Detected count spectra of GRB 221009A and the background spectrum. The count spectrum of the GRB at time t_s minus the background spectrum at time t_b is shown in black. The background spectrum at time t_b is shown in red. The count spectrum of the GRB at the peak time t_p = 13 : 20 : 49.5 UT on 2022 October 9, minus the background spectrum at time t_b, is shown in green and it reveals a spectral hardening at the highest energies present around the GRB peak.