Near-infrared polarimetry as a tool for testing properties of accreting super-massive black holes

TL;DR

This paper explores how near-infrared and X-ray polarimetry can be used to determine key properties like spin and inclination of accreting supermassive black holes, using hot spot models and new time-lag analysis methods.

Contribution

It introduces a novel method for analyzing polarization time-lags in hot spot models and demonstrates its potential to constrain black hole parameters with current and future polarimetric data.

Findings

Polarization signatures predicted to be measurable soon.

Spin and inclination of Sgr A* can be constrained with current data.

Method shows promise for future high-precision polarimetry.

Abstract

Several massive black holes exhibit flux variability on time scales that correspond to source sizes of the order of few Schwarzschild radii. We survey the potential of near-infrared and X-ray polarimetry to constrain physical properties of such black hole systems, namely their spin and inclination. We have focused on a model where an orbiting hot spot is embedded in an accretion disk. A new method of searching for the time-lag between orthogonal polarization channels is developed and applied to an ensemble of hot spot models that samples a wide range of parameter space. We found that the hot spot model predicts signatures in polarized light which are in the range to be measured directly in the near future. However, our estimations are predicted upon the assumption of a Keplerian velocity distribution inside the flow where the dominant part of the magnetic field is toroidal. We also found that if the right model of the accretion flow can be chosen for each source (e.g. on the basis of magnetohydrodynamics simulations) then the black hole spin and inclination can be constrained to a small two-dimensional area in the spin-inclination space. The results of the application of the method to the available near-infrared polarimetric data of Sagittarius A* (Sgr A*) is presented. It is shown that even with the currently available data the spin and inclination of Sgr A* can be constrained. Next generations of near-infrared and X-ray polarimeters should be able to exploit this tool.