Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

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 ($k_B{T}_{\mathrm{eff}}\approx 1-2\mathrm{keV}$). 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 $1-3\times 10^{20}{\mathrm{cm}}^{-2}$, 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.

10 pages, 8 figures, to be published in ApJ

References in corpus (3)

• Discovery of the Neutron Star Spin Frequency in EXO 0748-676
• Atomic Spectral Features During Thermonuclear Flashes on Neutron Stars
• Discrete X-ray Signatures of a Photoionized Plasma Above the Accretion Disk of the Neutron Star EXO 0748-676

Cited by in corpus (22)

• Measuring the neutron star equation of state using X-ray timing
• Rotating Stars in Relativity
• Exposing the Nuclear Burning Ashes of Radius Expansion Type I X-ray Bursts
• The Effects of Variations in Nuclear Processes on Type I X-Ray Burst Nucleosynthesis
• Dense matter with eXTP
• Surface Emission from Neutron Stars and Implications for the Physics of their Interiors
• Nuclear astrophysics: the unfinished quest for the origin of the elements
• Measurement of neutron star parameters: a review of methods for low-mass X-ray binaries
• The Impact of Uncertainties in Reaction Q-values on Nucleosynthesis in Type I X-Ray Bursts
• The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation
• Discovery of a 552 Hz burst oscillation in the low-mass X-ray binary EXO 0748-676
• Absorption features in the spectra of X-ray bursting neutron stars
• Rotational Broadening of Atomic Spectral Features from Neutron Stars
• The Shapes of Atomic Lines from the Surfaces of Weakly Magnetic Rotating Neutron Stars and Their Implications
• The Incompatibility of Rapid Rotation with Narrow Photospheric X-ray Lines in EXO 0748-676
• Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24
• Mass-loss and composition of wind ejecta in type I X-ray bursts
• Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars
• Constraints on physics of neutron stars from X-ray observations
• The Sensitivity of Nucleosynthesis in Type I X-ray Bursts to Thermonuclear Reaction-Rate Variations
• Simulations of stellar winds from X-ray bursts. Characterization of solutions and observable variables
• Search For Gravitational Redshifted Absorption Lines In LMXB Serpens X-1