Abstract
Abstract We have found significant correlations between the arrival directions of the highest energy photons (Eγ > 10 GeV) observed by EGRET and the positions of the BL Lac type objects (BL Lacs). The observed correlations imply that not less than 3 per cent of extragalactic photons at these energies originate from BL Lacs. Some of the BL Lacs that are correlated with the highest energy photons have no counterparts in the EGRET source catalogue, i.e. do not coincide with strong emitters of gamma-rays at lower energy. A study of correlating BL Lacs suggests that they may form a subset which is statistically different from the total BL Lac catalogue; we argue that they are prominent candidates for TeV gamma-ray sources. Our results demonstrate that the analysis of positional correlations is a powerful approach, indispensable in cases when low-number statistics limit or even prohibit the standard case-by-case identification.
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