  POLAR is a novel compact Compton X-ray polarimeter designed to measure the
linear polarization of the prompt emission of Gamma Ray Bursts (GRB) and other
strong transient sources such as soft gamma repeaters and solar flares in the
energy range 50-500 keV. A detailed measurement of the polarization from
astrophysical sources will lead to a better understanding of the source
geometry and emission mechanisms. POLAR is expected to observe every year
several GRBs with a minimum detectable polarization smaller than 10%, thanks to
its large modulation factor, effective area, and field of view. POLAR consists
of 1600 low-Z plastic scintillator bars, divided in 25 independent modular
units, each read out by one flat-panel multi-anode photomultiplier. The design
of POLAR is reviewed, and results of tests of one modular unit of the
engineering and qualification model (EQM) of POLAR with synchrotron radiation
are presented. After construction and testing of the full EQM, we will start
building the flight model in 2011, in view of the launch foreseen in 2013.
