First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole

Abstract

In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations; radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. All models fail at least one constraint. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. A number of models fail only the variability constraints. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. They have accretion rate (5.2–9.5) × 10−9 M ⊙ yr−1, bolometric luminosity (6.8–9.2) × 1035 erg s−1, and outflow power (1.3–4.8) × 1038 erg s−1. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature; exploratory, nonthermal model sets tend to have higher 2.2 μm flux density; and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations.

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].…”

“…Consensus is currently lacking regarding the spin and observer inclination angle of Sgr A , with different analyses returning differing constraints. For instance, the EHT observations of Sgr A are consistent with a large spin and low inclination angle [20,24], but are very far from ruling out the antipodal region of parameter space. Earlier work based on quasi-periodic oscillations of emissions in radio, infrared, and X-rays [195] or millimeter VLBI [196,197], exclude an extremal spin.…”

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

“…Second, and perhaps most importantly, there is no clear consensus on the value of SgrA * 's spin and inclination angle. The EHT images are in principle consistent with large spin and low inclination angle, but are far from being inconsistent with low spin and large inclination angle [11,15]. Independent works based on radio, infrared, and X-ray emission, as well as millimeter VLBI, exclude extremal spin (1 − a 1), but have been unable to place strong constraints otherwise [149][150][151] constraints across a wide range of spin values [152,153].…”

“…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].…”