Evidence of a mid-atomic number atmosphere in the neutron star 1E 1207.4-5209

Recently, Sanwal et al. reported the first clear detection of absorption features in an isolated neutron star, 1E 1207.4-5209. Remarkably, their spectral modeling demonstrates that the atmosphere cannot be hydrogen. They speculated that the neutron star atmosphere is indicative of ionized helium in an ultrastrong (similar to1.5 x 10(14) G) magnetic field. We have applied our recently developed atomic model (Mori & Hailey) for strongly magnetized neutron star atmospheres to this problem. We find that this model, along with some simple atomic physics arguments, severely constrains the possible composition of the atmosphere. In particular, we find that the absorption features are naturally associated with He-like oxygen or neon in a magnetic field of similar to10(12) G, comparable to the magnetic field derived from the spin parameters of the neutron star. This interpretation is consistent with the relative line strengths and widths and is robust. Our model predicts possible substructure in the spectral features, which has now been reported by XMM-Newton (Mereghetti et al.). However, we show that the Mereghetti et al. claim that the atmosphere is iron or some comparable high-Z element at similar to10(12) G is easily ruled out by the Chandra and XMM-Newton data.