γ-Rays from γ Geminorum?

Abstract

THE γ-ray source CG195+4 (Geminga) discovered by SAS 2 and further observed by COS B has a period of 59 s which is increasing at a rate Ṗ = 2×10⁻⁹ (refs 1,2). If the periodicity is produced by a neutron star which is spinning down at the observed rate, then the rate at which rotational energy is being lost is L ∼ 3×10³² erg s⁻¹ (ref. 3). The observed γ-ray flux of 3×10⁻⁶ photon cm⁻² s⁻¹ with energy E>100 MeV implies that the distance, d, to the source is given by d ≃ 73f^1/2 pc where f(≳ 1) is the fraction of the total energy output which is emitted as γ-rays. Theoretical studies of the evolution of binary systems which give rise to the binary X-ray sources have led to the suggestion that, before the neutron star starts to accrete and spin up (that is, before the X-ray source turns on) it spends a comparatively long time being spun down in the weak stellar wind of its companion⁴. This leads us to seek a candidate companion for the γ-ray source which is an early type star at a distance of ≳ 70 pc. As we report here, the only object which satisfies these constraints is γ Geminorum.