2020ApJ...889...52P

In the last two decades, approximately 200 quasars have been discovered at z> 6, hosting active supermassive black holes with masses M_•≳10⁹M_☉. 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> 6 black hole that was inferred thus far (J0100+2802, M_•~1.2×10¹⁰M_☉) was recently claimed to be lensed, with a magnification factor µ=450. Here, we perform a consistency check of this claim, finding that the detection of such a source requires a bright-end slope β≥ 3.7 for the intrinsic quasar luminosity function (LF), Φ(L)∝L^-β^. Commonly used values of β∼2.8 are rejected at > 3σ . 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 ≳25 % of the remaining sources are lensed for the intrinsic LF to differ significantly (i.e., > 3σ ) from the observed one. The presence of additional extremely magnified sources in the sample would lower the requirement to ∼4 % . Our results urge the community to perform more extended multiwavelength searches targeting z> 6 lensed quasars, also among known samples. This effort could vitally contribute to solving the open problem of the growth of the brightest z∼7 quasars.