Multiwavelength VLBI observations of Sagittarius A*

Abstract

The compact radio source Sgr\,A*, associated with the super massive black hole at the center of the Galaxy, has been studied with VLBA observations at 3 frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The total VLBI flux density of Sgr\,A* varies from day to day. The variability is correlated at the 3 observing frequencies with higher variability amplitudes appearing at the higher frequencies. For the modulation indices, we find 8.4\,% at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron self-absorption with a turnover frequency at or above 86\,GHz. The radio spectral index correlates with the flux density, which is harder (more inverted spectrum) when the source is brighter. The average source size does not appear to be variable over the 10-day observing interval. However, we see a tendency for the sizes of the minor axis to increase with increasing total flux, whereas the major axis remains constant. Towards higher frequencies, the position angle of the elliptical Gaussian increases, indicative of intrinsic structure, which begins to dominate the scatter broadening. At cm-wavelength, the source size varies with wavelength as ${\lambda }^{2.12\pm 0.12}$, which is interpreted as the result of interstellar scatter broadening. After removal of this scatter broadening, the intrinsic source size varies as ${\lambda }^{\mathrm{1.4...1.5}}$. The VLBI closure phases at 22, 43, and 86\,GHz are zero within a few degrees, indicating a symmetric or point-like source structure. In the context of an expanding plasmon model, we obtain an upper limit of the expansion velocity of about 0.1\,c from the non-variable VLBI structure. This agrees with the velocity range derived from the radiation transport modeling of the flares from the radio to NIR wavelengths.}

14pages, 14 Figures, Accepted for publication in A&A

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• BlackHoleCam: fundamental physics of the Galactic center
• A General Relativistic Null Hypothesis Test with Event Horizon Telescope Observations of the black-hole shadow in Sgr A*
• Towards the event horizon -- the supermassive black hole in the Galactic Center
• The Size, Shape, and Scattering of Sagittarius A* at 86 GHz: First VLBI with ALMA
• The Scattering and Intrinsic Structure of Sagittarius A* at Radio Wavelengths
• Imaging an Event Horizon: Mitigation of Scattering Toward Sagittarius A*
• Radio observations of active galactic nuclei with mm-VLBI
• The Black Hole Explorer: Motivation and Vision
• Dynamical Imaging with Interferometry
• The Intrinsic Two-Dimensional Size of Sagittarius A*
• Multiwavelength VLBI observations of Sagittarius A*
• Discovery of Substructure in the Scatter-Broadened Image of Sgr A*
• Stochastic Optics: A Scattering Mitigation Framework for Radio Interferometric Imaging
• Millimeter-band variability of the radio-quiet nucleus of NGC7469
• Evidence for nuclear radio jet and its structure down to <100 Schwarzschild radii in the center of the Sombrero galaxy (M 104, NGC 4594)
• The Intrinsic Shape of Sagittarius A* at 3.5-mm Wavelength
• Wisps in the Galactic center: NIR triggered observations of the radio source Sgr A* at 43 GHz
• The intrinsic structure of Sagittarius A* at 1.3 cm and 7 mm
• The internal Faraday screen of Sagittarius A*
• Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz
• Relative Astrometry of Compact Flaring Structures in Sgr A* with Polarimetric VLBI
• Multi-Epoch VERA Observations of Sagittarius A*: I. Images and Structural Variability
• No asymmetric outflows from Sagittarius A* during the pericenter passage of the gas cloud G2
• High-frequency very long baseline interferometry studies of NRAO 530
• VLBI imaging of black holes via second moment regularization
• First Space-VLBI Observations of Sagittarius A*
• Micro-arcsecond structure of Sagittarius A* revealed by high-sensitivity 86 GHz VLBI observations
• A comparative study of amplitude calibrations for East-Asia VLBI Network: a-priori and template spectrum methods
• Measuring the Direction and Angular Velocity of a Black Hole Accretion Disk via Lagged Interferometric Covariance
• Detecting Stars at the Galactic Centre via Synchrotron Emission
• Long-term monitoring of Sgr A* at 7 mm with VERA and KaVA
• Trigonometric parallax and proper motion of Sagittarius A* measured by VERA using the new broad-band back-end system OCTAVE-DAS
• Monitoring the Size and Flux Density of Sgr A* during the Active State in 2019 with East Asian VLBI Network
• Resolving Microlensing Events with Triggered VLBI
• The Galactic Center Black Hole Laboratory
• The Core-shift of Sagittarius A* as a Discriminant between Disk and Jet Emission Models with millimeter-VLBI
• SgrA* emission at 7mm: variability and periodicity
• Nonthermal hotspot orbiting the supermassive black hole Sgr A*