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 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 +0.20 −0.19 × 10 54 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 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 ∼ 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 η = 0.2, we derived a half-opening angle of 8.4 • ± 1.0 • , which is the highest ever measured for a z 6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1 × 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 GRB 210905A and its afterglow, we demonstrate that they are consistent within 2σ with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.

The Maximum Isotropic Equivalent Energy of Gamma-Ray Bursts

“…Consequently, the entire population of GRBs, which is a mixture of near-axis and far off-axis GRBs, is expected to satisfy the Amati correlation with a mean power index 1/2(1/ 2 + 1/3) ≈ 0.42, in agreement with that reported for the entire population of GRBs (nearand far off-axis) observed so far with secured redshift, E p , and E iso values (e.g., Tsvetkova et al 2021;Rossi et al 2022).…”

GRB 221009A: The BOAT

“…At a redshift of 10 this would be observed with an E peak of 100-300 keV. Other GRBs detected beyond z ≈ 6 show similar observed spectral hardness, e.g., the recent GRB 210905A at z ∼ 6.3 and with peak energy of 145 keV (Rossi et al 2022a). Thus, a population of GRBs with peak observed energies at Earth in the hundreds of keV should exist, which should be accounted for in the design of high-z GRB missions (Amati et al 2021;White et al 2021).…”

“…This may suggest GRB 221009A as intermediate between typical long and ultralong GRBs, providing some support for a single continuum. There are additional GRBs that may fall into a similar range, including GRB 840304 (discussed next) and GRB 210905A with a duration of 870 s at a redshift of z = 6.3 (Rossi et al 2022a).…”

The very high-z GRB 210905A

“…Knowing the value of the jet opening angle is crucially important because it enables us to estimate the 'true', collimation-corrected, energetics of the outflow [21,22]. In [7], following [23], we find an angle 𝜃 jet = 0.147 ± 0.017 rad, or 8.41 ± 0.97 degrees, using the isotropic energy 𝐸 𝛾,iso = 12.7 × 10 53 found in our analysis of the Konus-Wind and Swift/BAT data [7]. This value lies in the top ∼7% for the KW sample of 338 GRBs with known redshifts [24,25].…”

“…In the following we highlight the results presented in [7], where the follow-up of the bright GRB 210905A is presented in full fashion. This is the tenth burst with redshift 𝑧 ≳ 6 detected in the last 16 years (considering the photometric redshift cases too).…”