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Measurements of the mean free path of Lyman-continuum photons in theintergalactic medium during the epoch of reionization can help constrain thenature of the sources as well as sinks of hydrogen-ionizing radiation. A recentapproach to this measurement has been to utilize composite spectra of multiplequasars at $z\sim 6$, and infer the mean free path after correcting the spectrafor the presence of quasar proximity zones. This has revealed not only a steepdrop in the mean free path from $z=5$ to $z=6$, but also potentially a mildtension with reionization simulations. We critically examine such directmeasurements of the mean free path for biases due to quasar environment,incomplete reionization, and quasar proximity zones. Using cosmologicalradiative transfer simulations of reionization combined with one-dimensionalradiative transfer calculations of quasar proximity zones, we find that thebias in the mean free path due to overdensities around quasars is minimal at$z\sim 6$. Patchiness of reionization at this redshift also does not affect themeasurements significantly. Fitting our model to the data results in a meanfree path of $\lambda_{\mathrm{mfp}}=0.90^{+0.66}_{-0.40}$ pMpc at $z=6$, whichis consistent with the recent measurements in the literature, indicatingrobustness with respect to the modelling of quasar proximity zones. We alsocompare various ways in which the mean free path has been defined insimulations before the end of reionization. Overall, our finding is that recentmeasurements of the mean free path appear to be robust relative to severalsources of potential bias.

Robustness of direct measurements of the mean free path of ionizing photons in the epoch of reionization