Abstract
We study the time-variable linear polarisation of Sgr A* during a bright near-infrared 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 ≃30% is smaller than the one predicted by our model (≃50%). The emission is likely beam depolarised, indicating that the flaring emission region resolves the magnetic field structure close to the black hole.
Highlights
Using precision astrometry with the second generation beam combiner instrument GRAVITY at the Very Large Telescope Interferometer (VLTI) operating in the NIR (Gravity Collaboration 2017), we recently discovered continuous clockwise motion that is associated with three bright flares from Sgr A* (Gravity Collaboration 2018b, 2020b)
We forward modelled Q and U Stokes parameters obtained from ray-tracing calculations of a variety of hotspot models in different magnetic field geometries, transformed them into quantities as seen by the instrument, and fitted them directly to the polarised data taken with GRAVITY
We have shown that the hotspot model serves to qualitatively reproduce the features seen in the polarisation data measured with GRAVITY
Summary
There is overwhelming evidence that the Galactic Centre harbours a massive black hole, Sagittarius A* We take intrinsic Stokes parameters Q and U from numerical calculations of a hotspot orbiting a black hole in a given magnetic field geometry, transform them to the GRAVITY observables Q and U following Eq (1), and compare them to the data. This allows us to fit the July 28 polarisation data directly without having to make assumptions on Stokes V or interpolate between gaps of data due to the lack of simultaneous measurements of the Stokes parameters, but to make predictions for Q when it is the only quantity measured, as is the case for the other 2018 flares. The flare traces 1.5 loops during its 60−70 min evolution
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