really observed neutron stars (pulsars) are determined with a dispersion of values. To estimate expected optical properties of these objects (in a quiet state — before or after the burst), we should choose also a definite temperature of the thermal emission from the whole surface of such a compact object.

The limitation on the temperature $T_{bb}$ on the whole surface of the compact object for an analogous model (a “cold” single neutron star as a potential source of [$\gamma$]{}–ray burst) in connection with deep X-ray EXOSAT observations of [$\gamma$]{}–ray boxes has been already used in the interpretation of optical observations of the supposed candidates near the [$\gamma$]{}–ray box GRB 790325b (Hartmann et al., 1989; Motch et al., 1990). There is already quite a number of papers limiting the neutron star surface temperature by observations not only with the EINSTEIN and EXOSAT satellites (Pizzichini et al., 1986; Boer et al., 1988; Boer et al., 1991), but also by later ROSAT data in the soft X–rays (Boer et al., 1993b) (see also references in Boer et al., 1993a; Greiner et al., 1995). Here we use the results of ROSAT observations in the soft X–ray range (0.1 - 2.4 keV) for the same 