A tera–electron volt afterglow from a narrow jet in an extremely bright gamma-ray burst

Abstract

Some gamma-ray bursts (GRBs) have a tera–electron volt (TeV) afterglow, but the early onset of this has not been observed. We report observations with the Large High Altitude Air Shower Observatory of the bright GRB 221009A, which serendipitously occurred within the instrument field of view. More than 64,000 photons >0.2 TeV were detected within the first 3000 seconds. The TeV flux began several minutes after the GRB trigger, then rose to a peak about 10 seconds later. This was followed by a decay phase, which became more rapid ~650 seconds after the peak. We interpret the emission using a model of a relativistic jet with half-opening angle ~0.8°. This is consistent with the core of a structured jet and could explain the high isotropic energy of this GRB.

Constraining the Jet Composition of GRB 221009A with the Prompt TeV Emission Limit

“…in this scenario, we assume that the Lorentz factor of the shells increases with time regularly, ranging from 200 to 800, while the masses of each shell are the same (see Case II in panels (a) and (b) of Figure 1). This approach results in a much smoother synthetic light curve (see Figure 5 in the Appendix), matching better the observations of GRB 221009A (An et al 2023;Lesage et al 2023).…”

“…The light curve of GRB 221009A shows a rising phase from about T 0 + 220 s to T 0 + 227 s (An et al 2023;Lesage et al 2023). To reproduce this rising light curve, we explore two scenarios:…”

“…For the spectrum, we use the Band function (α = − 0.75 and β = − 2.2) for all pulses (An et al 2023;Lesage et al 2023; we chose α = − 0.75 as the observed slope; however, the discussion of the internal shock model's capacity to reproduce this slope is not addressed here). The peak energy E p of the Band function for each pulse is determined by the L γ -E p relationship: L N E 10 erg s 1 MeV Yonetoku et al 2004), where L γ is the isotropic luminosity and we assume the normalization factor N 0 = 1.…”

“…We set the initial distance d = cΔt c between shells to 5l. We fix the duration ΔT of the central engine to 10 s. Liu et al (2023) find that, during the early rising phase (T 0 + 210 s to T 0 + 219 s) of the main burst emission, at which the detector is not saturated, the minimum variability time is Δt v = 0.082 s. Lesage et al (2023) find that the minimum variability time is about 0.2 s during the time right before and after the main burst (see Figure 1 in Lesage et al 2023). We thus consider two cases of variability time of the central engine: Δt c = 82 ms and Δt c = 200 ms.…”

Constraints on the Intergalactic Magnetic Field Strength from γ-Ray Observations of GRB 221009A

“…At 13:16:59.99 UT (T 0 ) on 2022 October 9, the Fermi Gamma-ray Burst Monitor triggered and located GRB 221009A (Veres et al 2022;Lesage et al 2023), which was also detected by the Fermi Large Area Telescope (Fermi-LAT; Bissaldi et al 2022), Konus-Wind (Frederiks et al 2022), Swift-BAT-GUANO (Dichiara et al 2022), GECAM-C (Liu et al 2022;An et al 2023;Yang et al 2023), and LHAASO (LHAASO Collaboration 2023). Its redshift is reported to be z = 0.151 (de Ugarte Postigo et al 2022).…”