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INTEGRAL/IBIS and Swift/BAT will likely be able to expose a significant area of the extragalactic sky down to the 0.5 mCrab 4 flux level. BAT will achieve it thanks to its continuos all-sky coverage, while INTEGRAL will do it through its Key Programme observations (e.g. see the 6 Ms observation of the North Ecliptic Pole). Simbol-X, NuSTAR and NeXT ([65, 66, 67], respectively), all focusing hard X-ray telescopes, will be able to sample fluxes of 10-13 - 10-14 erg cm-2 s-1. Until that time, no other scheduled mission 5 will probe intermediate fluxes.

There is however a way to sample fluxes weaker than the ones sampled by the INTEGRAL/IBIS and Swift/BAT surveys and this is represented by the Swift+INTEGRAL X-ray (SIX) survey (Ajello et al. in prep.). The SIX survey is obtained as a natural combination of both surveys. Indeed, as it is shown in Tab. 2, the performances of both instruments, BAT and IBIS, are similar in terms of sensitivity for deep extragalactic exposures. The main difference is represented by the point spread function (PSF) which is better for INTEGRAL. However, as long as one avoids crowded regions (i.e. the Galactic plane) this issue does not matter. The advantage of the SIX survey is not only that it allows to combine the exposures to obtain a deeper one, but that it smoothes out the relative systematic errors that both instruments have. As a test field to demonstrate the potentiality of the SIX survey we chose the North Ecliptic Pole (NEP) field. This field has been proposed and approved as an INTEGRAL key-programme because of the absence of bright sources which could worsen the sensitivity. At the moment of this writing, the NEP field has been surveyed to 1.7 Ms (of the 6 Ms requested). Using two years of Swift/BAT survey data [69] we get an exposure on the same region of ~ 4 Ms. Left panel of Fig. 6 shows the difference between the BAT and the IBIS surveys. Indeed, while IBIS reaches a deeper sensitivity at the centre of the mosaic, the BAT survey has shallower, but much more uniform exposure on the whole area. The SIX survey joins the best of both worlds yielding already in this simple test a sensitivity better than 0.5 mCrab. As expected by the combination of two surveys with different systematic errors, the noise is well behaved and the distribution of signal-to-noise ratios is consistent with a normal Gaussian distribution (see right panel of Fig. 6). The number of detected sources above 5sigma is 18 which is a large number considering that it has been detected in an INTEGRAL mosaic ( ~ 0.5 sr) away from the Galactic plane. This test shows the potentiality of this approach fully as it has already produced the most sensitive hard X-ray survey to date. A much better sensitivity is, reasonably, expected when the NEP field will be surveyed by INTEGRAL to the planned 6 Ms.

Table 2. Comparison of the BAT and ISGRI instruments. Sensitivities are real in-flight performances and were derived for BAT and ISGRI by Ajello et al. (2008a) and Bassani et al. (2006) respectively.


PSF (arcmin) 22 12
FOV (deg2) 4500 400
Energy range (keV) 15-200 16-300
5sigma Sensitivity in 1 Ms (mCrab) 0.9 0.8

Figure 6

Figure 6. Performances of the SIX survey on a test field. The field chosen is the North Ecliptic Pole (INTEGRAL Key programme) surveyed by INTEGRAL to 1.7 Ms. The left panel shows the BAT (black), IBIS (blue) and SIX (red) sky coverages. The limiting sensitivity of the SIX mosaic, in this example, is better than 0.5 mCrab. The right panel shows the distribution of pixel significances in the SIX-NEP mosaic. The dashed line is an overlaid Gaussian with sigma = 1.0. The long tail at positive significances is given by real sources detected in the mosaic.

4 As a reference 1 mCrab in the 15-55 keV band is equivalent to ~ 1.27 × 10-11 erg cm-2 s-1. Back.

5 The EXIST mission under study by NASA would probe the 5 × 10-13 erg cm-2 s-1 fluxes [68]. Back.

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