Constraining the Structure of Sagittarius A*'s Accretion Flow with Millimeter Very Long Baseline Interferometry Closure Phases

Citation:

Avery E. Broderick, Vincent L. Fish, Sheperd S. Doeleman, and Abraham Loeb. 2011. “Constraining the Structure of Sagittarius A*'s Accretion Flow with Millimeter Very Long Baseline Interferometry Closure Phases.” The Astrophysical Journal, 738.

Abstract:

Millimeter wave very long baseline interferometry (mm-VLBI) providesaccess to the emission region surrounding Sagittarius A* (Sgr A*), thesupermassive black hole at the center of the Milky Way, on sub-horizonscales. Recently, a closure phase of 0° ± 40° wasreported on a triangle of Earth-sized baselines (SMT-CARMA-JCMT)representing a new constraint upon the structure and orientation of theemission region, independent from those provided by the previouslymeasured 1.3 mm-VLBI visibility amplitudes alone. Here, we compare thisto the closure phases associated with a class of physically motivated,radiatively inefficient accretion flow models and present predictionsfor future mm-VLBI experiments with the developing Event HorizonTelescope (EHT). We find that the accretion flow models are capable ofproducing a wide variety of closure phases on the SMT-CARMA-JCMTtriangle and thus not all models are consistent with the recentobservations. However, those models that reproduce the 1.3 mm-VLBIvisibility amplitudes overwhelmingly have SMT-CARMA-JCMT closure phasesbetween ±30°, and are therefore broadly consistent with allcurrent mm-VLBI observations. Improving station sensitivity by factorsof a few, achievable by increases in bandwidth and phasing togethermultiple antennas at individual sites, should result in physicallyrelevant additional constraints upon the model parameters and eliminatethe current 180° ambiguity on the source orientation. Whenadditional stations are included, closure phases of order45°-90° are typical. In all cases, the EHT will be able tomeasure these with sufficient precision to produce dramatic improvementsin the constraints upon the spin of Sgr A*.

Recent Publications

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole.” ApJL, 875, L6, Pp. 1-44. Publisher's Version Abstract

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 42 ± 3 μas and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc2 = 3.8 ± 0.4 μas. Folding in a distance measurement of ${16.8}_{-0.7}^{+0.8}\,\mathrm{Mpc}$ gives a black hole mass of $M=6.5\pm 0.2{| }_{\mathrm{stat}}\pm 0.7{| }_{\mathrm{sys}}\times {10}^{9}\hspace{2pt}{M}_{\odot }$ . This measurement from lensed emission near the event horizon is consistent with the presence of a central Kerr black hole, as predicted by the general theory of relativity.

The Event Horizon Telescope Collaboration. 4/10/2019. “First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring.” ApJL, 875, L5, Pp. 1-31. Publisher's Version Abstract

The Event Horizon Telescope (EHT) has mapped the central compact radio source of the elliptical galaxy M87 at 1.3 mm with unprecedented angular resolution. Here we consider the physical implications of the asymmetric ring seen in the 2017 EHT data. To this end, we construct a large library of models based on general relativistic magnetohydrodynamic (GRMHD) simulations and synthetic images produced by general relativistic ray tracing. We compare the observed visibilities with this library and confirm that the asymmetric ring is consistent with earlier predictions of strong gravitational lensing of synchrotron emission from a hot plasma orbiting near the black hole event horizon. The ring radius and ring asymmetry depend on black hole mass and spin, respectively, and both are therefore expected to be stable when observed in future EHT campaigns. Overall, the observed image is consistent with expectations for the shadow of a spinning Kerr black hole as predicted by general relativity. If the black hole spin and M87's large scale jet are aligned, then the black hole spin vector is pointed away from Earth. Models in our library of non-spinning black holes are inconsistent with the observations as they do not produce sufficiently powerful jets. At the same time, in those models that produce a sufficiently powerful jet, the latter is powered by extraction of black hole spin energy through mechanisms akin to the Blandford-Znajek process. We briefly consider alternatives to a black hole for the central compact object. Analysis of existing EHT polarization data and data taken simultaneously at other wavelengths will soon enable new tests of the GRMHD models, as will future EHT campaigns at 230 and 345 GHz.

Sheperd S. Doeleman

Smithsonian Astrophysicist and Harvard Senior Research Fellow

Citation:

Abstract:

Recent Publications