The EPIC sensitivity limits depend on the sky area, i.e. the true X-ray background and on the `space weather' as described in §§ 184.108.40.206 and 220.127.116.11.
As the sensitivity limits also depend on the angular structure and the spectral characteristics of the source that is observed, it is strongly recommended to use simulations to get an estimate of the signal to noise which can be achieved with a certain instrument setup and exposure time.
Currently the best statistical results on the EPIC sensitivity limits
are based on the Lockman Hole data (Hasinger et al., 2001, A&A 365, L45; Brunner et al. 2008, A&A, 479, 283).
In Table 4 the sensitivity limits in the ``soft'',
``hard'', and ``very hard'' energy band are reported
|``Soft'' (0.5-2 keV)||0.19|
|``Hard'' (2-10 keV)||0.9|
|``Very hard'' (5-10 keV)||1.8|
Watson et al., 2001, A&A, 365, L51 used the nominal quiescent background values together with the measured XMM-Newton PSF to compute an EPIC point source sensitivity based on a simple 5 source detection criterion against assumed purely Poissonian background fluctuations, as shown in Fig. 373. These estimates are in reasonable agreement with the flux distribution of sources in the EPIC Serendipitous Source Catalogue (Watson et al., 2009, A&A, 493, 339).
The actual background in an observation depends critically on the fraction of background flares removed, i.e. the trade off between net background levels and net exposure time.
At very faint fluxes the effective sensitivity is potentially limited by confusion effects. However, recent estimates indicate that the confusion limit in the hard X-ray band (2-10 keV) is only reached after 2 Ms of observing time and will probably never be reachable in the 5-10 keV band.