  We investigated the complex index of refraction in the x-ray regime of atoms
in laser light. The laser (intensity up to 10^13 W/cm^2, wavelength 800nm)
modifies the atomic states but, by assumption, does not excite or ionize the
atoms in their electronic ground state. Using quantum electrodynamics, we
devise an ab initio theory to calculate the dynamic dipole polarizability and
the photoabsorption cross section, which are subsequently used to determine the
real and imaginary part, respectively, of the refractive index. The interaction
with the laser is treated nonperturbatively; the x-ray interaction is described
in terms of a one-photon process. We numerically solve the resolvents involved
using a single-vector Lanczos algorithm. Finally, we formulate rate equations
to copropagate a laser and an x-ray pulse through a gas cell. Our theory is
applied to argon. We study the x-ray polarizability and absorption near the
argon K edge over a large range of dressing-laser intensities. We find
electromagnetically induced transparency (EIT) for x rays on the Ar 1s --> 4p
pre-edge resonance. We demonstrate that EIT in Ar allows one to imprint the
shape of an ultrafast laser pulse on a broader x-ray pulse (duration 100ps,
photon energy 3.2keV). Our work thus opens new opportunities for research with
hard x-ray sources.
