Detection of burning ashes from thermonuclear X-ray bursts

TL;DR

This paper analyzes X-ray burst spectra from a neutron star, revealing that deviations from black body spectra are caused by photo-ionization edges due to nuclear burning ashes, which are transported to the photosphere and sometimes ejected into space.

Contribution

It demonstrates that non-Planckian spectra in X-ray bursts are caused by photo-ionization edges from nuclear ashes, providing insights into the composition and dynamics of the neutron star atmosphere.

Findings

Photo-ionization edges cause deviations from black body spectra.

Nuclear ashes are transported to the photosphere and sometimes ejected.

The photosphere can be entirely metal-rich during bursts.

Abstract

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