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Inspired by recent IRIS observations, we forward model the response of the FeXXI 1354 \AA\ line to fundamental, standing, linear fast sausage modes (FSMs)in flare loops. Starting with the fluid parameters for an FSM in a straighttube with equilibrium parameters largely compatible with the IRIS measurements,we synthesize the line profiles by incorporating the non-Equilibrium Ionization(NEI) effect in the computation of the contribution function. We find that boththe intensity and Doppler shift oscillate at the wave period ($P$). The phasedifference between the two differs from the expected value ($90^\circ$) onlyslightly because NEI plays only a marginal role in determining the ionicfraction of Fe XXI in the examined dense loop. The Doppler width modulations,however, posses an asymmetry in the first and second halves of a wave period,leading to a secondary periodicity at $P/2$ in addition to the primary one at$P$. This behavior results from the competition between the broadening due tobulk flow and that due to temperature variations, with the latter beingstronger but not overwhelmingly so. These expected signatures, with theexception of the Doppler width, are largely consistent with the IRISmeasurements, thereby corroborating the reported detection of a fundamentalFSM. The forward modeled signatures are useful for identifying fundamental FSMsin flare loops from measurements of the Fe \MyRoman{21} 1354 \AA\ line withinstruments similar to IRIS, even though a much higher cadence is required forthe expected behavior in the Doppler widths to be detected.

Synthetic Emissions of the Fe XXI 1354 \AA\ Line from Flare Loops Experiencing Fundamental Fast Sausage Oscillations