XMM-Newton Users Handbook

3.3.8 EPIC's sensitivity limits

The EPIC sensitivity limits depend on the sky area, i.e. the true X-ray background and on the `space weather' as described in §§ 3.3.7.1 and 3.3.7.2.

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 a feeling on 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

Table 4: Detection limits (i.e., minimum detectable flux at 4$\sigma$ in units of $10^{-15}$ erg cm$^{-2}$ s$^{-1}$) for different energy bands in the 1.16 Ms observation of the Lockman Hole (Hasinger et al., 2001, A&A 365, L45; Brunner et al. 2008, A&A, 479, 283).

Band	Lockman Hole
``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$\sigma$ source detection criterion against assumed purely Poissonian background fluctuations, as shown in Fig. 37⁴.

Figure 37: EPIC sensitivity (5$\sigma$ minimum detectable flux in erg cm$^{-2}$ s$^{-1}$ in respective bands) as a function of exposure time (from Watson et al., 2001). Sensitivity is computed for an assumed $\alpha = 1.7$ powerlaw spectrum with a column density $N_H = 3\times 10^{20}$ cm$^{-2}$. Solid curves are for the nominal background rates. Dashed curves are for background levels enhanced by a factor 3. The EPIC MOS curves correspond to the combination of the two cameras.

Empirical data from analysis of several XMM-Newton fields using the source detection software in the SAS are broadly consistent with these plots. 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. An investigation of a few example fields demonstrates that the effective sensitivity of typical observations is within a factor 2 of the values plotted in Fig. 37. A few observations are affected by enhanced background throughout; here the average background can be several times higher than the nominal values even after the removal of the largest flares.

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.