How are Black Holes imaged?

With the recent imaging of the black hole in the center of the Milky Way, known as Sagittarius A (Sgr A*), the world was given the first image ever captured of the black hole at the center of the galaxy. Initially predicted by Albert Einstein’s general theory of relativity in 1916, black holes have been one of astrophysics’ hard to observe phenomena, whose interest by the astrophysical community was sparked from the discovery of neutron stars by Jocelyn Bell Burnell in 1967. These remnants of gravitationally collapsed stars could not be directly observed until 2019, but were commonly identified by the effects they have on stellar objects close to them, such as the bending of light known as “gravitational lensing”. Larger black holes, such as Sgr A*, could also be indirectly observed through the paths of stars as they travelled around it.

Fig. 1: Animation of stars orbiting around Sgr A*.

Black holes are generally active in nature, and usually have glowing surroundings (known as accretion disks) from which they can be observed as a silhouettes. These silhouettes are about 2.5 times the actual size of the black hole, and so this makes it sound like a simple feat to accomplish. However, no singular telescope has been created (as of yet!) which would have enough resolution to be able to discern the black hole from its surroundings. Because of this, we have been limited to artists renditions of how black holes may look like until the day the Event Horizon Telescope released its first image of the black hole known as M87*, located ~55 million light-years from Earth and has a mass of ~6.5 billion Suns.

Fig. 2: Image of M87* obtained by the EHT.

The EHT is not one singular telescope, and is instead an international collaboration of 60 institutions in over 20 regions of the world. The EHT uses a technique known as Very Long Baseline Interferometry (VBLI), with the institutions each individually contribute telescopes which are synchronised as an array of telescopes to focus on the same object at the same time, acting as a telescope as large as the Earth itself. In such a VBLI set-up, the aperture of this “virtual” telescope is the distance between the two farthest telescopes from each other, which in the case of the EHT is the distance between Antarctica and Spain. This distance is almost the same as the diameter of the planet, and effectively acts as a telescope with an aperture the size of the planet, which allows the EHT to image black holes with a relatively large apparent size (the size of the black hole in the night sky from Earth) [1]. The only black holes that can really be imaged by the EHT would be supermassive black holes, such as the ones found at the center of most galaxies.

These telescopes, while not physically connected together, work by taking images of the same object timed using Hydrogen Maser atomic clocks, which precisely timestamp the image obtained. Weather forecasts would be used to time an image capture, specifically for ranges of days with considerable clear skies at as many sites as possible. Once a suitable range of days is determined, the telescopes throughout the EHT would take images of the object over the course of several days. The data obtained would be in the ~350TB/day range for each telescope in the EHT during the capture of M87*, and data was stored on high-performance helium-filled drives. By the end of the data-capturing period, all these data would be sent to supercomputers to combine the images together into one overall image. [2]

Fig. 3: The locations of the telescopes in the EHT for the capture of M87*.

M87* was the first of the two targets of the EHT, with it being a very active supermassive black hole. The black hole itself is one of the largest black holes in terms of apparent size from the Earth, and so it was one of the ideal targets of the EHT. Its active state also made it interesting to image, as it actively has matter falling into it and spewing out as jets of particles. After the imaging of M87*, the second target of the EHT was Sgr A*, which has a considerably more noisy environment due to it being in the center of our own galaxy. However the same procedure was used as for M87*, with more institutions having joined the EHT since the imaging of M87*, and the final result was released as of 12th May 2022.

Fig. 4: Compiled image of Sgr A* by EHT.

The image received of Sgr A* could be split into 4 clusters of similar features, their averages shown below the main image of the main Sgr A* image. Three of the four clusters show ring-like features, with different parts of the ring brighter than the others. The last cluster also contained images that fit the data, but were not ring-like. The bars associated with each image show to what proportion each cluster was present in the obtained data, with thousands of images in the first three, and only hundreds of images in the fourth.

Information References:

1. Lutz, O., 2019. How Scientists Captured the First Image of a Black Hole. [online] Jet Propulsion Laboratory. Available at: <https://www.jpl.nasa.gov/edu/news/2019/4/19/how-scientists-captured-the-first-image-of-a-black-hole/> [Accessed 12 May 2022].
2. Eventhorizontelescope.org. 2019. Press Release (April 10, 2019): Astronomers Capture First Image of a Black Hole. [online] Available at: <https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole> [Accessed 12 May 2022].
3. Eventhorizontelescope.org. 2022. Astronomers reveal first image of the black hole at the heart of our galaxy. [online] Available at: <https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy> [Accessed 12 May 2022].

Image References:

1. Youtube.com. 2015. Animation of the Stellar Orbits around the Galactic Center. [online] Available at: <https://www.youtube.com/watch?v=tMax0KgyZZU> [Accessed 12 May 2022].
2. Eventhorizontelescope.org. 2019. Press Release (April 10, 2019): Astronomers Capture First Image of a Black Hole. [online] Available at: <https://eventhorizontelescope.org/press-release-april-10-2019-astronomers-capture-first-image-black-hole> [Accessed 12 May 2022].
3. ESO.org. 2019. Locations of the EHT Telescopes. [online] Available at: <https://www.eso.org/public/images/eso1907p/> [Accessed 12 May 2022].
4. Eventhorizontelescope.org. 2022. Astronomers reveal first image of the black hole at the heart of our galaxy. [online] Available at: <https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy> [Accessed 12 May 2022].