Once-ionized Helium in Superstrong Magnetic Fields

It is generally believed that magnetic fields of some neutron stars, theso-called magnetars, are enormously strong, up to 10^{14} - 10^{15} G. Recentinvestigations have shown that the atmospheres of magnetars are possiblycomposed of helium. We calculate the structure and bound-bound radiativetransitions of the He^+ ion in superstrong fields, including the effects causedby the coupling of the ion's internal degrees of freedom to its center-of-massmotion. We show that He^+ in superstrong magnetic fields can produce spectrallines with energies of up to about 3 keV, and it may be responsible forabsorption features detected recently in the soft X-ray spectra of severalradio-quiet isolated neutron stars. Quantization of the ion's motion across amagnetic field results in a fine structure of spectral lines, with a typicalspacing of tens electron-volts in magnetar-scale fields. It also gives rise toion cyclotron transitions, whose energies and oscillator strengths depend onthe state of the bound ion.Comment: 12 pages, including 3 figures. Submitted to ApJ Letters (revised version