HEAD Division Meeting 1999, April 1999
Session 36. SNRs and Isolated NS
Oral, Thursday, April 15, 1999, 8:30am-10:01am, Colonial Room

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[36.06] Thermal Emission from Strongly Magnetized Neutron Stars

P. Arras, D. Lai (Cornell University)

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 T\gtrsim 105 K and moderately strong magnetic field, B\lesssim 1013 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>> 1013~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 (B\gtrsim 1014~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 {\rm keV}(B/1014 {\rm 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).

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