A blast from the infant Universe: The very high- z GRB 210905A

We present a detailed follow-up of the very energetic GRB210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. Following the detection by Swift and Konus-Wind, we obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of E-iso = 1.27(-0.19)(+0.20) x 10(54) erg, GRB210905A lies in the top similar to 7% of gamma-ray bursts (GRBs) in the Konus-Wind catalogue in terms of energy released. Its afterglow is among the most luminous ever observed, and, in particular, it is one of the most luminous in the optical at t >= 0.5 d in the rest frame. The afterglow starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at similar to 46.2 +/- 16.3 d (6.3 +/- 2.2 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to a constant contribution from the host galaxy and potentially from a foreground intervening galaxy. In particular, the host galaxy is only the fourth GRB host at z > 6 known to date. By assuming a number density n = 1 cm(-3) and an efficiency eta = 0.2, we derived a half-opening angle of 8.4 degrees +/- 1.0 degrees, which is the highest ever measured for a z greater than or similar to 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of similar or equal to 1 x 10(52) erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB210905A and its afterglow, we demonstrate that they are consistent within 2 sigma with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.