  We present a systematic measurement of the X-ray emission asymmetries in the
K-shell dielectronic, trielectronic, and quadruelectronic recombination of free
electrons into highly charged ions. Iron ions in He-like through O-like charge
states were produced in an electron beam ion trap, and the electron-ion
collision energy was scanned over the recombination resonances. Two identical
X-ray detectors mounted head-on and side-on with respect to the electron beam
propagation recorded X-rays emitted in the decay of resonantly populated
states. The degrees of linear polarization of X-rays inferred from observed
emission asymmetries benchmark distorted-wave predictions of the Flexible
Atomic Code (FAC) for several dielectronic recombination satellite lines. The
present method also demonstrates its applicability for diagnostics of energy
and direction of electron beams inside hot anisotropic plasmas. Both
experimental and theoretical data can be used for modeling of hot astrophysical
and fusion plasmas.
