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Truth or delusion? A possible gravitational lensing interpretation of the ultra-luminous quasar SDSS J010013.02+280225.8 at $z=6.30$

Gravitational lensing sometimes dominates the observed properties of apparently very bright objects. We present morphological properties in the high-resolution (FWHM $\sim0.''15$) Atacama Large Millimeter/submillimeter Array (ALMA) 1-mm map for an ultra-luminous quasar (QSO) at $z=6.30$, SDSS J010013.02+280225.8 (hereafter J0100+2802), whose black hole mass $M_{\rm BH}$ is the most massive ($\sim$ 1.2$\times10^{10}M_{\odot}$) at $z>6$ ever known. We find that the continuum emission of J0100+2802 is resolved into a quadruple system within a radius of $0.''2$, which can be interpreted as either multiple dusty star-forming regions in the host galaxy or multiple images due to strong gravitational lensing. The Mg$_{\rm II}$ absorption and the potential Ly$α$ line features have been identified at $z=2.33$ in the near-infrared spectroscopy towards J0100+2802, and a simple mass model fitting well reproduces the positions and flux densities of the quadruple system, both of which are consistent with the latter interpretation. Although a high-resolution map taken in the Advanced Camera for Survey (ACS) on board Hubble Space Telescope (HST) shows a morphology with an apparently single component, in our fiducial lens mass model it can simply be explained by a $\sim50$ pc scale offset between the ALMA and HST emission regions. In this case, the magnification factor for the observed HST emission is obtained to $\sim450$, reducing the intrinsic $M_{\rm BH}$ estimate to even below $10^{9}\,M_{\odot}$. The confirmation or the rejection of the gravitational lensing scenario is important for our understanding of the super-massive black holes in the early Universe.