Resolved Magnetic-Field Structure and Variability Near the Event Horizon of Sagittarius A* (1512.01220v1)

Abstract: Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.

• The paper used the Event Horizon Telescope to spatially resolve partially ordered magnetic fields and their variability near the event horizon of Sagittarius A*.
• Interferometric observations with the EHT revealed high polarization fractions up to 70%, suggesting resolved, structured magnetic fields rather than scattered or random configurations.
• The findings support toroidal magnetic field configurations arising from differential rotation and provide crucial data for comparison with GRMHD simulations of black hole accretion.

Resolved Magnetic-Field Structure and Variability near the Event Horizon of Sagittarius A*

This paper presents significant advancements in the understanding of magnetic field structures and dynamics in the vicinity of the supermassive black hole at the center of our galaxy, known as Sagittarius A* (Sgr A*). Utilizing the Event Horizon Telescope (EHT) and its global Very Long Baseline Interferometry (VLBI) array at 1.3 mm wavelength, the authors have achieved spatial resolution of the linearly polarized emission from Sgr A*, unveiling partially ordered magnetic fields near the event horizon.

Key Insights and Results

1. Observational Findings: Interferometric observations have allowed for spatial resolution on scales of approximately 6 Schwarzschild radii, detecting partially ordered magnetic field structures near the black hole's event horizon. Intrinsic intra-hour variability in these fields has been localized, providing direct evidence of dynamic processes at play.
2. Polarization and Magnetic Field Interpretation: The paper details interferometric measurements of fractional polarization that enable insights into the magnetized environment surrounding Sgr A*. The observations revealed high polarization fractions, up to 70%, considerably exceeding earlier measurements with larger interferometers that found only 5-10%. This is indicative of the resolving power of the EHT to dissect structured components within compact emission regions, rather than random and scattered fields.
3. Implications for Magnetic Field Configurations: The observed data suggests toroidal magnetic field configurations as a result of differential rotation and shearing in the accretion disk. This aligns with predictions of ordered field production despite the turbulence driven by the Magnetorotational Instability (MRI). The evidence points to a complex interplay between disk rotation, magnetic field dynamics, and turbulence.
4. Position of Polarization Centroid: The relative astrometry with polarimetric VLBI allowed the authors to determine alignment between polarized and total flux regions. The spatial coherence and direction of polarization reveal substantial magnetic field activity localized to event-horizon scales.

Implications and Future Research

The findings advance our theoretical understanding of accretion processes and magnetic field dynamics near black holes. The polarization data offer insights into the complex physical conditions and mechanisms that govern electromagnetic emissions from accretion flows. This has implications for modeling relativistic jets and hot accretion flows, bringing clarity to the magnetic structures instrumental in these phenomena.

The paper heralds further development in 1.3-mm polarimetric VLBI. Prospects include enhanced imaging capabilities and refined observations of magnetic structures, variability studies on gravitational timescales, and more precise constraints on plasma properties through comparison to General Relativistic Magnetohydrodynamics (GRMHD) simulations.

Speculation on Future Developments

As VLBI technology and theoretical models evolve, the precision of magnetic field measurements and resolution of emission regions will improve, potentially allowing comprehensive mapping and modeling of black hole accretion ecosystems. The EHT array and similar future endeavors will be critical in revealing the microphysics of accretion, shedding light on black hole growth and evolution within galaxy cores. The work lays foundational insights encouraging further exploration into the coupling between magnetic fields, energetic jets, and the dynamic environment enshrouding supermassive black holes.