetic photometry. In Sect. 3, we discuss the colour transformations between the XMM-OM optical filters and the Johnson U, B, V system. We treat the temperature and the interstellar absorption determination in Sects. 4 and 5 whereas the selection of quasar candidates is addressed in Sect. 6. Section 7 outlines the main conclusions of our study.