Deciphering the ∼18 TeV Photons from GRB 221009A

Abstract

On 2022 October 9, an extremely powerful gamma-ray burst, GRB 221009A, was detected by several instruments. Despite being obstructed by the Milky Way galaxy, its afterglow outburst outshone all other GRBs seen before. LHAASO detected several thousand very high energy photons extending up to 18 TeV. Detection of such energetic photons is unexpected due to the large opacity of the universe. It is possible that in the afterglow epoch, the intrinsic very high energy photon flux from the source might have increased manifolds, which could compensate for the attenuation by pair production with the extragalactic background light. We propose such a scenario and show that very high energy photons can be observed on the Earth from the interaction of very high energy protons with the seed synchrotron photons in the external forward shock region of the GRB jet.

The Radio to GeV Afterglow of GRB 221009A

“…Thus, in the (presumed) absence of a high-energy cutoff in the electron spectrum, the synchrotron spectrum will dominate over SSC at 1 TeV, and hence the intrinsic spectrum is expected to be ≈ −p/2 ≈ −1.1 in F ν . The synchrotron flux is a factor of ≈40 higher than the rough observed flux computed above; absorption due to γ-γ pair production against the extragalactic background light is expected to attenuate the observed spectrum (although, see also Sahu et al 2023), and this deficit is of the same order of magnitude as, but smaller than, that inferred for GRB 190114C (MAGIC Collab 2019). Thus, it is conceivable that the VHE emission for this GRB was produced by the FS, although a full analysis requires the VHE data.…”

The Radio to GeV Afterglow of GRB 221009A

“…Thus, in the (presumed) absence of a highenergy cutoff in the electron spectrum, the synchrotron spectrum will dominate over SSC at 1 TeV, and hence the intrinsic spectrum is expected to be ≈ −p/2 ≈ −1.1 in F ν . The synchrotron flux is a factor of ≈ 40 higher than the rough observed flux computed above; absorption due to γ-γ pair production against the Extragalactic Background Light (EBL) is expected to attenuate the observed spectrum (although, see also Sahu et al 2023), and this deficit is of the same order of magnitude as, but smaller than, that inferred for GRB 190114C (MAGIC Collab. 2019).…”

Jitter Mechanism as a Kind of Coherent Radiation: Constrained by the GRB 221009A Emission at 18 TeV

“…We take the jitter radiation as a kind of coherent emission. In our scenario, we suggest that GRBs are both (Cao et al 2022;Ren et al 2022;Sahu et al 2023).…”

“…In the meanwhile, the effect of the strong absorption of the extragalactic background light (EBL) should be considered. For example, when we take the EBL models of Franceschini et al (2008) and Domínguez et al (2011), the intrinsic flux can be reduced by an order of 10 −8 -10 −9 (Sahu et al 2023). Here, we take the minimum order of 1.0 × 10 −9 in our calculation.…”

“…The solid line (purple) indicates the calculation result with the parameters of γ = 1.0 × 10 7 and A = 3.0 × 10 −5 cm −3 . The dashed line (gray) is the LHAASO sensitivity (2000 s) at 18 TeV(Cao et al 2022;Ren et al 2022;Sahu et al 2023).…”