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

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.

13 pages, 14 figures, to be published in A&A

References in corpus (12)

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• An Update on Monitoring Stellar Orbits in the Galactic Center
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• Sgr A* near-infrared flares from reconnection events in a magnetically arrested disc
• The Galactic Center cloud G2 and its gas streamer
• Polarized NIR and X-ray Flares from SgrA*
• NuSTAR detection of high-energy X-ray emission and rapid variability from Sagittarius A* flares
• Simultaneous NIR/sub-mm observation of flare emission from SgrA*
• The Submillimeter Polarization of Sgr A*
• The Flux Distribution of Sgr A*
• Polarized light from Sgr A* in the near-infrared $K_s$-band

Cited by in corpus (35)

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• Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*
• Constraining particle acceleration in Sgr A* with simultaneous GRAVITY, Spitzer, NuSTAR and Chandra observations
• Magnetically Modified Spherical Accretion in GRMHD: Reconnection-Driven Convection and Jet Propagation
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• Magnetic reconnection plasmoid model for Sagittarius A* flares
• Reconnection-driven flares in 3D black hole magnetospheres -- A scenario for hot spots around Sagittarius A*
• Deep Images of the Galactic Center with GRAVITY
• The Galactic Center as a laboratory for theories of gravity and dark matter
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• Polarized image of an equatorial emitting ring around a 4D Gauss-Bonnet black hole
• Fitting the light curves of Sagittarius A* with a hot-spot model
• Relative depolarization of the black hole photon ring in GRMHD models of Sgr A* and M87*
• A "coronal-mass-ejection'' model for flares in Sagittarius A*
• General relativistic effects and the near-infrared and X-ray variability of Sgr A* I
• How Spatially Resolved Polarimetry Informs Black Hole Accretion Flow Models
• Hot-spots around Sagittarius A*: Joint fits to astrometry and polarimetry
• The future ability to test theories of gravity with black-hole shadows
• Parameter study for hot spot trajectories around Sgr$A\ast$
• Magnetically arrested disk flux eruption events to describe SgrA* flares
• Forward Ray Tracing and Hot Spots in Kerr Spacetime
• Constraints on metric-affine gravity black holes from the stellar motion at the Galactic Center
• Electromagnetic signatures of strong-field gravity from accreting black holes
• Accurate analytical modeling of light rays in spherically symmetric spacetimes: Applications in the study of black hole accretion disks and polarimetry
• General Relativistic effects and the NIR variability of Sgr A* II: A systematic approach to temporal asymmetry
• Observational signatures from higher-order images of moving hotspots in accretion disks
• Relativistic Reflection Modeling in AGN and Related Variability from PCA: A Brief Review
• Characterization of black hole accretion through image moment invariants
• Modified electron trajectory due to the axion dark matter background
• Full-polarization millimeter wavelength variability of Sagittarius A* during the 2018 EHT campaign
• Flaremodel: An open-source Python package for one-zone numerical modelling of synchrotron sources
• Fast and accurate analytical formulas for light propagation in general static, spherically symmetric spacetimes
• Sagittarius A* near-infrared flares polarization as a probe of space-time I: Non-rotating exotic compact objects