A Candidate Kiloparsec-scale Quasar Pair at z = 5.66

We report the discovery of a close quasar pair candidate at z = 5.66, J2037-4537. J2037-4537 is resolved into two quasar images at the same redshift in ground-based observations. Follow-up spectroscopy shows significant differences in both the continuum slopes and emission line properties of the two images. The two quasar images have a projected separation of 1"24 (7.3 kpc at z = 5.66) and a redshift difference of Δz ≲ 0.01. High-resolution images taken by the Hubble Space Telescope do not detect the foreground lensing galaxy. The observational features of J2037-4537 strongly disfavor the lensing hypothesis. If J2037-4537 is a physical quasar pair, it indicates a quasar clustering signal of ~10⁵ at a separation of ~10 proper kpc (pkpc), and gives the first observational constraint on the pair fraction of z > 5 quasars, f_pair(r < 30 pkpc) > 0.3%. The properties of J2037-4537 are consistent with those of merger-triggered quasar pairs in hydrodynamical simulations of galaxy mergers.