Detecting Changing Polarization Structures in Sagittarius A* with High Frequency VLBI

Sagittarius A* is the source of near infrared, X-ray, radio, and(sub)millimeter emission associated with the supermassive black hole atthe Galactic Center. In the submillimeter regime, Sgr A* exhibitstime-variable linear polarization on timescales corresponding to 10Schwarzschild radii of the presumed 4 × 10⁶ M_sun black hole. In previous work, we demonstrated thepotential for total-intensity (sub)millimeter-wavelength very longbaseline interferometry (VLBI) to detect time-variable—andperiodic—source structure changes in the Sgr A* black hole systemusing nonimaging analyses. Here, we extend this work to include fullpolarimetric VLBI observations. We simulate full-polarization(sub)millimeter VLBI data of Sgr A* using a hot spot model that isembedded within an accretion disk, with emphasis on nonimagingpolarimetric data products that are robust against calibration errors.Although the source-integrated linear polarization fraction in themodels is typically only a few percent, the linear polarization fractionon small angular scales can be much higher, enabling the detection ofchanges in the polarimetric structure of Sgr A* on a wide variety ofbaselines. The shortest baselines track the source-integrated linearpolarization fraction, while longer baselines are sensitive topolarization substructures that are beam-diluted by connected-elementinterferometry. The detection of periodic variability in sourcepolarization should not be significantly affected even if instrumentalpolarization terms cannot be calibrated out. As more antennas areincluded in the (sub)millimeter-VLBI array, observations with fullpolarization will provide important new diagnostics to help disentangleintrinsic source polarization from Faraday rotation effects in theaccretion and outflow region close to the black hole event horizon.