First Sagittarius A* Event Horizon Telescope Results. IV. Variability, Morphology, and Black Hole Mass

Abstract

In this paper we quantify the temporal variability and image morphology of the horizon-scale emission from Sgr A*, as observed by the EHT in 2017 April at a wavelength of 1.3 mm. We find that the Sgr A* data exhibit variability that exceeds what can be explained by the uncertainties in the data or by the effects of interstellar scattering. The magnitude of this variability can be a substantial fraction of the correlated flux density, reaching ∼100% on some baselines. Through an exploration of simple geometric source models, we demonstrate that ring-like morphologies provide better fits to the Sgr A* data than do other morphologies with comparable complexity. We develop two strategies for fitting static geometric ring models to the time-variable Sgr A* data; one strategy fits models to short segments of data over which the source is static and averages these independent fits, while the other fits models to the full data set using a parametric model for the structural variability power spectrum around the average source structure. Both geometric modeling and image-domain feature extraction techniques determine the ring diameter to be 51.8 ± 2.3 μas (68% credible intervals), with the ring thickness constrained to have an FWHM between ∼30% and 50% of the ring diameter. To bring the diameter measurements to a common physical scale, we calibrate them using synthetic data generated from GRMHD simulations. This calibration constrains the angular size of the gravitational radius to be 4.8 − 0.7 + 1.4 μas, which we combine with an independent distance measurement from maser parallaxes to determine the mass of Sgr A* to be 4.0 − 0.6 + 1.1 × 10 6 M ⊙.

Superradiant evolution of the shadow and photon ring of Sgr A$^\star$

“…Because of the short dynamical timescale ∼ 20 s, the EHT has pushed VLBI techniques to image the dynamical evolution of Sgr A by reconstructing the source's emission region [18,19]. Assuming General Relativity, the estimated dynamical mass and distance are consistent with the angular diameter of the shadow (51.8 ± 2.3) µas recently reported by the EHT collaboration [20][21][22][23][24][25][26][27][28][29].…”

“…For this purpose, we define the shadow centre from the BH as r C ≡ |r max − r min |/2. Alternative definitions, including the shadow's geometrical mean r G and the ring center defined by EHT [9,22,23] would lead to quantitatively similar results. On the other hand, the impact of the evolution on other observables, such as the shadow size ∆R ≡ |r max + r min |/2, the ring diameter, and the circularity in Ref.…”

Horizon-scale tests of gravity theories and fundamental physics from the Event Horizon Telescope image of Sagittarius A$^*$

“…Horizon-scale images of supermassive black holes and their shadows have opened a new unparalleled window onto tests of gravity and fundamental physics in the very strong-field regime, including the possibility that astrophysical BHs may be described by alternatives to the Kerr metric. In this work, we have used the horizonscale images of SgrA * provided by the Event Horizon Telescope [11][12][13][14][15][16][17][18][19][20] to test some of the most popular and well-motivated scenarios deviating from the Kerr metric. Compared to horizon-scale images of M87 * , there are significant advantages in the use of images of SgrA * , as we discussed towards the end of Sec.…”

“…The first groundbreaking SMBH images were delivered by the Event Horizon Telescope (EHT), a millimeter VLBI array with Earthscale baseline coverage [2], which in 2019 resolved the near-horizon region of the SMBH M87 * [3][4][5][6][7][8], before later revealing its magnetic field structure [9,10]. This was recently followed by the EHT's first images of Sagittarius A * (SgrA * ), the SMBH located at the Milky Way center [11][12][13][14][15][16][17][18][19][20].…”

A Universal Power-law Prescription for Variability from Synthetic Images of Black Hole Accretion Flows

“…With the advent of millimeter and submillimeter very long baseline interferometry (VLBI), it has become possible to image a handful of targets with resolutions sufficient to resolve the lensed event horizon, silhouetted against the luminous plasma in its vicinity. Images of the two horizon-scale EHT targets, M87 * and Sgr A * , have now been published, revealing in unprecedented detail the astrophysical processes at work in the innermost regions of accretion flows and in the region responsible for launching jets (Event Horizon Telescope Collaboration et al 2019a, 2019b, 2019c, 2019d, hereafter M87 * Papers I-VI; Event Horizon Telescope Collaboration et al 2022a, 2022b, 2022c, 2022d.…”