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When neutron stars (NS) accrete gas from low-mass binary companions,explosive nuclear burning reactions in the NS envelope fuse hydrogen and heliuminto heavier elements. The resulting thermonuclear (type-I) X-ray burstsproduce energy spectra that are fit well with black bodies, but a significantnumber of burst observations show deviations from Planck spectra. Here wepresent our analysis of RXTE/PCA observations of X-ray bursts from the NSlow-mass X-ray binary HETE J1900.1-2455. We have discovered that thenon-Planckian spectra are caused by photo-ionization edges. Theanti-correlation between the strength of the edges and the colour temperaturesuggests that the edges are produced by the nuclear burning ashes that havebeen transported upwards by convection and become exposed at the photosphere.The atmosphere model fits show that occasionally the photosphere can consistentirely of metals, and that the peculiar changes in black body temperature andradius can be attributed to the emergence and disappearance of metals in thephotosphere. As the metals are detected already in the Eddington-limited phase,it is possible that a radiatively driven wind ejects some of the burning ashesinto the interstellar space.

Detection of burning ashes from thermonuclear X-ray bursts