Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars

Motivated by the measurement of redshifted Fe Hα 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 (kBTeff ≈ 1-2 keV). We discuss the effects of Stark broadening, resonant scattering, and NLTE (level population) on the formation of hydrogenic Fe Hα, Lyα, and Paα lines. From the observed equivalent width (EW) of the Fe Hα line, we find an implied Fe column of × 1020 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 0748-676 must be slow, as confirmed by the recent measurement of a 45 Hz burst oscillation by Villareal & Strohmayer. We also show that the Fe Lyα EW ≈ 15-20 eV (redshifted 11-15 eV) and the Paα EW ≈ 4-7 eV (redshifted 3-5 eV) when the Hα EW is 10 eV (redshifted 8 eV). The Lyα line is rotationally broadened to a depth of ≈10%, making it difficult to observe with Chandra. We also show that radiative levitation can likely support the Fe column needed to explain the line.

[1]  H. Bethe,et al.  Quantum Mechanics of One- and Two-Electron Atoms , 1957 .

[2]  L. Bildsten,et al.  Atomic Spectral Features during Thermonuclear Flashes on Neutron Stars , 2003, astro-ph/0303147.

[3]  Dimitrios Psaltis,et al.  Spectral Lines from Rotating Neutron Stars , 2002, astro-ph/0209225.

[4]  Pierre Chayer,et al.  Radiative Levitation in Hot White Dwarfs: Equilibrium Theory , 1995 .

[5]  D. F. Gray,et al.  The Observation and Analysis of Stellar Photospheres , 2021 .

[6]  M. Turner,et al.  Spectral and temporal features in bursts from 2S 1636−536 observed with EXOSAT , 1984 .

[7]  A. Fabian,et al.  The formation of iron features in the cooling spectrum of X-ray bursts , 1987 .

[8]  A. Parmar,et al.  The bursting behavior of the transient X-ray burst source EXO 0748-676 - A dependence between the X-ray burst properties and the strength of the persistent emission , 1986 .

[9]  N. Schulz,et al.  Discrete X-Ray Signatures of a Photoionized Plasma above the Accretion Disk of the Neutron Star EXO 0748–676 , 2003, astro-ph/0302391.

[10]  John T. Jefferies,et al.  Spectral line formation , 1968 .

[11]  N. Arav.  The ``Ghost of LY alpha '' as Evidence for Radiative Acceleration in Quasars , 1996 .

[12]  NASAGSFC,et al.  Discovery of the Neutron Star Spin Frequency in EXO 0748–676 , 2004, astro-ph/0409384.

[13]  A. Różańska,et al.  Model Atmospheres and X-Ray Spectra of Bursting Neutron Stars: Hydrogen-Helium Comptonized Spectra , 2004 .

[14]  G. Vauclair,et al.  Diffusion in main-sequence stars: radiation forces, time scales, anomalies. , 1976 .

[15]  J. P. Harrington.  The Scattering of Resonance-line Radiation in the Limit of Large Optical Depth , 1973 .

[16]  Nicholas E. White,et al.  The Discovery of 3.8 Hour Periodic Intensity Dips and Eclipses from the Transient Low-Mass X-Ray Binary EXO 0748-676 , 1986 .

[17]  G. Rybicki,et al.  Radiative processes in astrophysics , 1979 .

[18]  G. Michaud,et al.  Diffusion Processes in Peculiar a Stars , 1970 .

[19]  W. Lewin,et al.  UvA-DARE ( Digital Academic Repository ) X-ray bursts at extreme mass accretion rates from GX 17 + 2 , 2002 .

[20]  Gilles Fontaine,et al.  The Presence of Helium in Hot DA White Dwarfs: The Role of Radiative Levitation and the Case for Stratified Atmospheres , 1988 .

[21]  A. Schuster.  Radiation through a foggy atmosphere , 1903 .

[22]  S. Chandrasekhar.  The formation of absorption lines in a moving atmosphere , 1945 .

[23]  W. Lewin,et al.  Compact stellar X-ray sources , 2006 .

[24]  L. Bildsten.  The Fate of Accreted CNO Elements in Neutron Star Atmospheres , 1992 .

[25]  Pressure Broadening of Absorption Lines in Neutron Star Atmospheres and Prospects for Measuring Neutron Star Masses and Radii , 1997 .

[26]  P. Joss.  X-ray bursts and neutron-star thermonuclear flashes , 1977, Nature.

[27]  W. M. Howard,et al.  Model Atmospheres for X-Ray Bursting Neutron Stars (abstract) , 1986 .

[28]  Walter H. G. Lewin,et al.  X-ray bursts , 1993 .

[29]  F. Paerels,et al.  Gravitationally redshifted absorption lines in the X-ray burst spectra of a neutron star , 2002, Nature.

[30]  G. Pavlov,et al.  Spectra of X-ray bursts at near-Eddington luminosities , 1991 .