Discovery of a relation between the height of the radiatively-supported disk and the blackbody luminosity of LMXB

We present results of a survey of the spectra of the Atoll and Z-track classes of Low Mass X-ray Binaries based on ASCA data. It is shown that all sources in the survey are well-fitted by the same two-component emission model that we have previously shown is able to describe both the non-dip and dip spectra of the dipping class of LMXB. This model consists of point-like blackbody emission which we have associated with the neutron star plus extended Comptonized emission probably from the accretion disk corona. In all sources studied, a blackbody component is required. We demonstrate that in most cases the Comptonization cut-off energy is much higher than 10 keV. A previously unknown relationship between the blackbody luminosity and the total luminosity is revealed such that in the brighter sources, the blackbody luminosity approaches 50% of the total, whereas in less bright sources, it decreases to a smaller fraction. Additional data points from results previously published by us from BeppoSAX and ASCA data support this result. As the total luminosity increases the blackbody emitting area increases by a factor of ∼700 while the blackbody temperature decreases by ∼40% so that T decreases by less than one order of magnitude. Thus the large changes in blackbody luminosity are shown to be due primarily to major changes in blackbody emitting area, the change in temperature having much less effect. Assuming that the emission is from an equatorial strip on the neutron star, it is shown that the half-height of the strip agrees well with the half-height of the radiatively-supported inner accretion disk, this agreement spanning three orders of magnitude in each parameter. This strongly suggests a simple geometric explanation of the level of blackbody emission in low mass X-ray binaries in which the disk height determines the emitting area on the star thus determining the emission level. This implies that matter flows readily between the inner accretion disk and the star at all vertical positions above the orbital plane. Our results strongly support the neutron star as the origin of the blackbody emission and it is shown that the results are inconsistent with a blackbody origin in the accretion disk. Subject headings: accretion, accretion disks — binaries: close — stars: circumstellar matter — X-rays: