2025A&A...704A.166V

We report the discovery of an extremely high-velocity outflow (EHVO) in the most luminous QSO (L_Bol ∼ 2.29 × 10⁴⁸ erg/s), named SMSS J2157-3602, at z = 4.692. Combined XSHOOTER and NIRES observations reveal that the EHVO reaches a maximum velocity of v_max ∼ 0.13c and persists over rest-frame timescales of a few months up to one year. SMSS J2157-3602 also exhibits one of the highest balnicity index values discovered for an EHVO so far. In addition, the blueshifted CIV emission traces a high-velocity ($ v_\mathrm{{CIV}}^{50}\sim 4660 $ km/s) outflow from the broad-line region (BLR). Thanks to an XMM-Newton observation, we were also able to reveal the X-ray weak nature of this QSO, which likely prevents the overionization of the innermost disk atmosphere and facilitates the efficient launch of the detected EHVO and BLR winds. The extraordinary luminosity of SMSS J2157-3602 and the extreme velocity of the EHVO make it a unique laboratory for testing active galactic nucleus (AGN) driven feedback under extreme conditions. Current uncertainties on the outflow's location and column density strengthen the case for a dedicated follow-up, which will be essential to assess the full feedback potential of this remarkable quasar.