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Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrichphotosphere with metals, which may imprint the photoionization edges on theburst spectra. We report here the discovery of absorption edges in the spectraof the type I X-ray burst from the NS low-mass X-ray binary GRS 1747$-$312 inTerzan 6 during observations by the Rossi X-ray Timing Explorer. We find thatthe edge energy evolves from $9.45\pm0.51$ keV to $\sim6$ keV and then back to$9.44\pm0.40$ keV during the photospheric radius expansion phase and remains at$8.06\pm0.66$ keV in the cooling tail. The photoionization absorption edges ofhydrogen-like Ni, Fe or Fe/Ni mixture and the bound-bound transitions of metalsmay be responsible for the observed spectral features. The ratio of themeasured absorption edge energy in the cooling tail to the laboratory value ofhydrogen-like Ni(Fe) edge energy allows us to estimate the gravitationalredshift factor $1+z=1.34\pm0.11$($1+z=1.15\pm0.09$). The evolution of thespectral parameters during the cooling tail are well-described by themetal-rich atmosphere models. The combined constraints on the NS mass andradius from the direct cooling method and the tidal deformability stronglysuggest very high atmospheric abundance of the iron group elements and limitthe distance to the source to $11\pm1$ kpc.

Evidence for the photoionization absorption edge in a photospheric radius expansion X-ray burst from GRS 1747$-$312 in Terzan 6