Properties of the ionised plasma in the vicinity of the neutron-star
  X-ray binary EXO 0748-676
  We present the spectral analysis of a large set of XMM-Newton observations of
EXO 0748-676, a bright dipping LMXB. In particular, we focus on the dipping
phenomenon as a result of changes in the properties of the ionised gas close to
the source. Using the high-resolution spectra collected with the RGS, we
explored two simple geometrical scenarios for which we derived physical
quantities of the absorbing material like the density, size, and mass. We find
that the continuum is absorbed by a neutral gas, and by both a collisionally
(temperature T~70 eV) and photoionised (ionisation parameter log\xi~2.5)
absorbers. Emission lines from OVII and OVIII are also detected. This is the
first time that evidence of a collisionally ionised absorber has been found in
a low-mass X-ray binary. The collisionally ionised absorber may be in the form
of dense (n>10^14 cm^-3) filaments, located at a distance r>10^11 cm. During
dips, the photoionised absorber significantly increases its column density
(factor 2--4) while becoming less ionised. This strengthens the idea that the
colder material of the accretion stream impinging the disc is passing on our
line of sight during dips. We find that the distance from the neutron star to
the impact region (~ 5x10^10 cm) is similar to the size of the neutron star's
Roche lobe. The gas observed during the persistent state may have a flattened
geometry. Finally, we explore the possibility of the existence of material
forming an initial, hotter portion of a circumbinary disc.
