The Performance of the TPC polarimeter for PRAXyS

W.B. Iwakiri,  J.K. Black, R. Cole, T. Enoto,  A.Hayato,  J.E. Hill, K. Jahoda, P. Kaaret, T. Kitaguchi, M. Kubota, H. Marlowe, R. McCurdy, Y. Takeuchi, T. Tamagawa

We present the performance of the Time Projection Chamber (TPC) polarimeter for the Polarimeter for Relativistic Astrophysical X-ray Sources (PRAXyS) Small Explorer using polarized and unpolarized X-ray sources.  PRAXyS is designed to make highly sensitive measurement of the linear X-ray polarization of astronomical sources in 2 – 10 keV energy band. Performance tests of the polarimeter were carried out at the Brookhaven National Laboratory, National Synchrotron Light Source (BNL-NSLS) and at NASA’s Goddard Space Flight Center. The polarimeter was tested with linearly polarized, monochromatic X-rays at 11 different energies between 2.5 and 8.0 keV. At maximum sensitivity, the measured modulation factors at 2.7, 4.5 and 8.0 keV are 27%, 43% and 59%, respectively and the measured angle of polarization is consistent with the expected value at all energies. Measurements with a broadband, unpolarized X-ray source placed a limit of less than 1% on false polarization in the PRAXyS polarimeter.