A stochastic forward model for the intergalactic dispersion-measure distribution of Fast Radio Bursts

Abstract: Fast Radio Bursts probe ionised baryons through their observed dispersion measures. We present \turbofrb, a semi-analytic stochastic forward model for the intergalactic dispersion-measure distribution, $P({\rm DM}_{\rm IGM}\mid z)$, that resolves the diffuse IGM, halo, and filament contributions as explicit physical channels, with the halo and filament encounter rates coupled by a latent line-of-sight environmental variable. Only four effective parameters are calibrated against hydrodynamical ray-traced IllustrisTNG benchmark. The model matches the benchmark mean DM to the percent level and yields a per-redshift Jensen-Shannon divergence of at most $5\times10<sup>{-3}$ across $z = 0.5$-$2.5$. The per-sightline channel decomposition makes explicit what closed-form parametric descriptions cannot show: the diffuse IGM sets the body of the distribution, while halos and filaments populate the high-DM tail. Applied to representative localised FRBs, the forward likelihood quantifies host-excess events independently of their astrophysical signatures and recovers the injected $H_0$ within $1σ$ in a closed-loop consistency test. The \turbofrb package is available at \href{https://github.com/jefersonfortunato/turbofrb}{github.com/jefersonfortunato/turbofrb}.