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While theoretical arguments predict that most of the early growth ofsupermassive black holes (SMBHs) happened during heavily obscured phases ofaccretion, current methods used for selecting $z>6$ quasars (QSOs) are stronglybiased against obscured QSOs, thus considerably limiting our understanding ofaccreting SMBHs during the first Gyr of the Universe from an observationalpoint of view. We report the $Chandra$ discovery of the first heavily obscuredQSO candidate in the early universe, hosted by a close ($\approx5$ kpc) galaxypair at $z=6.515$. One of the members is an optically classified type 1 QSO,PSO167-13. The companion galaxy was first detected as a [C II] emitter by ALMA.An X-ray source is significantly ($P=0.9996$) detected by $Chandra$ in the 2-5keV band, with $<1.14$ net counts in the 0.5-2 keV band, although the currentpositional uncertainty does not allow a conclusive association with eitherPSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratioarguments, we estimated an obscuring column density of$N_H>2\times10^{24}\,\mathrm{cm^{-2}}$ and$N_H>6\times10^{23}\,\mathrm{cm^{-2}}$ at $68\%$ and $90\%$ confidence levels,respectively. Thus, regardless of which of the two galaxies is associated withthe X-ray emission, this source is the first heavily obscured QSO candidate at$z>6$.

Discovery of the first heavily obscured QSO candidate at $z>6$ in a close galaxy pair