Relativistic Lines and Reflection from the Inner Accretion Disks Around Neutron Stars

• Published in: The Astrophysical journal Vol. 720; no. 1; pp. 205 - 225
• Main Authors: Cackett, Edward M, Miller, Jon M, Ballantyne, David R, Barret, Didier, Bhattacharyya, Sudip, Boutelier, Martin, Miller, M. Coleman, Strohmayer, Tod E, Wijnands, Rudy
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01-09-2010; IOP
• Subjects: ACCRETION DISKS; Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASYMMETRY; BOUNDARY LAYERS; COSMIC RADIO SOURCES; Earth, ocean, space; ENERGY RANGE; Exact sciences and technology; LAYERS; LUMINOSITY; METRICS; NEUTRON STARS; OPTICAL PROPERTIES; PHYSICAL PROPERTIES; PULSARS; RELATIVISTIC RANGE; SCHWARZSCHILD METRIC; SIMULATION; SPECTRA; STARS; X-RAY SPECTRA; Blackbody; Neutron stars; Accretion; X-rays: binaries; Schwarzschild metric; Binary X ray source; Cosmic x-ray sources; Luminosity; X-ray spectra; Relativistic effect; Accretion disks; stars: neutron; Continuum; Reflection spectrum; X-ray binary stars; Low-mass stars; K line; Models; Modelling; X ray pulsar; Emission line; Millisecond pulsar; Boundary layers; X ray emission
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/720/1/205

A number of neutron star low-mass X-ray binaries (LMXBs) have recently been discovered to show broad, asymmetric Fe K emission lines in their X-ray spectra. These lines are generally thought to be the most prominent part of a reflection spectrum, originating in the inner part of the accretion disk where strong relativistic effects can broaden emission lines. We present a comprehensive, systematic analysis of Suzaku and XMM-Newton spectra of 10 neutron star LMXBs, all of which display broad Fe K emission lines. Of the 10 sources, 4 are Z sources, 4 are atolls, and 2 are accreting millisecond X-ray pulsars (also atolls). The Fe K lines are fit well by a relativistic line model for a Schwarzschild metric, and imply a narrow range of inner disk radii (6-15 GM/c {sup 2}) in most cases. This implies that the accretion disk extends close to the neutron star surface over a range of luminosities. Continuum modeling shows that for the majority of observations, a blackbody component (plausibly associated with the boundary layer) dominates the X-ray emission from 8 to 20 keV. Thus it appears likely that this spectral component produces the majority of the ionizing flux that illuminates the accretion disk. Therefore, we also fit the spectra with a blurred reflection model, wherein a blackbody component illuminates the disk. This model fits well in most cases, supporting the idea that the boundary layer illuminates a geometrically thin disk.