The Future of X-ray Polarimetry

The physical view of the X-ray emission from blazars has improved substantially with recent measurements using the Imaging X-ray Polarimetry Explorer (IXPE). There are several prospects for further advancing our understanding of the processes that occur in blazar jets and the environments in which they propagate. First, deeper observations with IXPE will be useful for examining kpc-scale jets. Next, we have only two years of data tracking secular changes that could indicate long-term evolution or redirection of the jets, so continued campaigns on existing targets are needed. Clearly, instruments with larger effective areas would give us better signal for time-dependent variations on day and intra-day time scales that are just starting to be investigated with IXPE. Instruments with more effective area could detect deviations from uniform polarization angle rotations, providing evidence of relativistic aberration effects in the X-ray emitting regions. Also, larger effective areas give the opportunity to measure energy-dependent polarization, which are predicted in the stratified shock model. Instruments currently in development to operate below 1 keV or above 10 keV could provide critical tests of the stratified shock model in high spectral peak blazars and examine the Compton dominated components in low and intermediate spectral peak blazars. Finally, I will address the challenge of measuring circular polarization in the X-ray band. Funding for this work was provided in part by contract 80MSFC17C0012 from the Marshall Space Flight Center (MSFC) to MIT in support of IXPE, a NASA Astrophysics Small Explorers mission.