Is Geminga a very close neutron star binary?

Abstract

Geminga (2CG195+4) is one of the brightest sources in the sky at photon energies around 1 GeV (refs 1–3). It has been tentatively identified4 with an unusual soft X-ray and optical object for which the γ-ray-to-X-ray-to-optical luminosity ratios are ∼106:103:1, which may now have been confirmed by the discovery5 of a ∼60-s periodic variation in the X-ray source, similar to that reported in γ rays. The period has been increasing over the past 10 yr at a rate such that it doubles in ∼400–800 yr. The implied rotational energy loss would supply the γ-ray luminosity of Geminga if the spinning object were a neutron star at a distance of ∼10 pc, consistent with the very low observed X-ray absorption. A lone pulsar with a spin period of ∼60 s cannot spin down at the required rate. The observed period would need to be due to precession or some other process. We show here that the presence of a close neutron star companion provides a suitable mechanism. All neutron star binaries may evolve through a Geminga-like phase.