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 Δθ = (8.76 ± 0.11)″. The redshifts of the two quasars are consistent within the errors and measured as z = (1.76 ± 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 ± 1) kpc. For both targets we detected Si iv, C iv, C iii], and Mg ii emission lines. However, the two quasars show significantly different optical colors, one being among the most reddened quasars at z > 1.5 and the other with colors consistent with typical quasar colors 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 colors, having a separation ≲10″, which extends the catalog of known small-separation quasar pairs.