Sagittarius A* black hole observations
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Event Horizon Telescope Imaging of Sagittarius A* Black Hole
The Event Horizon Telescope (EHT) achieved the first direct imaging of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, in 2017. These observations revealed a bright, thick ring of emission with a diameter of about 51.8 microarcseconds, showing modest brightness asymmetry and a dimmer interior, consistent with theoretical predictions for a black hole of approximately 4 million solar masses. The EHT images provide direct evidence for the presence of a supermassive black hole and connect predictions from stellar orbit measurements to event horizon-scale features and variability 16.
Structure and Size of Sagittarius A* at Multiple Wavelengths
Very Long Baseline Interferometry (VLBI) observations at different wavelengths, including 1.3 cm, 7 mm, and 3.5 mm, have been used to study the intrinsic structure of Sgr A*. These studies show that the source can be described by a symmetric Gaussian model, with the intrinsic size scaling with observing wavelength as a power law. The unscattered source at 3.5 mm has a major-axis size of about 120 microarcseconds, and the morphology is largely symmetrical. These results help constrain models of the accretion flow and indicate that nonthermal electrons are needed to explain the observed sizes 25.
Polarization and Magnetic Field Structure
The EHT also produced the first resolved linear and circular polarization images of Sgr A*. The emission ring is highly polarized, with a prominent spiral pattern in the electric vector polarization angle and a peak fractional polarization of about 40% in the western part of the ring. The circular polarization shows a dipole structure along the ring. These polarization measurements provide strong constraints on the magnetic field structure near the event horizon and support the presence of ordered magnetic fields in the accretion flow .
Variability and Dynamics Near the Event Horizon
Sgr A* exhibits rapid structural changes and variability in its emission, posing challenges for imaging. Dynamic reconstructions using EHT data with high temporal resolution have revealed intricate temporal dynamics, including features that propagate around the black hole. These findings confirm the time-averaged structure seen in earlier EHT results and provide new insights into the dynamic environment near the event horizon .
Testing General Relativity and Alternative Theories
The EHT observations of Sgr A* allow for stringent tests of general relativity (GR) in the strong-field regime. The observed image size matches the predictions of the Kerr metric (the GR solution for rotating black holes) within about 10%. These results rule out many alternative scenarios, such as black holes with surfaces or certain types of black hole mimickers, and place strong constraints on deviations from GR. However, some alternative models, including certain wormhole and naked singularity scenarios, are not yet fully excluded 610.
Multiwavelength and Gamma-Ray Observations
Sgr A* has been observed across the electromagnetic spectrum, from radio to gamma rays. Fermi-LAT observations have detected a gamma-ray point source coinciding with Sgr A*, with properties consistent with the black hole's bolometric luminosity. The gamma-ray emission is likely produced by cosmic rays accelerated by or near the black hole . Multiwavelength VLBI campaigns have also revealed variability and flaring activity, providing further insights into the accretion and outflow processes 89.
Conclusion
Recent observations of Sagittarius A* with the EHT and other instruments have provided unprecedented views of the black hole's shadow, structure, polarization, and variability. These results confirm the presence of a supermassive black hole at the center of the Milky Way, support the predictions of general relativity, and offer new constraints on the physics of accretion, magnetic fields, and alternative gravity theories. Ongoing and future observations will continue to refine our understanding of this unique cosmic laboratory.
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First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way
The first Event Horizon Telescope observations confirm the presence of a supermassive black hole at the center of the Milky Way galaxy, supporting the theory of general relativity.
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