Atomic Transition Lines During Type I X-Ray Bursts

Philip Chang

University of California Santa Barbara


The discovery of the first atomic transition lines from a bursting neutron star (NS) by Cottam, Paerels and Mendez is the solid measurement of gravitational redshift from a NS surface. I will review the observation and discuss the basic physics of the neutron star atmosphere and line formation in this context. I will highlight the important effects of Stark broadening, resonant scattering and NLTE effects (level population) on the formation of the Fe H\alpha and Ly\alpha lines. With the inclusion of these effects, I reproduce the appropriate equivalent width of the observed line with Fe columns that are consistent with solar metallicity accretion. I also compare the fully relativisitic rotationally broadened line profile to the data and find that the NS spin is \nu_{\rm s}\sin i (R/10\,{\rm km}) = 32-28+52\,{\rm Hz} with 95% confidence, in agreement with the 44.7 Hz spin detected by Villarreal and Strohmayer. However, fine structure splitting of the line precludes a meaningful constraint on the radius of this NS. Finally I highlight future prospects for detecting other features on more rapidly rotationing NSs and their application in determining both the redshift and radius to contraint the nuclear equation of state.

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