  Chandra/HETG observations of the recurrent nova RS Ophiuchi at day 13.9 of
its 2006 outburst reveal a spectrum covering a large range in plasma
temperature and characterized by asymmetric and blue-shifted emission lines. We
investigate the origin of these asymmetries and broadening of emission lines.
We perform 3-D hydrodynamic simulations of the blast wave from the 2006
outburst, propagating through the inhomogeneous CSM. The model takes into
account the thermal conduction (including the effects of heat flux saturation)
and the radiative cooling. From the simulations, we synthesize the X-ray
emission and derive the spectra as they would be observed with Chandra/HETG.
Our model reproduces the observed X-ray emission in a natural way if the CSM in
which the outburst occurred is characterized by an equatorial density
enhancement. Our ``best-fit'' model predicts that most of the early X-ray
emission originates from a small region propagating in the direction
perpendicular to the line-of-sight and localized just behind the interaction
front between the blast wave and the equatorial density enhancement. The model
predicts asymmetric and blue-shifted line profiles remarkably similar to those
observed. These asymmetries are due to substantial X-ray absorption of
red-shifted emission by ejecta material. The comparison of high quality data of
Chandra/HETG with detailed hydrodynamic modeling has allowed us to unveil, for
the first time, the details of the structure emitting in the X-ray band in
early phases of the outburst evolution, contributing to a better understanding
of the physics of interactions between nova blasts and CSM in recurrent novae.
This may have implications for whether or not RS Ophiuchi is a Type Ia SN
progenitor system.
