Performance characterization of the Hard X-ray Photoelectric Polarimeter

With the successful launch of the Imaging X-ray Polarimeter Explorer (IXPE; sensitive between 2-8 keV) in December 2021 the X-ray polarimetry window to the universe has finally been opened. We expect that IXPE will raise as many scientific questions, requiring higher sensitivity and broader bandpass, as it will answer. Polarimetric observations above 10 keV have the potential to pin down the accretion geometry and emission mechanism in accretion-powered pulsars, provide the most stringent constraints on the origin and location of high-energy emission of rotation-powered pulsars, and constrain the coronal properties in active galactic nuclei.

As part of an on-going program of X-ray polarimeter development at NASA Goddard Space Flight Center, we have built the Hard X-ray Photoelectric Polarimeter (HXPP) designed for astrophysical observations in the 10-60 keV band. HXPP’s detection method is based on the photoelectric effect and a gas micro-pattern time projection chamber technique. This allows us to tune polarimeter sensitivity to the bandpass of interest by selection of gas type and gas pressure. Here we will report on our efforts to choose the HXPP’s optimal operating parameters, its performance tests in the laboratory test bed, the first experimental results from the calibration campaigns at the Argonne National Laboratory’s Advanced Photon Source synchrotron beam and our plans for the future. Our ultimate goal is to enable future balloon- and space-borne missions to make polarization sensitive observations above 10 keV of the persistent X-ray emission produced in the regions of extreme gravitational and magnetic fields surrounding compact objects.