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

Astrophysics Group, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK

Corresponding author: l.lucy@imperial.ac.uk

Received: 7 May 2003
Accepted: 29 July 2003

Abstract

The Hei λ5876 Å 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 ~1 MeV gives rise to a λ5876 Å 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 λ6678 Å. Compared to the corresponding triplets, singlet lines are weak because of the loss of excitation when photons emitted in the series ionize hydrogen atoms, an effect absent in the hydrogen-free atmospheres of type Ib SNe.