Reality or Mirage? Observational Test and Implications for the Claimed Extremely Magnified Quasar at z = 6.3

In the last two decades, approximately 200 quasars have been discovered at $z\gt 6$ , hosting active supermassive black holes with masses ${M}_{\bullet }\gtrsim {10}^{9}\,{M}_{\odot }$ . While these sources reflect only the tip of the iceberg of the black hole mass distribution, their detection challenges standard growth models. The most massive $z\gt 6$ black hole that was inferred thus far (J0100+2802, ${M}_{\bullet }\approx 1.2\times {10}^{10}\,{M}_{\odot }$ ) was recently claimed to be lensed, with a magnification factor $\mu =450$ . Here, we perform a consistency check of this claim, finding that the detection of such a source requires a bright-end slope $\beta \geqslant 3.7$ for the intrinsic quasar luminosity function (LF), ${\rm{\Phi }}(L)\propto {L}^{-\beta }$ . Commonly used values of $\beta \sim 2.8$ are rejected at $\gt 3\sigma $ . If the claim is confirmed, it is very unlikely that all the remaining 51 sources in the Sloan Digital Sky Survey sample are not magnified. Furthermore, it suffices that $\gtrsim 25 \% $ of the remaining sources are lensed for the intrinsic LF to differ significantly (I.e., $\gt 3\sigma $ ) from the observed one. The presence of additional extremely magnified sources in the sample would lower the requirement to $\sim 4 \% $ . Our results urge the community to perform more extended multiwavelength searches targeting $z\gt 6$ lensed quasars, also among known samples. This effort could vitally contribute to solving the open problem of the growth of the brightest $z\sim 7$ quasars.