Polarization Signatures of Inspiraling Hotspots around Kerr Black Holes

TL;DR

This paper develops a framework to simulate polarized emission from inspiraling hotspots around Kerr black holes, revealing unique polarization signatures that can help probe black hole accretion physics.

Contribution

It introduces a parametric model for inspiraling hotspots in Kerr spacetime, extending standard assumptions and predicting distinctive polarization signatures.

Findings

Inspiral motion produces a precessing, unwinding Q-U polarization pattern.

The polarization signatures depend on black hole spin, inclination, and magnetic field.

The model can be used with current and future interferometric polarization observations.

Abstract

Polarimetric interferometry is a powerful tool for probing both black hole accretion physics and the background spacetime. Current models aimed at explaining the observed multiwavelength flares in Sgr A* often assume hotspots moving on geodesic, Keplerian orbits. In many scenarios, though, a hotspot may instead follow an inspiraling trajectory, potentially transitioning into a plunge toward the black hole. In this work, we present a general framework to simulate the polarized emission from generic equatorial inspiraling hotspots in Kerr spacetime using a parametric four-velocity profile. This parametrization defines a continuous family of flows, ranging from Cunningham's disk model (fixed radius orbits outside the innermost stable circular orbit and plunging motion within the innermost stable circular orbit) to purely radial motion, thereby extending the standard assumptions. Within this framework, we show that inspiral motion produces a distinctive observational signature: a precessing, unwinding evolution of the polarimetric Stokes Q-U looping pattern, in sharp contrast with the closed Q-U loops associated with stable orbits at a fixed radius. We then explore how the morphology of these signatures depends on black hole spin, observer inclination, and magnetic-field configuration. The presented model can be applied to current and near-future interferometric observations of linear polarization, offering a new avenue to probe the physics of matter spiraling inward and the relativistic velocities of plunging plasma.