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At low redshifts, the observed baryonic density falls far short of the totalnumber of baryons predicted. Cosmological simulations suggest that thesebaryons reside in filamentary gas structures, known as the warm-hotintergalactic medium (WHIM). As a result of the high temperatures of thesefilaments, the matter is highly ionised such that it absorbs and emits far-UVand soft X-ray photons. Athena, the proposed European Space Agency X-rayobservatory, aims to detect the `missing' baryons in the WHIM up to redshiftsof $z=1$ through absorption in active galactic nuclei and gamma-ray burstafterglow spectra, allowing for the study of the evolution of these large-scalestructures of the Universe. This work simulates WHIM filaments in the spectraof GRB X-ray afterglows with Athena using the SImulation of X-ray TElescopes(SIXTE) framework. We investigate the feasibility of their detection with theX-IFU instrument, through O VII ($E=573$ eV) and O VIII ($E=674$ eV) absorptionfeatures, for a range of equivalent widths imprinted onto GRB afterglow spectraof observed starting fluxes ranging between $10^{-12}$ and $10^{-10}$ ergcm$^{-2}$ s$^{-1}$, in the 0.3-10 keV energy band. The analyses of X-IFUspectra by blind line search show that Athena will be able to detect O VII-OVIII absorption pairs with EW$_\mathrm{O VII} > 0.13$ eV and EW$_\mathrm{OVIII} > 0.09$ eV for afterglows with $F>2 \times 10^{-11}$ erg cm$^{-2}$s$^{-1}$. This allows for the detection of $\approx$ 45-137 O VII-O VIIIabsorbers during the four-year mission lifetime. The work shows that to obtainan O VII-O VIII detection of high statistical significance, the local hydrogencolumn density should be limited at $N_\mathrm{H}<8 \times 10^{20}$ cm$^{-2}$.

Detection capabilities of the Athena X-IFU for the warm-hot intergalactic medium using gamma-ray burst X-ray afterglows