The very high-z GRB 210905A

Abstract

e present the follow-up of the very high-redshift GRB 210905A and its luminous X-ray and optical afterglow. Our analysis covers the prompt and afterglow emission from a few minutes up to 20 Ms after burst. Its afterglow is among the most luminous ever observed, and, in particular, it is the most luminous known in the optical at $t\gtrsim0.5$ d in the rest frame. Its spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. Our analysis reveals the presence of a jet break at $\sim46.2\pm16.3$ d ($6.3\pm2.2$ d rest-frame). This break is very clear in the X-ray light curve but it is hidden in the $H$ band due to a constant contribution from the host galaxy, which is only the fourth but the brightest GRB host at $z>6$ known to date. The half-opening angle of $8.4^\circ\pm1.0^\circ$ is the highest ever measured for a $z\gtrsim6$ burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of $\simeq1\times10^{52}$ erg is also among the highest ever measured and the highest in the \kw{} catalogue. The total jet energy is 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. Finally, we show that the energetics and luminosity of both GRB 210905A and its afterglow are consistent with those of less distant bursts, suggesting that they share the same powering mechanisms and progenitors.