50 ks simulated X-IFU energy spectra of a GRB afterglow (z_GRB = 2) imprinted by O VII (EW_O VII = 0.35 eV) and O VIII (EW_O VIII = 0.23 eV) WHIM absorbers. Both spectra are binned using the optimal binning scheme of Kaastra & Bleeker (2016). Left: GRB absorbed starting flux 5 × 10⁻¹² erg cm⁻¹ s⁻¹ and z_WHIM = 0.4388. Right: GRB absorbed starting flux 1 × 10⁻¹⁰ erg cm⁻¹ s⁻¹ and z_WHIM = 0.0382. The insets show the energy range around the WHIM absorption features. Note: The absorption feature located at ≈0.53 eV is the O resonance line of Galactic origin and is contained within the tbabs model component.