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

Rossi, A. and Frederiks, D.D. and Kann, D.A. and De Pasquale, M. and Pian, E. and Lamb, G. and D'Avanzo, P. and Izzo, L. and Levan, A.J. and Malesani, D.B. and Melandri, A. and Nicuesa Guelbenzu, A. and Schulze, S. and Strausbaugh, R. and Tanvir, N.R. and Amati, L. and Campana, S. and Cucchiara, A. and Ghirlanda, G. and Della Valle, M. and Klose, S. and Salvaterra, R. and Starling, R.L.C. and Stratta, G. and Tsvetkova, A.E. and Vergani, S.D. and D'Aì, A. and Burgarella, D. and Covino, S. and D'Elia, V. and Postigo, A.D.U. and Fausey, H. and Fynbo, J.P.U. and Frontera, F. and Guidorzi, C. and Heintz, K.E. and Masetti, N. and Maiorano, E. and Mundell, C.G. and Oates, S.R. and Page, M.J. and Palazzi, E. and Palmerio, J. and Pugliese, G. and Rau, A. and Saccardi, A. and Sbarufatti, B. and Svinkin, D.S. and Tagliaferri, G. and Van Der Horst, A.J. and Watson, D.J. and Ulanov, M.V. and Wiersema, K. and Xu, D. and Zhang, J. (2022) A blast from the infant Universe: The very high- z GRB 210905A. Astronomy and Astrophysics, 665. ISSN 1432-0746

Abstract

We present a detailed follow-up of the very energetic GRB 210905A 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 Eiso = 1.270.19+0.20- 1054 erg, GRB 210905A lies in the top 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 t0.5 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 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 cm3 and an efficiency η=0.2, we derived a half-opening angle of 8.4±1.0, which is the highest ever measured for a z6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1- 1052 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 GRB 210905A and its afterglow, we demonstrate that they are consistent within 2 with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift. © 2022 EDP Sciences. All rights reserved.