The nature of the -ray source 2EG J2020+4026 (2CG078+2)

Abstract

The second EGRET catalogue places the bright γ-ray source 2EG J2020+4026 inside the γ-Cygni supernova remnant. We report the discovery of a point-like X-ray source, denoted RX J2020.2+4026, which lies close to the centre of the remnant and is within the EGRET error box. RX J2020.2+4026 is also close to the position of an m_v = 15 KOV star which could emit the observed X-ray flux. No γ-ray emission would be expected from this star. The stellar association with the X-ray source is, however, by no means certain and offers no simple explanation for 2EG 2020+4026. Assuming that the X-ray object is the counterpart of the γ-ray source, the F_γ/F_x ratio is ∼ 6000, similar to that of Geminga and higher than the ratios found in all sources apart from pulsars. The fluxes from the γ- and X-ray sources are constant. The EGRET γ-ray spectrum has the characteristics typical of young pulsars. A pulsar with the age and distance of the γ-Cygni SNR is also compatible with the γ-ray flux of 2EG 2020+4026. We therefore propose that 2EG 2020+4026 is a pulsar. No radio point source is known at this position and the pulsar may be a Geminga-like, radio-quiet object.