Hard X-ray surveys are particularly well suited for the discovery of the
most extreme blazars, i.e. those displaying the most powerful jets, the
most luminous accretion disks and the largest black hole masses. These
extreme characteristics are a direct consequence of the spectral energy
distribution (SED) for these powerful AGN, showing a Compton peak in the
hard X-ray/soft gamma-ray range, i.e. in the energy range where instruments
like INTEGRAL/IBIS show their optimum performance.
Until today, 9 blazars with a redshift z > 2 have been detected by
INTEGRAL/IBIS and 3 of these are above z = 3. The most recent discovery is
that of IGR J12319-0749 reported to be a flat spectrum radio quasar at
z = 3.1. This source harbours a super- massive black hole of 2.8 x 109
solar masses, and an accretion disk with a luminosity close to or above
10% of the Eddington limit. Figures 1a and 1b show the source optical
spectrum (top left), and the non-simultaneous rest frame SED (top right),
respectively.
Another 8 sources have been reported by Swift/BAT increasing the sample
of hard X-ray selected blazars at high redshift (z > 2) to 17. Figure 2
compares this BAT/IBIS sample with a similar one detected by Fermi: it
is evident that observations in hard X-rays allowed the detection of
objects at high redshifts (Figure 2a) and with large black hole masses
(Figure 2b) with the same or even better capability than Fermi.
As the exploration of the hard X-ray sky will continue, we expect that
more high redshift blazars like IGR J12319-0749 will be discovered and
studied in great detail.
