The Characteristic Shape of Damping Wings During Reionization

Spectroscopic analysis of Ly$\alpha$ damping wings of bright sources at $z>6$ is a promising way to measure the reionization history of the universe. However, the theoretical interpretation of the damping wings is challenging due to the inhomogeneous nature of the reionization process and the proximity effect of bright sources. In this Letter, we analyze the damping wings arising from the neutral patches in the radiative transfer cosmological simulation suite Cosmic Reionization on Computers (CROC). We find that the damping wing profile remains a tight function of volume-weighted neutral fraction $\left< x_{\rm HI} \right>_{\rm v}$, especially when $\left< x_{\rm HI} \right>_{\rm v}>0.5$, despite the patchy nature of reionization and the proximity effect. This small scatter indicates that with a well-measured damping wing profile, we could constrain the volume-weighted neutral fraction as precise as $\Delta \left< x_{\rm HI} \right>_{\rm v} \lesssim 0.1$ in the first half of reionization.