ipole - semianalytic scheme for relativistic polarized radiative transport

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We describe ipole, a new public ray-tracing code for covariant, polarized radiative transport. The code extends the ibothros scheme for covariant, unpolarized transport using two representations of the polarized radiation field: in the coordinate frame, it parallel transports the coherency tensor; in the frame of the plasma it evolves the Stokes parameters under emission, absorption, and Faraday conversion. The transport step is implemented to be as spacetime- and coordinate- independent as possible. The emission, absorption, and Faraday conversion step is implemented using an analytic solution to the polarized transport equation with constant coefficients. As a result, ipole is stable, efficient, and produces a physically reasonable solution even for a step with high optical depth and Faraday depth. We show that the code matches analytic results in flat space, and that it produces results that converge to those produced by Dexter's grtrans polarized transport code on a complicated model problem. We expect ipole will mainly find applications in modeling Event Horizon Telescope sources, but it may also be useful in other relativistic transport problems such as modeling for the IXPE mission.