Detectability of ONSs

The detection of ONSs is a long-sought goal of X-ray astronomy such that it was included as a possible target for the Einstein mission, but the first systematic study of the observability of these sources with ROSAT was carried out by Treves & Colpi (1991), who found that, in the most favourable case of polar cap accretion, thousands of ONSs should appear in the ROSAT PSPC All Sky Survey. A complete analysis by Blaes & Madau (1993) and Zane et al. (1995) essentially confirmed the results of Treves & Colpi (1991).