Polarimetric signatures of hot spots in black hole accretion flows

Abstract

Context. The flaring events observed in the Sagittarius A* supermassive black hole system can be attributed to the nonhomogeneous nature of the near-horizon accretion flow. Bright regions in this flow may be associated with density or temperature anisotropies, corresponding to so-called “bright spots” or “hot spots.” Such orbiting features may explain observations at infrared wavelengths, as well as recent findings at millimeter wavelengths. Aims. In this work, we study the emission from an orbiting equatorial bright spot, imposed on a radiatively inefficient accretion flow background, to find polarimetric features indicative of the underlying magnetic field structure and other system variables, including inclination angle, spot size, black hole spin, and more. Specifically, we investigate the impact of these parameters on the Stokes 𝒬 − 𝒰 signatures that commonly exhibit a typical double loop (pretzel-like) structure. Methods. Our semi-analytical model, describing the underlying plasma conditions and the orbiting spot, is built within the framework of the numerical radiative transfer code ipole, which calculates synchroton emission at 230 GHz. Results. We showcase the wide variety of 𝒬 − 𝒰 loop signatures and the relation between inner and outer loops. For the vertical magnetic field topology, the inner 𝒬 − 𝒰 loop is explained by the suppression of the synchrotron emission as seen by the distant observer. For the radial and toroidal magnetic field topologies, the inner 𝒬 − 𝒰 loop corresponds to the part of the orbit where the spot is receding with respect to the observer. Conclusions. Based on our models, we conclude that it is possible to constrain the underlying magnetic field topology with an analysis of the 𝒬 − 𝒰 loop geometry, particularly in combination with circular polarization measurements.