Disclosing the properties of low-redshift dual AGN through XMM-Newton
  and SDSS spectroscopy
  We report on an optical (SDSS) and X-ray (XMM) study of an optically selected
sample of four dual AGN systems at projected separations of 30--60~kpc. All
sources are detected in the X-ray band (0.3-10 keV); seven objects are
optically identified as Seyfert, while one source, optically classified as a
LINER, is likely powered by accretion in virtue of its relatively high X-ray
luminosity (1.2$\times10^{41}$ erg/s). Six of the eight objects are obscured in
X-rays with N$_{\rm H} \geq$ 10$^{23}$ cm$^{-2}$; three of these, whose X-ray
spectrum is dominated by a reflection component, are likely Compton-thick
(N$_{\rm H} \geq$ 10$^{24}$ cm$^{-2}$). This finding is in agreement with the
hypothesis that galaxy encounters are effective in driving gas inflow toward
the nuclear region, thus increasing the obscuration. We compare the absorption
properties in our dual AGN with those in larger samples observed in X-rays but
selected in different ways (optical, IR and hard X-rays). We find that the
obscured (N$_{\rm H} \geq$ 10$^{22}$ cm$^{-2}$) AGN fraction within the larger
sample is 84$\pm$4 per cent (taking into account the 90 per cent error on the
N$_{\rm H}$ measure) up to large pair separations ($\sim$100~kpc). This is
statistically higher than the fraction of obscured AGN in isolated galaxies
found in X-ray surveys.
