Constraints on reionization from the z=7.5 QSO ULASJ1342+0928

The recent detection of ULASJ1342+0928, a bright QSO at z = 7.54, provides a powerful probe of the ionization state of the intervening intergalactic medium, potentially allowing us to set strong constraints on the epoch of reionization (EoR). Here we quantify the presence of Ly α damping wing absorption from the EoR in the spectrum of ULASJ1342+0928. Our Bayesian framework simultaneously accounts for uncertainties on (i) the intrinsic QSO emission (reconstructing the Ly α profile from a covariance matrix of emission lines) and (ii) the distribution of H ii regions during reionization (from three different 1.6 3 Gpc 3 simulations spanning plausible EoR morphologies). Our analysis is complementary to the Banados et al. discovery and accompanying Davies et al. method paper as it focuses solely on the damping wing imprint redward of Ly α (1218 < < 1230 Å) and uses a different methodology for (i) and (ii). We recover weak evidence for damping wing absorption from our intermediate EoR model yielding a volume-weighted neutral hydrogen fraction at z = 7.5: barx-mathrmH, scriptscriptstyle I = 0.21 +0.17-0.19 (68 per cent). These constraints depend weakly on the EoR morphology. Our limits are lower than those of the previous two analyses, though they are consistent at 1/4 1-1.5σ. We attribute differences to (i) a lower intrinsic amplitude for our recovered Ly α profile and (ii) only considering transmission redward of Ly α, reducing the available constraining power but making the results less model-dependent. Our results are consistent with previous estimates of the EoR history, supporting a moderately extended EoR.