A High-Precision Optical Polarimeter to Measure Inclinations of High-Mass X-Ray Binaries

We present commissioning data for the POLISH instrument obtained on the Hale 5-m telescope. The goal of this high-precision polarimeter is to constrain orbital inclination of high-mass X-ray binaries and therefore to obtain independent mass estimates for their black-hole companions. We have obtained photon shot-noise limited precision on standard stars, and we have measured the polarization of bright stars at the part-per-million level on a nightly basis. Systematic effects have been reduced to less than 1% of the measured polarization for polarized sources and to the part-per-million level for weakly polarized sources. The high sensitivity of this instrument to asymmetry suggests that valuable contributions will be made in many other fields, including studies of extrasolar planets, debris disks, and stellar astrophysics.