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

Abstract

We present a detailed follow-up of the very energetic GRB 210905A at a high redshift ofz = 6.312 and its luminous X-ray and optical afterglow. Following the detection bySwiftand 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 ofEiso= 1.27−0.19+0.20× 1054erg, GRB 210905A lies in the top ∼7% of gamma-ray bursts (GRBs) in the Konus-Windcatalogue 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 att ≳ 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 ∼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 theHband 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 atz > 6 known to date. By assuming a number densityn = 1 cm−3and an efficiencyη = 0.2, we derived a half-opening angle of 8.4 ° ±1.0°, which is the highest ever measured for az ≳ 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of ≃1 × 1052erg 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 GRB 210905A and its afterglow, we demonstrate that they are consistent within 2σwith those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.