Speaker
Description
GRB 221009A, a relatively nearby (redshift z= 0.1505) and exceptionally bright gamma-ray burst, has been detected with the LHAASO-WCDA instrument in the energy range of 0.2-7 TeV, as well as with the LHAASO-KM2A array up to the energy of $\approx 13$ TeV. The unprecedentedly high fluence of TeV gamma-rays from GRB 221009A allows us, for the first time for GRBs, to set constraints on the strength B of the extragalactic magnetic field (EGMF), excluding the values of $B<10^{-18}$ G [Dzhatdoev et al., MNRAS Lett., 527, L95 (2024)]. The observation of $\sim 10$ TeV gamma-rays and the subsequent search for $\sim 1-100$ GeV pair echo allows to probe the “far” ($\sim 100$ Mpc) environment of GRB 221009A.
Gamma-ray bursts are typically situated in star-forming regions. Therefore, the “near” (<100 pc) environment of GRBs is, as a rule, occupied by a significant amount of gas (typical column density $\sim 10^{21}-10^{23}$ 1/cm$^{2}$). Furthermore, GRBs are expected to be capable of accelerating protons/nuclei up to the energy of at least 1 PeV/nucleon in the fireball's rest frame. The protons/nuclei accelerated during the GRB prompt phase could interact with dense photon fields of the prompt emission producing neutrons. These neutrons escape from the magnetic fields of the fireball freely and interact with the interstellar matter of the star-forming region, eventually resulting in an observable flux of multi-TeV gamma-rays. We show that for certain values of relevant parameters the intensity of these gamma-rays could contribute significantly to the observable spectrum at $E > 10$ TeV. Thus, the “near” (<100 pc) environment of GRBs could be probed with TeV gamma-ray observations as well.
We show that, contrary to some previous studies, the intrinsic (intergalactic absorption-corrected) gamma-ray spectrum of GRB 221009A reveals a surprising cutoff or a break above the energy of several TeV. The nature of the multi-TeV gamma-rays is not clear due to a possible strong influence of the Klein-Nishina effect, severely limiting the observable gamma-ray intensity at the energy in excess of several TeV. A significant part of \approx 10 TeV gamma-rays from GRB 221009A could have been produced by an unconventional mechanism, for example, the one discussed above. Finally, we discuss the relevance of the presented findings to the models of intrinsic gamma-ray production in GRB fireballs and to intergalactic gamma-ray propagation models, as well as to the searches for axion-like particles and Lorentz invariance violation effects.
