Unraveling the Mystery of GRB 221009A: Secondary Gamma Rays and Cosmic Magnetic Field

The brightest gamma-ray burst (GRB) ever recorded, GRB 221009A, has puzzled astronomers with its unexpected flux of multi-TeV gamma rays. These high-energy photons, observed at a redshift of 0.151, may challenge conventional models because they should have been absorbed by extragalactic background light (EBL) during their journey across the universe, unless the EBL level is low enogh. A recent study proposes an elegant solution which doesn't impose any constraints on EBL: these gamma rays may not originate directly from the GRB but are instead secondary particles produced by ultra-high energy cosmic rays interacting with photon backgrounds along the line of sight.

The study argues that cosmic rays (protons) accelerated by the GRB jet interact with EBL and cosmic microwave background (CMB) photons during their travel. These interactions produce secondary gamma rays through processes like pion decay and pair production. Unlike primary gamma rays, which are heavily absorbed, secondary gamma rays are generated closer to Earth, avoiding significant attenuation.

To model these processes, the researchers used the publicly available numerical code CRbeam, which simulates cosmic-ray propagation, electromagnetic cascades, and deflections in magnetic fields. This powerful tool is accessible for broader scientific use via the MMODA astrophysics platform and the EuroScienceGateway Galaxy workflow, enabling researchers to reproduce and extend these findings. The simulations confirmed that secondary gamma rays could explain the highest-energy LHAASO data points if extragalactic magnetic field EGMF is weaker than (10^{-16}) Gauss. Stronger fields would deflect protons, delaying their arrival and dispersing the secondary gamma-ray signal beyond detectable limits.
Thus the explanation also provides a unique probe for studying EGMFs, which are crucial for understanding cosmic structure formation and early-universe conditions.