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EHT Picture of the black hole in M87
The picture from the Event Horizon Telescope:
https://apod.nasa.gov/apod/ap190411.html Why is the center black? Does the accretion disk just happen to be in a plane normal to our line-of-sight? (The near-perfect circularity of the image implies this may be so, but the varying intensity implies not.) In particular, if the accretion disk is not in a plane normal to our line-of-sight, why don't we see light from the portion of it between us and the black hole? And why isn't the image elliptical? From the image, can they infer anything about the spin of the black hole? How about from observing stars orbiting nearby? Tom Roberts [[Mod. note -- (Tom very likely knows all this, but others may not.) 1. VERY IMPORTANT: Because the observed photons (mm-wavelength radio waves) originated close to the black hole, their paths were strongly bent by the black hole's gravity. So, the appearance of the image is very different than a geometric projection of the actual physical positions from which the photons were emitted. 2. The observations basically measured the 2-D Fourier transform of the sky's radio brightness, at a finite set of spatial frequencies corresponding to the inter-telescope baselines projected on the sky plane (see figure 2 of paper 1 in the list below, or paper 4 for many more details). Reconstructing an image from this data is a tricky inverse problem (see paper 4 for details). 3. Yes, this tells us a bit about the black hole spin and its orientation. See paper 5 in the list below for details. 4. The first 5 research papers describing this are open-access at the Astrophysical Journal Letters website: (There's mention of a paper 6 but I haven't found it yet) First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole https://iopscience.iop.org/article/1...41-8213/ab0ec7 First M87 Event Horizon Telescope Results. II. Array and Instrumentation https://iopscience.iop.org/article/1...41-8213/ab0c96 First M87 Event Horizon Telescope Results. III. Data Processing and Calibration https://iopscience.iop.org/article/1...41-8213/ab0c57 First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole https://iopscience.iop.org/article/1...41-8213/ab0e85 First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring https://iopscience.iop.org/article/1...41-8213/ab0f43 -- jt]] |
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