Serendipitous discovery of a physical binary quasar at z=1.76
  Binary quasars are extremely rare objects, used to investigate clustering on
very small scales at different redshifts. The cases where the two quasar
components are gravitationally bound, known as physical binary quasars, can
also exhibit enhanced astrophysical activity and therefore are of particular
scientific interest. Here we present the serendipitous discovery of a physical
pair of quasars with an angular separation of $\Delta\theta = (8.76 \pm 0.11)$
arcsec. The redshifts of the two quasars are consistent within the errors and
measured as $z = (1.76\pm 0.01)$. Under the motivated assumption that the pair
does not arise from a single gravitationally lensed quasar, the resulting
projected physical separation was estimated as $(76 \pm 1)$ kpc. For both
targets we detected Si VI, C VI, C III], and Mg II emission lines. However, the
two quasars show significantly different optical colours, one being among the
most reddened quasars at $z > 1.5$ and the other with colours consistent with
typical quasar colours at the same redshift. Therefore it is ruled out that the
sources are a lensed system. This is our second serendipitous discovery of a
pair of two quasars with different colours, having a separation $\lesssim 10$
arcsec, which extends the very limited catalogue of known quasar pairs. We
ultimately argue that the number of binary quasars may have been significantly
underestimated in previous photometric surveys, due to the bias arising from
paired quasars with very different colours.
