Evidence for low-energy γ-ray emission close to CG135+1

Abstract

The COS-B source CG135+1 (ref. 1) is an extremely interesting object because of its possible identification with the QSO 0241+622. Radio observations have identified a strong point source within 2 arc s (ref. 2) of QSO 0241+622. X-ray emissions have also been observed from this region of the sky by the Uhuru (ref. 3), SAS-3 (ref. 2) and HEAO-A (ref. 4) telescopes. The HEAO-A (A1) data indicate that there are two soft X-ray sources in this region of the sky, H0241+62 which contains the QSO, and H0235+60 which includes the radio source GT0236+61 (ref. 5). The OSO-7 data in the energy range 1–40 keV (ref. 6) and measurements using the high-energy X-ray experiment on Ariel-V between 0.26 and 1.2 MeV (ref. 7) have indicated that the photon emission from this region of the sky is of an extremely hard nature, with a power law spectral index of γ≈1.0. However, the poor angular resolution of these telescopes make it uncertain whether the high-energy X-ray fluxes are due to the same celestial object. We now report the results of a balloon flight carried out on 8 October 1978 from Palestine, Texas, in which the region of the sky including the COS-B source CG135+1 was scanned by the Milan/Southampton telescope (MISO telescope). A 5σ excess in the counting rate of the telescope in the energy range 0.15–20 MeV was observed. The origin of this low-energy γ-ray emission is compatible with the measured position of CG135+1.