  A study of the luminous dipping LMXB X1624-490 was made using BeppoSAX. From
modelling the MECS radial profile in several energy bands, the optical depth to
dust scattering at 1 keV was found to be 2.4+/-0.4. The non-dip and dip spectra
of X1624-490 in the energy band 1-100 keV are well-described by emission
consisting of a point-like blackbody from the neutron star plus extended
Comptonized emission from an ADC. The blackbody temperature was 1.31+/-0.07 keV
and the Comptonized emission had photon power law index 2.0+0.5/-0.7 and
cut-off energy ~12 keV. Good fits to the dip spectra were obtained by allowing
the Comptonized emission to be progressively covered by an extended absorber
while the blackbody was rapidly absorbed and a constant halo component
accounted for dust scattering into the line-of-sight. From the dip ingress
time, a diameter of the extended Comptonized emission region of 5.3+/-0.8x10^10
cm was derived, consistent with a hot, X-ray emitting corona extending to ~50%
of the accretion disk radius. The source luminosity for a distance of 15 kpc is
7.3x10^37 erg/s, an appreciable fraction of the Eddington limit making
X1624-490 the most luminous dipping LMXB. The half-height of the blackbody
emitting region on the neutron star of 6.8+/-1.8 km agrees well with the
half-height of the radiatively supported inner accretion disk of 6.3+/-2.9 km.
Together with similar agreement recently obtained for 13 other LMXB in an ASCA
survey (Church & Balucinska-Church 2000), this strongly supports the
identification of the neutron star as the origin of the blackbody emission in
LMXB and suggests a mechanism in which the disk height is the main factor
determining the level of blackbody emission. Finally, from RXTE ASM data, we
derive an improved orbital period of 20.87+/-0.01 hr.
