The formation of helium lines in the spectrum of COM J1740-5340

The He I (lambda)5876 A 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 (gamma)-rays emitted by the pulsarʹs magnetosphere. Numerical calculations, similar to those carried out previously for type Ib SNe, indicate that a pulsar beam with photon energies ~1 MeV gives rise to a (lambda)5876 A line of the observed strength if the beamʹs spin-down conversion efficiency approaches 1%. However, a significant difficulty for the proposed mechanism is the strength of the singlet line at (lambda)6678 A. Compared to the corresponding triplets, singlet lines are weak because of the loss of excitation when photons emitted in the series n1P0 (longrightarrow) 1(1)s ionize hydrogen atoms, an effect absent in the hydrogen-free atmospheres of type Ib SNe.