An extra hard spectral component peaking at sub-GeV in the prompt emission of GRB 260226A

The prompt emission spectra of gamma-ray bursts (GRBs) have long been empirically described by the Band function over the keV--MeV range, whereas several Fermi/LAT-detected GRBs show evidence for an extra hard component at higher energies. Here we present a joint GBM--LAT study of GRB 260226A, a rare LAT seeded onboard trigger GRB, and find that an extra sub-GeV component is present. In the time-resolved analysis, we find that a cutoff power-law model fits the spectral data of the extra component better than other models at earlier times, while at later times, the broken power-law model is preferred (or at least equally good). Interpreting the early cutoff as the gamma gamma absorption implies that the Lorentz factor at early time is lower and increases with time. The low ratio between the peak energy of the extra component and that of the Band component is difficult to explain with a one-zone synchrotron self-Compton (SSC) scenario. Two-zone emission models, such as external inverse Compton scattering of the photosphere emission by relativistic electrons accelerated in internal shocks, could provide a possible explanation.