Discovery of Substructure in the Scatter-Broadened Image of Sgr A*

Abstract

We have detected substructure within the smooth scattering disk of the celebrated Galactic Center radio source Sagittarius A* (SgrA*). We observed this structure at 1.3 cm wavelength with the Very Long Baseline Array together with the Green Bank Telescope, on baselines of up to 3000 km, long enough to completely resolve the average scattering disk. Such structure is predicted theoretically, as a consequence of refraction by large-scale plasma fluctuations in the interstellar medium. Along with the much-studied ${\theta }_{\mathrm{d}}\propto {\lambda }^2$ scaling of angular broadening ${\theta }_{\mathrm{d}}$ with observing wavelength $\lambda$, our observations indicate that the spectrum of interstellar turbulence is shallow, with an inner scale larger than 300 km. The substructure is consistent with an intrinsic size of about 1 mas at 1.3 cm wavelength, as inferred from deconvolution of the average scattering. Further observations of the substructure can set stronger constraints on the properties of scattering material and on the intrinsic size of SgrA*. These constraints will guide understanding of effects of scatter-broadening and emission physics of the black hole, in images with the Event Horizon Telescope at millimeter wavelengths.

5 pages, 5 figures, accepted by Astrophysical Journal Letters; minor corrections to the text and figures are introduced

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• The Black Hole Explorer: Motivation and Vision
• The Radio Fundamental Catalogue. I. Astrometry
• PSR B0329+54: Substructure in the scatter-broadened image discovered with RadioAstron on baselines up to 330,000 km
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• The intrinsic structure of Sagittarius A* at 1.3 cm and 7 mm
• Persistent Non-Gaussian Structure in the Image of Sagittarius A* at 86 GHz
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• Enabling Transformational ngEHT Science via the Inclusion of 86 GHz Capabilities
• Tracing Milky Way scattering by compact extragalactic radio sources
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• Analysis of Orbital Configurations for Millimetron Space Observatory
• First Space-VLBI Observations of Sagittarius A*
• Trigonometric parallax and proper motion of Sagittarius A* measured by VERA using the new broad-band back-end system OCTAVE-DAS
• The Core-shift of Sagittarius A* as a Discriminant between Disk and Jet Emission Models with millimeter-VLBI
• Direct VLBI Detection of Interstellar Turbulence Imprint on a Quasar: TXS 2005+403
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