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