The second COS-B catalogue of high-energy y-ray sources

Abstract

The detection by COS-B of 13 high-energy y-ray sources (Hermsen et al. 1977) showed for the first time that copious y-ray emission is produced in numerous localized regions. The mysterious nature of those objects has persisted owing to a lack of unambiguous identification, except for PSR 0531 + 21 and PSR 0833 - 45. COS-B has continued to operate successfully since its launch in August 1975 and sufficient data have now been accumulated to permit a more systematic search for y-ray sources in the Galaxy. The results of the survey provide the basis for unbiased investigation of the y-ray sources which is the first requirement for the derivation of the main characteristics of this population . The 32 observations, each typically of one month duration, used for this investigation were made in the period August 1975 to December 1978. The region of the sky covered is shown in figure 1. The angular resolution of COS-B below 100 MeV is inadequate to maintain a sufficiently uniform source visibility throughout the Galaxy because of the complex structure of the galactic y-ray emission, particularly in the intense regions of the inner Galaxy (Mayer Hasselwander et al . 1980). Therefore, only events of measured energy above 100 MeV have been used in this search. The measured arrival directions of the y-rays have been sorted into 0.5° x 0.5° bins. The resulting sky-map was analysed by using a cross-correlation method in which the distribution of the photon arrival directions was correlated with the distribution expected for a point source. This latter distribution, the intrinsic point-spread function of the instrument, was determined by calibration and confirmed by the flight data for the strong source PSR 0833 — 45. A y-ray source is thus defined as a significant excess which has a spatial distribution consistent with the point-spread function. Pertinent aspects of the complete analysis have been presented by Hermsen (1980).