Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

Details Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

2005-05-03

arxiv.org/abs/astro-ph/0505062v1

Astrophys.J. 629 (2005) 998-1007

Astronomy and Astrophysics

Astrophysics

10 pages, 8 figures, to be published in ApJ

Scientific paper

10.1086/431730

Motivated by the measurement of redshifted Fe H$\alpha$ lines during type I X-ray bursts on EXO 0748-676 (Cottam, Paerels & Mendez), we study the formation of atomic Fe lines above the photosphere of a bursting neutron star ($k_BT_{\rm eff} \approx 1-2 {\rm keV}$). We discuss the effects of Stark broadening, resonant scattering and NLTE (level population) on the formation of hydrogenic Fe H$\alpha$, Ly$\alpha$ and P$\alpha$ lines. From the observed equivalent width of the Fe H$\alpha$ line, we find an implied Fe column of $1-3 \times 10^{20} {\rm cm}^{-2}$, which is 3-10 times larger than the Fe column calculated from the accretion/spallation model of Bildsten, Chang & Paerels. We also estimate that the implied Fe column is about a factor of 2-3 larger than a uniform solar metallicity atmosphere. We discuss the effects of rotational broadening and find that the rotation rate of \EXO must be slow, as confirmed by the recent measurement of a 45 Hz burst oscillation by Villarreal & Strohmayer. We also show that the Fe Ly$\alpha$ EW $\approx$ 15-20 eV (redshifted 11-15 eV) and the P$\alpha$ EW $\approx$ 4-7 eV (redshifted 3-5 eV) when the H$\alpha$ EW is 10 eV (redshifted 8 eV). The Ly$\alpha$ line is rotationally broadened to a depth of $\approx 10%$, making it difficult to observe with {\it Chandra}. We also show that radiative levitation can likely support the Fe column needed to explain the line.

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