Results of a LMXB survey: Variation in the height of the neutron star blackbody emission region

We present results of a survey of tile spectra of LOW Mass X-ray Binaries u sing ASCA. It is shown that all sources in the survey are well-fitted by th e same two-component emission model that we have previously shown is able t o describe bath the non-dip and dip spect rs of the dipping class of LMXB. This model consists of point-like blackbody emission from the neutron star plus Comptonized emission from a disk-like accretion disk corona of radius typically 50 000 km, Additional data from results published elsewhere by us from BeppoSAX and ASCA are added to the survey. The lar-ge variation in bl ackbody luminosity of survey sources is shown to be due primarily to major changes in blackbody emitting area. Fitting a multi-temperature disk blackb ody plus Comptonization model to the survey spectra requires values of inne r disk radius substantially less then the neutron star radius in many cases , making disk origin of the blackbody highly unlikely. Assuming that the em ission is from an equatorial strip on the neutron star, it: is shown that t he half-height of the strip h agrees well with the half-height H of the rad iatively-supported inner accretion disk, this agreement spanning three orde rs of magnitude in each parameter. Possible mechanisms for the agreement ar e discussed, including radial accretion flow between inner disk and star, a nd accretion how "creep" on the surface of the neutron star.