Relativistic Iron Emission Lines in Neutron Star Low-Mass X-Ray Binaries as Probes of Neutron Star Radii

Using Suzaku observations of three neutron star low-mass X-ray binaries ( Ser X-1, 4U 1820-30, and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ("diskline''), allowing a measurement of the inner radius of the accretion disk and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5-16.5 km for a 1.4 M-circle dot neutron star. The inner disk radii that we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk origin for kHz QPOs. In addition, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

[1]  P. Kaaret,et al.  RXTE Studies of Long-Term X-Ray Spectral Variations in 4U 1820-30 , 2000, astro-ph/0005496.

[2]  F. Paerels,et al.  SUBMITTED TO APJL Preprint typeset using LATEX style emulateapj v. 10/09/06 NON-DETECTION OF GRAVITATIONALLY REDSHIFTED ABSORPTION LINES IN THE X-RAY BURST SPECTRA OF GS 1826–24 , 2022 .

[3]  R. M. Hjellming,et al.  Episodic ejection of relativistic jets by the X-ray transient GRO J1655 - 40 , 1995, Nature.

[4]  Charles D. Bailyn,et al.  Optical and Infrared Photometry of the Microquasar GRO J1655-40 in Quiescence , 2001 .

[5]  J. Lattimer,et al.  Neutron star observations: Prognosis for equation of state constraints , 2007 .

[6]  J. Orosz,et al.  Optical Observations of GRO J1655–40 in Quiescence. I. A Precise Mass for the Black Hole Primary , 1996, astro-ph/9610211.

[7]  C. Horowitz,et al.  Neutron star structure and the neutron radius of 208Pb. , 2001, Physical review letters.

[8]  T. Strohmayer,et al.  Detection with RHESSI of High-Frequency X-Ray Oscillations in the Tailof the 2004 Hyperflare from SGR 1806–20 , 2005, astro-ph/0512630.

[9]  A. Laor.  Line Profiles from a Disk around a Rotating Black Hole , 1991 .

[10]  R. Davies,et al.  Astronomical Society of the Pacific Conference Series , 2010 .

[11]  S. Anderson,et al.  Time-Resolved Ultraviolet Observations of the Globular Cluster X-Ray Source in NGC 6624: The Shortest Known Period Binary System , 1997, astro-ph/9703180.

[12]  J. M. Miller,et al.  Relativistic X-Ray Lines from the Inner Accretion Disks Around Black Holes , 2007, 0705.0540.

[13]  October I.  Physical Review Letters , 2022 .

[14]  L. Brenneman,et al.  Constraining Black Hole Spin via X-Ray Spectroscopy , 2006, astro-ph/0608502.

[15]  C. Reynolds,et al.  An X-ray spectral study of 24 type 1 active galactic nuclei , 1997 .

[16]  D. R. Ballantyne,et al.  The Evolution of the Accretion Disk around 4U 1820–30 during a Superburst , 2004, astro-ph/0401154.

[17]  Mario Vietri,et al.  Correlations in the Quasi-periodic Oscillation Frequencies of Low-Mass X-Ray Binaries and the Relativistic Precession Model , 1999, astro-ph/9907346.

[18]  T. Bulik,et al.  Evidence for a 2:3 resonance in Sco X-1 kHz QPOs , 2002, astro-ph/0206490.

[19]  A. Markowitz,et al.  Suzaku Observations of the Hard X-ray Variability of MCG-6-30-15: the Effects of Strong Gravity Around a Kerr Black Hole , 2006, astro-ph/0609521.

[20]  Constraints on the high-density nuclear equation of state from the phenomenology of compact stars and heavy-ion collisions , 2006, nucl-th/0602038.

[21]  L. Titarchuk.  Effects of Resonance in Quasi-periodic Oscillators of Neutron Star Binaries , 2002, astro-ph/0208423.

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

[23]  G. Hasinger,et al.  A study of the continuum and iron K line emission from low-mass X-ray binaries , 1986 .

[24]  M. Méndez.  On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries , 2006, astro-ph/0607433.

[25]  Tod E. Strohmayer,et al.  Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-Ray Binary Serpens X-1 , 2007, 0708.3648.

[26]  Evaluating Spectral Models and the X-Ray States of Neutron Star X-Ray Transients , 2007, astro-ph/0702089.

[27]  G. Rybicki,et al.  A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae , 2005, astro-ph/0506563.

[28]  Jonathan E. Grindlay,et al.  Constraints on Neutron Star Properties from X-Ray Observations of Millisecond Pulsars , 2006, astro-ph/0612791.

[29]  V. Pandharipande,et al.  Equation of state of nucleon matter and neutron star structure , 1998, nucl-th/9804027.

[30]  Ryuichi Fujimoto,et al.  The X-Ray Observatory Suzaku , 2007 .

[31]  Tod E. Strohmayer,et al.  A Remarkable 3 Hour Thermonuclear Burst from 4U 1820–30 , 2001, astro-ph/0108420.

[32]  T. Oosterbroek,et al.  Simultaneous BeppoSAX and RXTE observations of the X-ray burst sources GX 3+1 and Ser X-1 , 2000, astro-ph/0010609.

[33]  P. Kaaret,et al.  Relations between Timing Features and Colors in the X-Ray Binary 4U 0614+09 , 2000, astro-ph/0001480.

[34]  J. Wilms,et al.  On the Inability of Comptonization to Produce the Broad X-Ray Iron Lines Observed in Seyfert Nuclei , 2000 .

[35]  L. Stella,et al.  A Broad Iron Line in the Chandra High Energy Transmission Grating Spectrum of 4U 1705–44 , 2005 .

[36]  T. Strohmayer,et al.  Correlation between Energy Spectral States and Fast Time Variability and Further Evidence for the Marginally Stable Orbit in 4U 1820–30 , 1998, astro-ph/9804228.

[37]  K. Mitsuda,et al.  Iron K Emission Lines in the Energy Spectra of Low-Mass X-Ray Binaries Observed with ASCA , 2000 .

[38]  Evidence for Black Hole Spin in GX 339-4: XMM-Newton EPIC-pn and RXTE Spectroscopy of the Very High State , 2003, astro-ph/0312033.

[39]  A. Peacock,et al.  Exosat observations of broad iron K line emission from Scorpius X-1 , 1985 .

[40]  J. Lattimer,et al.  Neutron Star Structure and the Equation of State , 2000, astro-ph/0002232.

[41]  Philip Chang,et al.  Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754 , 2006, astro-ph/0612145.

[42]  K. Nandra,et al.  ASCA Observations of Seyfert 1 Galaxies. II. Relativistic Iron Kα Emission , 1996, astro-ph/9606169.

[43]  Tod E. Strohmayer,et al.  Constraints on Neutron Star Parameters from Burst Oscillation Light Curves of the Accreting Millisecond Pulsar XTE J1814-338 , 2005 .

[44]  Nicholas E. White,et al.  X-ray fluorescence from the inner disc in Cygnus X-1 , 1989 .

[45]  Frederick K. Lamb,et al.  Sonic-Point Model of Kilohertz Quasi-periodic Brightness Oscillations in Low-Mass X-Ray Binaries , 1996, astro-ph/9609157.