Gamma-Ray Sources Observed by Cos-B

The nature of the fine-scale structure in the gamma-ray distribution is not yet disclosed. Considerable debate is going on whether these structures which appear point-like in the data, are mainly diffuse in nature or are genuinely compact objects. Most of the uncertainty is due to the experimental limitations. A status report is presented on the experimental study of the fine-scale structure measured by COS-B in the energy range 50 MeV-5 GeV. All identified gamma-ray sources are discussed: (i) the temporal and spectral characteristics of the radio pulsars PSR053+21 and PSR0833-45; (ii) COS-B upper limits on the gamma-ray flux from the binary system Cyg X-3; (iii) the ϱ Oph molecular cloud now shown to be resolved in gamma rays as is the case for the Orion complex; (iv) the evidence on the detection of 3C273 in three COS-B observations. The 2CG catalogue of high-energy (point-like) gamma-ray sources contains 25 sources, of which 21 are not yet unambiguously identified. Their average properties are discussed. The error region of the unidentified source 2CG195+04 (Geminga) is studied at other wavelengths in greatest detail. This search for a counterpart is summarized, showing the possibility that a nearby (≲, 100 pc) neutron star is the counterpart. In the appendix is presented the cross-correlation method which is applied in the search for gamma-ray sources, as well as the appearances of the sources in the data.