Simultaneous Multi-Wavelength Observations of Sgr A*

Abstract

We observed Sgr A* using the Very Large Array (VLA) and the Giant Metrewave Radio Telescope (GMRT) at multiple cm and mm wavelengths on 17 June 2003. The measured flux densities of Sgr A*, together with those obtained from the Submillimeter Array (SMA) and the Keck II 10 m telescope on the same date, are used to construct a simultaneous spectrum of Sgr A* from 90 cm to 3.8 $\mu$m. The simultaneous spectrum shows a spectral break at about 3.6 cm, a possible signature of synchrotron self-absorption of the strong radio outburst which occurred near epoch 17 July 2003. At 90 cm, the flux density of Sgr A* is $0.22\pm 0.06$ Jy, suggesting a sharp decrease in flux density at wavelengths longer than 47 cm. The spectrum at long cm wavelengths appears to be consistent with free-free absorption by a screen of ionized gas with a cutoff $\sim$100 cm. This cutoff wavelength appears to be three times longer than that of $\sim$30 cm suggested by Davies, Walsh & Booth based on observations in 1974 and 1975. Our analysis suggests that the flux densities of Sgr A* at wavelengths longer than 30 cm could be attenuated and modulated by stellar winds from massive stars close to Sgr A*.

15 pages, 4 figures, accepted by ApJ Letter

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• The Submillimeter Bump in Sgr A* from Relativistic MHD Simulations
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• Variable accretion and emission from the stellar winds in the Galactic centre
• A General Relativistic Null Hypothesis Test with Event Horizon Telescope Observations of the black-hole shadow in Sgr A*
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• Results from an extensive simultaneous broadband campaign on the underluminous active nucleus M81*: further evidence for mass-scaling accretion in black holes
• How to hide large scale outflows: size constraints on the jets of Sgr A*
• Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission
• The Submillimeter Polarization of Sgr A*
• Rapid Variability of Sgr A* across the Electromagnetic Spectrum
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• The Galactic Center Weather Forecast
• General relativistic MHD simulations of non-thermal flaring in Sagittarius A*
• Multiwavelength VLBI observations of Sagittarius A*
• The Circular Polarization of Sagittarius A* at Submillimeter Wavelengths
• Self-consistent spectra from radiative GRMHD simulations of accretion onto Sgr A*
• Spectral Energy Distribution of the inner accretion flow around Sgr A* -- Clue for a weak outflow in the innermost region
• Millimeter Radio Continuum Emissions as the Activity of Super Massive Black Holes in Nearby Early-Type Galaxies and Low-Luminosity Active Galactic Nuclei
• Testing RIAF model for Sgr A* using the size measurements
• An Inverse Compton Scattering Origin of X-ray Flares from Sgr A*
• Coronal radiation of a cusp of spun-up stars and the X-ray luminosity of Sgr A*
• Linearly Polarized Millimeter and Submillimeter Continuum Emission of Sgr A* Constrained by ALMA
• J0906+6930: a radio-loud quasar in the early Universe
• The impact of non-thermal electrons on event horizon scale images and spectra of Sgr A*
• Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*
• An accretion-jet model for M87: interpreting the spectral energy distribution and Faraday rotation measure
• Sub-mm Jet Properties of the X-Ray Binary Swift J1745$-$26
• Far Infrared Variability of Sagittarius A*: 25.5 Hours of Monitoring with $Herschel$
• ALMA and VLA observations of emission from the environment of Sgr A*
• Nuclear Radio Jet from a Low-Luminosity Active Galactic Nucleus in NGC 4258
• A single HII region model of the strong interstellar scattering towards Sgr A*
• The possible submillimeter bump and accretion-jet in the central supermassive black hole of NGC 4993
• Effects of Hydrogen vs. Helium on Electromagnetic Black Hole Observables
• The Effects of Tilt on the Time Variability of Millimeter and Infrared Emission from Sagittarius A*
• Detection of a 20 minute time lag observed from Sgr A* between 8 and 10 GHz with the VLA
• Submillimetre Transient Science in the Next Decade: EAO Submillimetre Futures White Paper Series, 2019