Rotational Broadening of Atomic Spectral Features from Neutron Stars

The discovery of the first gravitationally redshifted spectral line from a neutron star (NS) by Cottam et al. has triggered theoretical studies of the physics of atomic line formation in NS atmospheres. Chang et al. showed that the hydrogenic Fe Hα line formed above the photosphere of a bursting NS is intrinsically broad. We now include rotational broadening within general relativity and compare the resulting profile to that observed during type I bursts from EXO 0748-676. We show that the fine-structure splitting of the line precludes a meaningful constraint on the radius. Our fitting of the data show that the line-forming Fe column is log(NFe, n=2/cm-2) = 17.9 and gravitational redshift 1 + z = 1.345 with 95% confidence. We calculate the detectability of this spectral feature for a large range of spins and inclinations, assuming that the emission comes from the entire surface. We find that at 300 and 600 Hz only 10%-20% and 5%-10% of NSs would have spectral features as deep as those seen in EXO 0748-676.

[1]  R. Romani,et al.  Light Curves of Rapidly Rotating Neutron Stars , 2000, astro-ph/0004411.

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

[3]  F. Haberl,et al.  The Bursting Behavior of 4U 1728–34: The Parameters of a Neutron Star and the Geometry of a Neutron Star-Disk System , 2003, astro-ph/0307215.

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

[5]  D. Leahy,et al.  Pulse Shapes from Rapidly Rotating Neutron Stars: Equatorial Photon Orbits , 2004, astro-ph/0409261.

[6]  A. Beloborodov.  Gravitational Bending of Light Near Compact Objects , 2002, astro-ph/0201117.

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

[8]  Juri Poutanen,et al.  On the Nature of the X-ray Emission of Accreting Millisecond Pulsar SAX J1808.4-3658 , 2002, astro-ph/0303084.

[9]  J. Hartle,et al.  Slowly Rotating Relativistic Stars. II. Models for Neutron Stars and Supermassive Stars , 1968 .

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

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

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

[13]  L. Bildsten,et al.  Formation of Resonant Atomic Lines during Thermonuclear Flashes on Neutron Stars , 2005, astro-ph/0505062.

[14]  M. Miller,et al.  Bounds on the Compactness of Neutron Stars from Brightness Oscillations during X-Ray Bursts , 1997, astro-ph/9711325.

[15]  J. Hartle.  Slowly Rotating Relativistic Stars. I. Equations of Structure , 1967 .