Studies of Black Holes and Jets with the Event Horizon Telescope EHT logo

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Role of Alan Marscher & Svetlana Jorstad in Making the Event Horizon Telescope & Imaging of the Black Hole in the Galaxy M87

As was announced on April 10, 2019, an array of high-precision radio telescopes called the Event Horizon Telescope (EHT) produced the first image of a black hole. Two Boston university astronomers, Professor Alan Marscher and Senior Research Scientist Dr. Svetlana Jorstad, played important roles in making the images from EHT data obtained in April 2017.

The primary goal of the international Event Horizon Telescope collaboration is to image the super-massive black holes at the centers of both our Milky Way Galaxy (in the constellation Sagittarius; the black hole region is called Sgr A*) and the relatively nearby galaxy M87, a giant elliptical galaxy at the center of the Virgo cluster of galaxies in the constellation Virgo. The collaboration, which involves 207 scientists from around the world, is led by Dr. Sheperd Doeleman, who has joint appointments at the Harvard-Smithsonian Center for Astrophysics and at MIT's Haystack Observatory.
Although, by definition, light cannot escape from black holes, light that is made by the high-energy particles (plasma) in the material near the black hole can leave the system. The light bends around the black hole before traveling in our direction. Computer simulations predict that an image of the region containing the black hole should contain a dark circle in the center – where the black hole is – surrounded by a ring of light. The dark area is nicknamed the "black hole shadow."
In order to see the ring of light and the shadow, the resolution of the image needs to be comparable to the size of the event horizon of the black hole – the boundary inside of which light cannot escape. This is currently possible only with a technique called "very long baseline interferometry" or VLBI, which combines signals from cosmic objects measured at a number of high-precision radio dishes scattered around the Earth. The data represent a Fourier transform of the image, so after the data are calibrated, an algorithm needs to be run on a computer to do an inverse transform to make the image. At Boston University, the research group led by Professor Marscher and Dr. Jorstad routinely uses VLBI to make images of the centers of galaxies where matter falling onto super-massive black holes creates jets of high-energy particles that radiate across the entire electromagnetic spectrum from radio to gamma-rays. In fact, these active galactic nuclei (AGNs) are the most luminous objects in the universe that live more than a few minutes. (Gamma-ray bursts, which are more luminous for 1-2 minutes, are also thought to be produced by black holes, but those are new black holes with masses in the 5-100 solar mass range resulting from the collapse of the cores of extremely massive stars.) But at the microwave wavelengths of 3 and 7 millimeters that the group uses for its VLBI observations of AGNs, the resolution is too coarse to see the black hole shadow. The EHT gains about 3 times finer resolution by observing at a wavelength of about 1 millimeter, allowing it to make images of the black hole shadow, if it exists as predicted. This is possible for two black holes: one at the center of M87, which is an AGN, with a mass estimated at about 6 billion times the Sun's mass, and Sgr A* at the center of our Milky Way galaxy, with a mass of 4 million times that of the Sun.

Observing with VLBI at a wavelength as short as 1 millimeter is extremely challenging. The Earth's atmosphere absorbs some of the 1 mm waves from cosmic objects, even for telescopes at high altitudes. In addition, VLBI works best if there are a lot of radio dishes involved, but there are a limited number of such dishes available for observations at 1 millimeter - eight in the 2017 EHT observations that were reported on April 10. Making images from a limited amount of VLBI data is very difficult. The EHT director called on Marscher and Jorstad, as experts in VLBI imaging at 7 and 3 mm, to participate in the imaging team and to provide some of their 7 and 3 mm data for practicing new imaging techniques. Marscher serves as the leader of one of four EHT imaging teams, while Jorstad is co-chair of an EHT working group on AGNs. After two week-long workshops at Harvard's Black Hole Initiative center, the imaging working group settled on multiple techniques for making high-quality images from EHT data. The imaging teams made images independently and compared the results, finding that the images made with the different techniques were very similar. This success led to the images reported on April 10 (see figure below).
The images were obtained from observations on 4 different days. Since the size of the event horizon only changes over times of millions of years, the radius of the black hole shadow should be the same for all 4 images. As you can see in the images shown below, that is indeed the case.

EHT images of the black hole in M87
 Images of the black hole at the center of M87 at the microwave wavelength of 1.3 mm. The shadow is obvious, and has the same radius on all 4 days of observation. The white circle shows the resolution of the EHT. The plasma (charged particles and magnetic fields)  is orbiting around the black hole at near-light speed, so that the part coming more toward us (on the southern side) is beamed in our direction, causing it to be brighter.

In order to interpret the images, theorists in the EHT collaboration ran computer simulations with different assumptions and sets of parameters. They made simulated images at ultra-fine resolution from their models. They then proceeded to make artificial data with the same properties as the actual EHT data, and produced images from the artificial data. The figure below presents some of the simulated images, which match the actual images extremely well.

Simulations of the black hole shadow
Top: Simulated images of microwave emission from the plasma surrounding the event horizon. The resolution is that of the simulations. Bottom: How the same images would appear as observed by the EHT. Note the similarity with the actual images shown above.

The images of the black hole shadow in M87 have led to the awarding of the following major prizes to the Event Horizon Telescope Collaboration:

* The inaugural Diamond Achievement Award bestowed by the US National Science Foundation in May 2019
* The 2020 Breakthrough Prize in Fundamental Physics
* The 2020 Rossi Prize by the High Energy Astrophysics Division of the American Astronomical Society
* The 2020 Nelson P. Jackson Aerospace Award by the National Space Club & Foundation

Although the EHT also observed Sgr A*, the collaboration is still in the process of analyzing the data. The problem is that the black hole is more than 1000 times smaller than that in M87. It is small enough that the plasma surrounding the event horizon is likely to change during the observations. Making an image in that case is more difficult than reconstructing a photo when the subject moves during the exposure. So, more work needs to be done to determine whether making an image is possible and, if so, to make the image. The collaboration hopes to figure this out by the end of summer 2019.

Marscher and Jorstad plan to continue working on EHT data, with the hopes of imaging the region a little farther from the black hole in AGNs, where the jets of high-energy particles originate. This will require the addition of more radio dishes to the EHT, which should happen for observations in spring 2020. This should also allow the EHT team to make larger images that can capture the connection between the region near the event horizon in M87 and the jets of particles. Another expected advancement is the addition of linear polarization to the 2017 images. This will reveal the magnetic field pattern near the black hole and in the jet. The magnetic field is thought to control much of the dynamics of the particles just outside the event horizon and in the jet, but the details can currently only be imagined through computer simulations based on a lot of guesswork.

Hubble Space Telescope image of M87 with its jet 
Visible-light Hubble Space Telescope image of M87, revealing its jet. Source:

The VLBI research of Prof. Marscher and Dr. Jorstad is supported by NASA and National Science Foundation grants.

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Publications of the Event Horizon Telescope Collaboration
Pages or Article no.
The Event Horizon General Relativistic Magnetohydrodynamic Code Comparison Project
Porth, O., The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Supplement Series

26 (40 pages)

First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L6 (44 pages)

First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L5 (31 pages)

First M87 Event Horizon Telescope Results. IV. Imaging the Central Supermassive Black Hole
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L4 (52 pages)

First First M87 Event Horizon Telescope Results. III. Data Processing and Calibration
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L3 (32 pages)

First M87 Event Horizon Telescope Results. II. Array and Instrumentation
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L2 (28 pages)

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole
The Event Horizon Telescope Collaboration (including Jorstad, S.G. & Marscher, A.P.) 2019 Astrophysical Journal Letters

L1 (17 pages)