Dynamically important magnetic fields near the event horizon of Sgr A*

TL;DR

This study models the polarized near-infrared emission from a hotspot orbiting Sgr A* to infer the magnetic field structure near the event horizon, revealing dynamically important poloidal magnetic fields.

Contribution

It introduces a relativistic ray-tracing model incorporating time-dependent instrument calibration to interpret polarimetric data of Sgr A* flares, highlighting the role of magnetic field geometry.

Findings

The linear polarization angle rotates during the flare.

A poloidal magnetic field component is necessary to match the observed period.

The observed polarization fraction is lower than model predictions, indicating beam depolarization.

Abstract

We study the time-variable linear polarisation of Sgr A* during a bright NIR flare observed with the GRAVITY instrument on July 28, 2018. Motivated by the time evolution of both the observed astrometric and polarimetric signatures, we interpret the data in terms of the polarised emission of a compact region ('hotspot') orbiting a black hole in a fixed, background magnetic field geometry. We calculated a grid of general relativistic ray-tracing models, created mock observations by simulating the instrumental response, and compared predicted polarimetric quantities directly to the measurements. We take into account an improved instrument calibration that now includes the instrument's response as a function of time, and we explore a variety of idealised magnetic field configurations. We find that the linear polarisation angle rotates during the flare, which is consistent with previous results. The hotspot model can explain the observed evolution of the linear polarisation. In order to match the astrometric period of this flare, the near horizon magnetic field is required to have a significant poloidal component, which is associated with strong and dynamically important fields. The observed linear polarisation fraction of $\simeq 30\%$ is smaller than the one predicted by our model ($\simeq 50\%$). The emission is likely beam depolarised, indicating that the flaring emission region resolves the magnetic field structure close to the black hole.