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 Lx~1-5x10^{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 contribution of the soft spectral component to the total luminosity. For a blackbody 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, theoretical calculations predict that in the case of a Keplerian accretion disk around a slowly rotating neutron star (with radius of 3Rg) ~2/3 of the total gravitational energy released in the system can be released in the boundary layer between the inner part of the accretion disk and the neutron 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 disk exceeds $R_{in} > 10-25 R_g$. (2) The characteristic temperature in the inner part of the accretion flow including the boundary layer exceeds $\sim$2 keV independently upon the assumption about the density in the accretion flow. The electron scattering gives the dominant contribution to the opacity in this region.