Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999head....4.3606a&link_type=abstract
American Astronomical Society, HEAD meeting #4, #36.06; Bulletin of the American Astronomical Society, Vol. 31, p.739
Astronomy and Astrophysics
Astronomy
1
Scientific paper
The outermost layers of some neutron stars are likely to be dominated by light elements such as hydrogen and helium, as a result of fast gravitational settling of heavier elements. These layers directly mediate thermal radiation from the stars, and determine the characteristics of the X-ray/EUV spectra. For a neutron star with surface temperature Tgtrsim 10(5) K and moderately strong magnetic field, B≲ 10(13) G, the envelope is nondegenerate, consisting of magnetized atoms and molecules, and the surface material gradually transforms into a degenerate Coulomb plasma as density increases. For higher field strength, B>> 10(13) G, there exists a first-order phase transition from the nondegenerate gaseous phase to the condensed metallic phase. The column density of saturated vapor above the metallic hydrogen decreases rapidly as the magnetic field increases and/or temperature decreases. Thus the thermal radiation can directly emerge from the degenerate metallic hydrogen surface. We have carried out preliminary study of the X-ray/EUV emission emerging directly from the condensed phase in superstrong magnetic fields (Bgtrsim 10(14) G). Deviations from a blackbody spectrum are expected due to plasma processes. Observable spectral features are possible, such as the proton cyclotron resonance at an energy 0.6 keV(B/10(14) G). In addition, the total efficiency of radiation will be lowered from the blackbody value. These features may provide useful diagnostics for the physical conditions in magnetars (SGRs and anomalous X-ray pulsars).
Arras Phil
Lai Dong
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