2020ApJ...904L..32D

Since their discovery 20 years ago, the observed luminosity function of z >= 6 quasars has been suspected to be biased by gravitational lensing. Apart from the recent discovery of UHS J0439+1634 at z ≃ 6.52, no other strongly lensed z >= 6 quasar has been conclusively identified. The hyperluminous z ≃ 6.33 quasar SDSS J0100+2802, believed to host a supermassive black hole of ∼10¹⁰ M_☉, has recently been claimed to be lensed by a factor of ∼450, which would negate both its extreme luminosity and black hole mass. However, its Lyα-transparent proximity zone is the largest known at z > 6, suggesting an intrinsically extreme ionizing luminosity. Here we show that the lensing hypothesis of z >= 6 quasars can be quantitatively constrained by their proximity zones. We first show that our proximity zone analysis can recover the strongly lensed nature of UHS J0439+1634, with an estimated magnification µ=28.0_–11.7^+18.4(_–18.3^+44.9) at 68% (95%) credibility that is consistent with previously published lensing models. We then show that the large proximity zone of SDSS J0100+2802 rules out lensing magnifications of µ > 4.9 at 95% probability, and conclusively rule out the proposed µ > 100 scenario. Future proximity zone analyses of existing z >= 6 quasar samples have the potential to identify promising strongly lensed candidates, constrain the distribution of z >= 6 quasar lensing, and improve our knowledge of the shape of the intrinsic quasar luminosity function.