The Intrinsic Two-Dimensional Size of Sagittarius A*

TL;DR

This paper presents the first direct measurement of the two-dimensional structure of Sagittarius A* at 7mm, revealing an elliptical shape aligned with jet or accretion disk models, and examines variability related to high-energy flares.

Contribution

It provides the first direct imaging of Sagittarius A*'s intrinsic 2D structure at 7mm, supporting jet and accretion disk interpretations and analyzing flare-related variability.

Findings

Intrinsic source size: 35.4 x 12.6 R_S with a 95° position angle.

No significant size change (>15%) over five epochs.

Flares in NIR and X-ray do not always correlate with radio variability.

Abstract

We report the detection of the two-dimensional structure of the radio source associated with the Galactic Center black hole, Sagittarius A*, obtained from Very Long Baseline Array (VLBA) observations at a wavelength of 7mm. The intrinsic source is modeled as an elliptical Gaussian with major axis size 35.4 x 12.6 R_S in position angle 95 deg East of North. This morphology can be interpreted in the context of both jet and accretion disk models for the radio emission. There is supporting evidence in large angular-scale multi-wavelength observations for both source models for a preferred axis near 95 deg. We also place a maximum peak-to-peak change of 15% in the intrinsic major axis size over five different epochs. Three observations were triggered by detection of near infrared (NIR) flares and one was simultaneous with a large X-ray flare detected by NuSTAR. The absence of simultaneous and quasi-simultaneous flares indicates that not all high energy events produce variability at radio wavelengths. This supports the conclusion that NIR and X-ray flares are primarily due to electron excitation and not to an enhanced accretion rate onto the black hole.