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Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars | | P. Chang
; L. Bildsten
; I. Wasserman
; | | Date: |
3 May 2005 | | Journal: | Astrophys.J. 629 (2005) 998-1007 | | Subject: | astro-ph | | Abstract: | 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 ($k_BT_{
m eff} approx 1-2 {
m keV}$). 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 $1-3 imes 10^{20} {
m cm}^{-2}$, 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. | | Source: | arXiv, astro-ph/0505062 | | Services: | Forum | Review | PDF | Favorites | |
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