THE INCOMPATIBILITY OF RAPID ROTATION WITH NARROW PHOTOSPHERIC X-RAY LINES IN EXO 0748−676

X-ray observations of EXO 0748-676 during thermonuclear bursts revealed a set of narrow absorption lines that potentially originate from the stellar photosphere. The identification of these lines with particular atomic transitions led to the measurement of the surface gravitational redshift of the neutron star and to constraints on its mass and radius. However, the recent detection of 552 Hz oscillations at 15% rms amplitude revealed the spin frequency of the neutron star and brought into question the consistency of such a rapid spin with the narrow width of the absorption lines. Here, we calculate the amplitudes of burst oscillations and the width of absorption lines emerging from the surface of a rapidly rotating neutron star for a wide range of model parameters. We show that no combination of neutron-star and geometric parameters can simultaneously reproduce the narrowness of the absorption lines, the high amplitude of the oscillations, and the observed flux at the time the oscillations were detected. We, therefore, conclude that the observed absorption lines are unlikely to originate from the surface of this neutron star.

[1]  D. Psaltis,et al.  Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer , 2006, astro-ph/0608259.

[2]  R. Narayan,et al.  General Relativistic Constraints on Emission Models of Anomalous X-Ray Pulsars , 2000, astro-ph/0004266.

[3]  Massachusetts Institute of Technology,et al.  DISCOVERY OF A 552 Hz BURST OSCILLATION IN THE LOW-MASS X-RAY BINARY EXO 0748−676 , 2009, 0910.5546.

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

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

[6]  D. Leahy,et al.  Light Curves for Rapidly Rotating Neutron Stars , 2006, astro-ph/0609325.

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

[8]  F. Özel,et al.  Soft equations of state for neutron-star matter ruled out by EXO 0748 - 676 , 2006, Nature.

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

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

[11]  C. Ftaclas,et al.  Hot spots on neutron stars - The near-field gravitational lens , 1982 .

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

[13]  T. Strohmayer,et al.  363 Hz Oscillations during the Rising Phase of Bursts from 4U 1728–34: Evidence for Rotational Modulation , 1997 .

[14]  F. Ozel,et al.  The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X-Ray Bursts , 2002, astro-ph/0204501.

[15]  University of Michigan,et al.  The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation , 2008 .

[16]  Columbia,et al.  Discovery of X-ray burst triplets in EXO 0748-676 , 2007, astro-ph/0701575.

[17]  Astrophysics,et al.  Absorption features in the spectra of X-ray bursting neutron stars , 2008, 0809.2170.

[18]  S. Morsink,et al.  Rotational Broadening of Atomic Spectral Features from Neutron Stars , 2006 .

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

[20]  D. Psaltis,et al.  Photon Propagation around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars , 2000, astro-ph/0004387.