Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models

Citation:

Avery E. Broderick, Vincent L. Fish, Michael D. Johnson, Katherine Rosenfeld, Carlos Wang, Sheperd S. Doeleman, Kazunori Akiyama, Tim Johannsen, and Alan L. Roy. 2016. “Modeling Seven Years of Event Horizon Telescope Observations with Radiatively Inefficient Accretion Flow Models.” The Astrophysical Journal, 820.

Abstract:

An initial three-station version of the Event Horizon Telescope, amillimeter-wavelength very-long baseline interferometer, has observedSagittarius A* (Sgr A*) repeatedly from 2007 to 2013, resulting in themeasurement of a variety of interferometric quantities. Of particularimportance is that there is now a large set of closure phases measuredover a number of independent observing epochs. We analyze theseobservations within the context of a realization of semi-analyticradiatively inefficient disk models, implicated by the low luminosity ofSgr A*. We find a broad consistency among the various observing epochsand between different interferometric data types, with the latterproviding significant support for this class of model of Sgr A*. The newdata significantly tighten existing constraints on the spin magnitudeand its orientation within this model context, finding a spin magnitudeof a={0.10}_-0.10-0.10^+0.30+0.56, an inclinationwith respect to the line of sight of θ ={60^\circ}_-{8^{^\circ}-{13}^{^\circ}}^+{5^{^\circ}+{10}^{^\circ}}, and aposition angle of ξ ={156^\circ }_-{17^{^\circ}-{27}^{^\circ}}^+{10^{^\circ}+{14}^{^\circ}} east of north. These are in goodagreement with previous analyses. Notably, the previous 180°degeneracy in the position angle has now been conclusively broken by theinclusion of the closure-phase measurements. A reflection degeneracy inthe inclination remains, permitting two localizations of the spin vectororientation, one of which is in agreement with the orbital angularmomentum of the infrared gas cloud G2 and the clockwise disk of youngstars. This may support a relationship between Sgr A*'s accretion flowand these larger-scale features.

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