Restrictions on the parameters of boundary layer and accretion disk of X-ray bursters in the low state

Using ASCA observations of several X-ray bursters in the low spectral state (L-x similar to 1-5 x 10(36) erg/s) restrictions on the parameters of the accretion disk and the boundary layer were obtained. The low state spectra of X-ray bursters observed with ASCA often can be well described by a power law with photon index of 1.7-2.2 and are not consistent with significant c ontribution of the soft spectral component to the total luminosity. For a b lackbody spectrum with temperature of 0.2-2 keV the upper limits correspond to < 10-20% of the total 0.5-10 keV luminosity. On the other hand, theoret ical calculations predict that in the case of a Keplerian accretion disk ar ound a slowly rotating neutron star with radius of 3R(g) similar to 2/3 of the total gravitational energy released in the system can be released in th e boundary layer between the inner part of the accretion disk and the neutr on star surface. More accurate analysis of the ASCA spectra shows that: (1) The inner radius of the standard geometrically thin optically thick part of the accretion d isk exceeds R-in >10-25R(g). (2) The characteristic temperature in the inne r part of the accretion Row including the boundary layer exceeds similar to 2 keV independently upon the assumption about the density in the accretion flow. The electron scattering gives the dominant contribution to the opaci ty in this region.