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Title:
Modelling mid-Z element atmospheres for strongly magnetized neutron stars
Authors:
Mori, Kaya; Ho, Wynn C. G.
Affiliation:
AA(Department of Astronomy and Astrophysics, University of Toronto, 50 St George Street, Toronto, Ontario, Canada M5S 3H4; Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George Street, Toronto, Ontario, Canada M5S 3H8), AB(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA; Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 377, Issue 2, pp. 905-919. (MNRAS Homepage)
Publication Date:
05/2007
Origin:
MNRAS
MNRAS Keywords:
atomic processes , magnetic fields , stars: atmospheres , stars: neutron
Abstract Copyright:
(c) 2007 The Authors. Journal compilation © 2007 RAS
DOI:
10.1111/j.1365-2966.2007.11663.x
Bibliographic Code:
2007MNRAS.377..905M

Abstract

We construct models for strongly magnetized neutron star atmospheres composed of mid-Z elements (carbon, oxygen and neon) with magnetic fields B = 1012-1013 G and effective temperatures Teff = (1 - 5) × 106 K; this is done by first addressing the physics relevant to strongly magnetized plasmas and calculating the equation of state and polarization-dependent opacities. We then obtain the atmosphere structure and spectrum by solving the radiative transfer equations in hydrostatic and radiative equilibrium. In contrast to hydrogen opacities at the relevant temperatures, mid-Z element opacities are dominated by numerous bound-bound and bound-free transitions. Consequently, temperature profiles are closer to grey profiles, and photosphere densities are lower than in the hydrogen case. Mid-Z element atmosphere spectra are significantly softer than hydrogen atmosphere spectra and show numerous absorption lines and edges. The atmosphere spectra depend strongly on surface composition and magnetic field but weakly on surface gravity. Absorption lines are primarily broadened by motional Stark effects and the (unknown) surface magnetic field distribution. When magnetic field variation is not severe, substructure in broad absorption features can be resolved by (phase-resolved) CCD spectroscopy from Chandra and XMM-Newton. Given the multiple absorption features seen in several isolated neutron stars (INSs), it is possible to determine the surface composition, magnetic field, temperature and gravitational redshift with existing X-ray data; we present qualitative comparisons between our model spectra and the neutron stars 1E1207.4-5209 and RX J1605.3+3249. Future high-resolution X-ray missions such as Constellation-X will measure the gravitational redshift with high accuracy by resolving narrow absorption features; when combined with radius measurements, it will be possible to uniquely determine the mass and radius of INSs.
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