First M87 Event Horizon Telescope Results. VII. Polarization of the Ring
  In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon
region around the supermassive black hole at the core of the M87 galaxy. These
1.3 mm wavelength observations revealed a compact asymmetric ring-like source
morphology. This structure originates from synchrotron emission produced by
relativistic plasma located in the immediate vicinity of the black hole. Here
we present the corresponding linear-polarimetric EHT images of the center of
M87. We find that only a part of the ring is significantly polarized. The
resolved fractional linear polarization has a maximum located in the southwest
part of the ring, where it rises to the level of about 15%. The polarization
position angles are arranged in a nearly azimuthal pattern. We perform
quantitative measurements of relevant polarimetric properties of the compact
emission and find evidence for the temporal evolution of the polarized source
structure over one week of EHT observations. The details of the polarimetric
data reduction and calibration methodology are provided. We carry out the data
analysis using multiple independent imaging and modeling techniques, each of
which is validated against a suite of synthetic data sets. The gross
polarimetric structure and its apparent evolution with time are insensitive to
the method used to reconstruct the image. These polarimetric images carry
information about the structure of the magnetic fields responsible for the
synchrotron emission. Their physical interpretation is discussed in an
accompanying publication.
