Once-ionized helium in superstrong magnetic fields

It is generally believed that magnetic fields of some neutron stars, the so-called magnetars, are enormously strong, up to 10¹⁴-10 ¹⁵ G. Recent investigations have shown that the atmospheres of magnetars are possibly composed of helium. We calculate the structure and bound-bound radiative transitions of the He⁺ ion in superstrong fields, including the effects caused by the coupling of the ion's internal degrees of freedom to its center-of-mass motion. We show that He⁺ in superstrong magnetic fields can produce spectral lines with energies of up to ≈3 keV, and it may be responsible for absorption features detected recently in the soft X-ray spectra of several radio-quiet isolated neutron stars. Quantization of the ion's motion across a magnetic field results in a fine structure of spectral lines, with a typical spacing of tens of electron volts in magnetar-scale fields. It also gives rise to ion cyclotron transitions, whose energies and oscillator strengths depend on the state of the bound ion. The bound-ion cyclotron lines of He can be observed in the UV-optical range at B ≲ 10¹³ G, and they get into the soft X-ray range at B ≳ 10¹⁴ G.