The faintest sources that the XMM serendipitous survey will contain (with X-ray fluxes ~ 10-15erg cm-2 s-1) are expected to have optical magnitudes R ~24-25.
From soft X-ray surveys carried out with ROSAT we would expect the magnitude of unobscured objects at that X-ray flux level to be around 23-24 (see graph 1), but the fact that XMM is sensitive to hard X-rays means that a significant fraction of the objects will be obscured and optically fainter.
In practice this means that the spectroscopic identification of the faintest sources will require the use of optical and infrared spectrographs on 8-10m class telescopes.
Figure 1: Soft X-ray flux versus R magnitude for the ROSAT Lockman Hole
The ROSAT Lockman Hole survey also shows that fainter X-ray sources are redder, even when selected at soft X-ray energies (see graph 2). This is consistent with greater extinction along the line of sight (Hasinger et al 1999).
Figure 2: Colours of the optical counterparts of the ROSAT Lockman Hole
deep survey sources.
However this is only one part of the serendipitous sample that XMM will provide. Currently the faintest measured sources in hard X-rays have 2-10 keV fluxes higher than 10-14erg cm-2 s-1 (Georgantopoulos et al 1997) and only a handful of are identified. The situation is little better at fluxes ~10-13 erg cm-2 s-1, with less than 100 sources identified so far. Therefore there is a wide range in flux to be explored with the use of the XMM serendipitous survey, a large fraction of which relates to sources with optical magnitudes appropriate for 2m and 4m class telescopes.
10th March 2000
Text based on AXIS proposal by X.Barcons and the SSC consortium.
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