Spectral Features in the Thermal Emission from Isolated Neutron Stars: Dependence on Magnetic Field Strengths

We study several effects that influence the strength of the proton cyclotron and atomic features in the thermal spectra of magnetic neutron stars. We show that it is possible for vacuum polarization to strongly suppress the spectral lines when the magnetic field B\ga 10^14 G. For weaker fields (B\la 7\times 10^13 G), the surface spectrum is unaffected by vacuum polarization; thus the proton cyclotron absorption line can have a large equivalent width. Using an approximate calculation of the synthetic spectra, we show that variation of magnetic fields over the neutron star surface leads to broadening of the line. The recently detected absorption features from the isolated neutron stars RX J1308.6+2127, RX J1605.3+3249, and RX J0720.4-3125 can plausibly be explained by the proton cyclotron resonance, with possible blending due to atomic transitions, in the atmosphere of the star.