2003A&A...409..737L

The He I λ5876Å absorption line recently discovered in the spectrum of the companion to the millisecond pulsar PSR J1740-5340 is tentatively attributed to electron impact excitations due to the irradiation of its atmosphere by γ-rays emitted by the pulsar's magnetosphere. Numerical calculations, similar to those carried out previously for type Ib SNe, indicate that a pulsar beam with photon energies ∼1MeV gives rise to a λ5876Å line of the observed strength if the beam's spin-down conversion efficiency approaches 1%. However, a significant difficulty for the proposed mechanism is the strength of the singlet line at λ6678Å. Compared to the corresponding triplets, singlet lines are weak because of the loss of excitation when photons emitted in the series n¹P⁰->1¹S ionize hydrogen atoms, an effect absent in the hydrogen-free atmospheres of type Ib SNe.