The suppression of pulsar and gamma-ray burst annihilation lines by magnetic photon splitting

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Abstract
Two-photon pair annihilation lines appear in a small percentage of gamma-ray burst observations, and there is recent evidence that one may be present in the Crab pulsar spectrum. Neutron stars, relativistic and compact by nature, show great potential for the copious creation of electron-positron pairs in their magnetospheres; these rapidly cool, thermalize and then annihilate. It is therefore expected that many neutron star sources might display evidence of pair annihilation lines in the 400–500 keV range. Here it is shown that magnetic photon splitting, which operates effectivel at these energies and in the enormous neutron star magnetic fields, can destroy an annihilation feature by absorbing line photons and reprocessing them to lower energies. In so doing, photon splitting creates a soft gamma-ray bump and a broad quasi-power-law contribution to the X-ray continuum, which is too flat to conflict with the observed X-ray paucity in gamma-ray bursts. The destruction of the line occurs in neutron stars with surface fields of 5 × 1012 G or maybe even less, depending on the size of the emission region. This line suppression may not be easily observable in most sources, since it is strongly dependent on the direction of photon motion relative to the field lines. However, the appearance of a line in a source spectrum should provide very useful diagnostic constraints on the radiation propagation direction and the magnetic field strength in the emission region.

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