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

A. Rossi

;D. D. Frederiks;D. A. Kann;M. De Pasquale;E. Pian;G. Lamb;P. D???Avanzo;L. Izzo;A. J. Levan;D. B. Malesani;A. Melandri;A. Nicuesa Guelbenzu;S. Schulze;R. Strausbaugh;N. R. Tanvir;L. Amati;S. Campana;A. Cucchiara;G. Ghirlanda;M. Della Valle;S. Klose;R. Salvaterra;R. L. C. Starling;G. Stratta;A. Tsvetkova;S. D. Vergani;A. D???A??;D. Burgarella;S. Covino;V. D???Elia;A. de Ugarte Postigo;H. Fausey;J. P. U. Fynbo;F. Frontera;C. Guidorzi;K. E. Heintz;N. Masetti;E. Maiorano;C. G. Mundell;S. R. Oates;M. J. Page;E. Palazzi;J. Palmerio;G. Pugliese;A. Rau;A. Saccardi;B. Sbarufatti;D. S. Svinkin;G. Tagliaferri;A. J. van der Horst;D. J. Watson;M. V. Ulanov;K. Wiersema;D. Xu;J. Zhang

2022-01-01

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Abstract

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.