A Large-Area Optical Time Projection Chamber for Hard X-ray Polarimetry with Directional Imaging of Low-Energy Electron Recoils

We report on the development of a large-volume, wide field-of-view time projection chamber (TPC) for X-ray polarimetry, featuring a triple-GEM amplification stage and optical readout. Originally developed within the CYGNO program for directional dark matter searches, the system employs a scientific CMOS (sCMOS) camera and a photomultiplier tube (PMT) to collect secondary scintillation light produced during charge amplification. A prototype with a cylindrical active volume (radius 3.7 cm, height 5 cm) was tested at the INAF--IAPS calibration facility (Rome, Tor Vergata) to assess sensitivity to low-energy electron directionality. We fully reconstruct electrons in the 10-60 keV range, obtain angular resolutions as good as 15{\deg}, and infer modulation factors up to 0.9. These first results demonstrate robust photoelectron tracking at tens of keV with strong modulation, indicating that photoelectric-effect polarimetry can be extended to higher energies. This capability is promising for rapid transients (GRBs, solar flares) and would broaden the astrophysical reach of X-ray polarimetry.