SIMBAD references

We report on an optical (SDSS) and X-ray (XMM-Newton) 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 (0.3-10keV); seven objects are optically identified as Seyfert, while one source, optically classified as a low-ionization nuclear emission-line region (LINER), is likely powered by accretion in virtue of its relatively high X-ray luminosity (1.2 x 10⁴¹ erg s^–1). Six of the eight objects are obscured in X-rays with N_H >= 10²³ cm^–2; three of these, whose X-ray spectrum is dominated by a reflection component, are likely Compton-thick (N_H >= 10²⁴ 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_H >= 10²² cm^–2) AGN fraction within the larger sample is 84 ± 4 per cent (taking into account the 90 per cent error on the N_H measure) up to large pair separations (∼100 kpc). This is statistically higher than the fraction of obscured AGN in isolated galaxies found in X-ray surveys.