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Low-collisionality plasma in a magnetic field generically develops anisotropyin its distribution function with respect to the magnetic field direction.Motivated by the application to radiation from accretion flows and jets, weexplore the effect of temperature anisotropy on synchrotron emission. We deriveanalytically and provide numerical fits for the polarized synchrotron emissionand absorption coefficients for a relativistic bi-Maxwellian plasma (we do notconsider Faraday conversion/rotation). Temperature anisotropy can significantlychange how the synchrotron emission and absorption coefficients depend onobserving angle with respect to the magnetic field. The emitted linearpolarization fraction does not depend strongly on anisotropy, while the emittedcircular polarization does. We apply our results to black hole imaging of SgrA* and M87* by ray-tracing a GRMHD simulation and assuming that the plasmatemperature anisotropy is set by the thresholds of kinetic-scaleanisotropy-driven instabilities. We find that the azimuthal asymmetry of the230 GHz images can change by up to a factor of 3, accentuating ($T_\perp >T_\parallel$) or counteracting ($T_\perp < T_\parallel$) the image asymmetryproduced by Doppler beaming. This can change the physical inferences fromobservations relative to models with an isotropic distribution function, e.g.,by allowing for larger inclination between the line of sight and spin directionin Sgr A*. The observed image diameter and the size of the black hole shadowcan also vary significantly due to plasma temperature anisotropy. We describehow the anisotropy of the plasma can affect future multi-frequency and photonring observations. In Appendices we calculate kinetic anisotropy-driveninstabilities (mirror, whistler, and firehose) for relativistically hotplasmas.

Polarized anisotropic synchrotron emission and absorption and its application to Black Hole Imaging