Journal Article

Neutron star atmosphere composition: the quiescent, low-mass X-ray binary in the globular cluster M28

M. Servillat, C. O. Heinke, W. C. G. Ho, J. E. Grindlay, J. Hong, M. van den Berg and S. Bogdanov

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 423, issue 2, pages 1556-1561
Published in print June 2012 | ISSN: 0035-8711
Published online June 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20976.x
Neutron star atmosphere composition: the quiescent, low-mass X-ray binary in the globular cluster M28

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Using deep Chandra observations of the globular cluster M28, we study the quiescent X-ray emission of a neutron star in a low-mass X-ray binary in order to constrain the chemical composition of the neutron star atmosphere and the equation of state of dense matter. We fit the spectrum with different neutron star atmosphere models composed of hydrogen, helium or carbon. The parameter values obtained with the carbon model are unphysical and such a model can be ruled out. Hydrogen and helium models give realistic parameter values for a neutron star, and the derived mass and radius are clearly distinct depending on the composition of the atmosphere. The hydrogen model gives masses/radii consistent with the canonical values of 1.4 M and 10 km, and would allow for the presence of exotic matter inside neutron stars. On the other hand, the helium model provides solutions with higher masses/radii, consistent with the stiffest equations of state. Measurements of neutron star masses/radii by spectral fitting should consider the possibility of heavier element atmospheres, which produce larger masses/radii for the same data, unless the composition of the accretor is known independently.

Keywords: equation of state; stars: neutron; globular clusters: individual: M28; globular clusters: individual: NGC 6626; X-rays: binaries; X-rays: individual: CXOGlb J182432.8−245208

Journal Article.  4220 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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