The first image of the shadow of a black hole was released on 10 April 2019 by the Event Horizon Telescope (EHT) collaboration and the National Science Foundation (NSF). The target of their observation was the supermassive black hole located near the center of the Messier 87 (M87) galaxy, which is about 55 million light years from Earth. That black hole is estimated to have a mass 6.5 billion times greater than our Sun.
After further analysis of the historic M87 data, EHT astronomers have been able to measure the polarization of the radio frequency signals from the bright disk of the black hole. Polarization is a signature of the direction of the very strong magnetic fields in the hot glowing gas at the edge of a black hole, which can be seen in the following image released on 24 March 2021.
The ability to measure the polarization in fine detail provides a new tool for mapping the dynamic magnetic field structure of a black hole. The new image shows the magnetic fields in the swirling accretion disk, which contains matter that is falling into the black hole.
Researchers also measured polarization that is pointing directly toward or away from the black hole, perpendicular to the accretion disk. Very strong magnetic fields in these directions may be responsible for launching plasma jets into space, away from the black hole. Such jets have been observed emanating from some black holes.
These are exciting times in astronomy and astrophysics.
The Event Horizon Telescope (EHT) Collaboration reported a great milestone on 10 April 2019 when they released the first synthetic image showing a luminous ring around the shadow of the M87 black hole.
The bright emission ring surrounding the black hole was estimated to have an angular diameter of about 42 ± 3 μas (microarcseconds), or 1.67 ± 0.08 e-8 degrees, at a distance of 55 million light years from Earth. At the resolution of the EHT’s first black hole image, it was not possible to see much detail of the ring structure.
Significantly improved telescope performance is required to discern more detailed structures and, possibly, time-dependent behavior of spacetime in the vicinity of a black hole. The EHT Collaboration has a plan for improving telescope performance. A challenging new observational goal has been established by scientists who recently postulated the existence of a “photon ring” around a black hole. Let’s take a look at these matters.
2. Improving the performance of the EHT terrestrial observatory network
As I described in my 3 March 2017 post on the EHT, a very long baseline interferometry (VLBI) array with the diameter of the Earth (12,742 km, 1.27e+7 meters) operating in the EHT’s millimeter / submillimeter wavelength band (1.3 mm to 0.6 mm) has a theoretical angular resolution of 25 to 12 μas, with the better resolution at the shorter wavelength.
The EHT team plans to improve telescope performance in the following key areas:
Improve the resolution of the EHT
Observe at shorter wavelengths: The EHT’s first black hole image was made at a wavelength of 1.3 mm (230 GHz). Operating the telescopes in the EHT array at a shorter wavelength of 0.87 mm (frequency of 345 GHz) will improve angular resolution by about 40%. This upgrade is expected to start after 2020 and take 3 – 5 years to deploy to all EHT observatories.
Extend baselines: Adding more terrestrial radio telescopes will lengthen some observation baselines, up to the limit of the Earth’s diameter.
Improve the sensitivity of the EHT
Collect data at multiple frequencies (wide bandwidth): Black holes emit radiation at many frequencies. EHT sensitivity and signal-to-noise ratio can be improved by increasing the number of frequencies that are monitored and recorded during EHT observations. This requires multi-channel receivers and faster, more capable data processing and recording systems at all EHT observatories.
The size of individual radio telescopes in the EHT array vary from the 12 m Greenland Telescope with an aperture of about 113 square meters to the 50 m Large Millimeter Telescope (LMT) in Mexico with an aperture of about 2,000 square meters.
The telescope with the largest aperture is the phased ALMA array, which is comprised of up to 54 x 12 m telescopes with a effective aperture of about 7,200 square meters. The Greenland Telescope originally was a prototype for the ALMA array and was relocated to Greenland to support VLBI astronomy.
A phased array is an effective solution for VLBI observations because the requirements for mechanical precision and rigidity of the dish are easier to meet with a smaller radio telescope dish that can be manufactured in large numbers.
With higher angular resolution and improved sensitivity, and with more powerful signal processing to handle the greater volume of data, it may be possible for the EHT to “see” some detailed structures around a black hole. Multiple images of a black hole over a period of time could be used to create a dynamic set of images (i.e., a short “video”) that reveal time-dependent black hole phenomena.
3. Photon ring: New insight into the fine structure in the vicinity of a black hole
On 18 March 2020, a team of scientists postulated the existence of a “photon ring” closely orbiting a black hole. The scientists further postulated that the “glow” from the first few photon sub-rings may be directly observable with a VLBI array like the EHT.
The abstract and part of the summary of the paper are reproduced below.
Abstract: “The Event Horizon Telescope image of the supermassive black hole in the galaxy M87 is dominated by a bright, unresolved ring. General relativity predicts that embedded within this image lies a thin “photon ring,” which is composed of an infinite sequence of self-similar subrings that are indexed by the number of photon orbits around the black hole. The subrings approach the edge of the black hole “shadow,” becoming exponentially narrower but weaker with increasing orbit number, with seemingly negligible contributions from high-order subrings. Here, we show that these subrings produce strong and universal signatures on long interferometric baselines. These signatures offer the possibility of precise measurements of black hole mass and spin, as well as tests of general relativity, using only a sparse interferometric array.”
Summary: “In summary, precise measurements of the size, shape, thickness, and angular profile of the nth photon subring of M87 and Sgr A* may be feasible for n = 1 (the first ring) using a high-frequency ground array or low Earth orbits, for n = 2 (the second ring) with a station on the Moon, and for n = 3 (the third ring) with a station in L2 (Lagrange Point).”
The following short video (1:05 minutes) from the Center for Astrophysics | Harvard & Smithsonian shows an animation of photon behavior in the vicinity of a black hole and the formation of a photon ring.
The creators of the video explain:
“Black holes cast a shadow on the image of bright surrounding material because their strong gravitational field can bend and trap light. The shadow is bounded by a bright ring of light, corresponding to photons that pass near the black hole before escaping.”
“The ring is actually a stack of increasingly sharp subrings, and the n-th subring corresponds to photons that orbited the black hole n/2 times before reaching the observer. This animation shows how a black hole image is formed from these subrings and the trajectories of photons that create the image.”
4. EHT images black hole-powered relativistic jets
On 7 April, 2020, the EHT Collaboration reported that it had produced images with the finest detail ever seen of relativistic jets produced by a supermassive black hole. The target of their observation was Quasar 3C 279, which contains a black hole about one billion times more massive than our Sun, and is about 5 billion light-years away from Earth in the constellation Virgo.
With a resolution of 20 μas (microarcseconds) for observations at a wavelength of 1.3 mm, the EHT imaging revealed that two relativistic jets existed. As shown in the following figure, lower resolution imaging by the Global 3mm VLBI Array (GMVA) and a VLBI array observing at 7 mm wavelength did not show two distinct jets.
In their 7 April 2020 press release, the EHT Collaboration reported: “For 3C 279, the EHT can measure features finer than a light-year across, allowing astronomers to follow the jet down to the accretion disk and to see the jet and disk in action. The newly analyzed data show that the normally straight jet has an unexpected twisted shape at its base and revealing features perpendicular to the jet that could be interpreted as the poles of the accretion disk where the jets are ejected. The fine details in the images change over consecutive days, possibly due to rotation of the accretion disk, and shredding and infall of material, phenomena expected from numerical simulations but never before observed.”
The following short video (1:14 minutes) from the EHT Collaboration shows the 3C 279 quasar jets and their motion over the course of one week, from 5 April to 11 April 2017, as observed by the EHT.
5. Adding space-based EHT observatories
Imaging the M87 photon ring will be a challenging goal for future observations with an upgraded EHT. As indicated in the paper by Michael Johnson, et al., an upgraded terrestrial EHT array may be able to “see” the first photon sub-ring. However, space-based telescopes will be needed to significantly extend the maximum 12,742 km (7,918 miles) baseline of the terrestrial EHT array and provide a capability to image the photon ring in greater detail.
Here’s how the EHT terrestrial baseline would change with space-based observatories:
Low Earth orbit (LEO): Add 370 – 460 km (230 – 286 miles) for a single telescope in an orbit similar to the International Space Station
Geosynchronous orbit: Add 35,786 km (22,236 mi) for a single telescope, or up to twice that for multiple telescopes
Moon: Add Earth-Moon average distance: 384,472 km (238,900 miles)
L2 Lagrange point: Add about 1.5 million km (932,057 miles)
It seems to me that several EHT observatories in geosynchronous orbits could be a good solution that could be implemented sooner than an observatory on the Moon or at L2. Geosynchronous telescopes would greatly expand the EHT baseline and the spacecraft could make long observing runs from orbital positions that are relatively fixed in relation to the terrestrial EHT sites. In-orbit servicing would be more practical in geosynchronous orbit than at L2. In February 2020, Northrop-Grumman demonstrated the ability to remotely restore a large communications satellite that was running out of fuel in geosynchronous orbit. With remote servicing, a geosynchronous observatory could have a long operating life.
6. In conclusion:
With the ongoing improvements to the terrestrial EHT array and its data recording and processing systems, we should see many more black hole observations reported in the years ahead. I’m looking forward to direct observation of M87’s photon ring and the first look at the Sagittarius A* black hole near the center of our Milky Way galaxy. The time delay between data acquisition (i.e., from a series of observation runs of a particular target) and reporting is about three years. This is understandable given the mass of data that must be aggregated from the many EHT observatories to synthesize images of a target black hole. Hopefully, this time delay can be shortened in the years ahead.
Within the next decade, a plan to expand the EHT array to include orbital and/or lunar observatories could be in developed. Hopefully, funding for spacecraft development and deployment will follow.
7. For more information:
See the following sources for more information on the EHT and imaging a black hole:
The first image of a black hole was released on 10 April 2019 at a press conference in Washington D.C. held by the Event Horizon Telescope (EHT) team and the National Science Foundation (NSF). The subject of the image is the supermassive black hole located near the center of the Messier 87 (M87) galaxy. This black hole is about 55 million light years from Earth and is estimated to have a mass 6.5 billion times greater than our Sun. The image shows a glowing circular emission ring surrounding the dark region (shadow) containing the black hole. The brightest part of the image also may have captured a bright relativistic jet of plasma that appears to be streaming away from the black hole at nearly the speed of light, beaming generally in the direction of Earth.
The EHT is not one physical telescope. Rather, it an array of millimeter and sub-millimeter wavelength radio telescopes located around the world. The following map shows the eight telescopes that participated in the 2017 observations of M87. Three additional telescopes joined the EHT array in 2018 and later.
All of the EHT telescopes are used on a non-dedicated basis by an EHT team of more than 200 researchers during a limited annual observing cycle. The image of the M87 black hole was created from observations made during a one week period in April 2017.
The long baselines between the individual radio telescopes give the “synthetic” EHT the resolving power of a physical radio telescope with a diameter that is approximately equal to the diameter of the Earth. A technique called very long-baseline interferometry (VLBI) is used to combine the data from the individual telescopes to synthesize the image of a black hole. EHT Director, Shep Doeleman, referred to VLBI as “the ultimate in delayed gratification among astronomers.” The magnifying power of the EHT becomes real only when the data from all of the telescopes are brought together and the data are properly combined and processed. This takes time.
At a nominal operating wavelength of about 1.3 mm (frequency of 230 GHz), EHT angular resolution is about 25 microarcseconds (μas), which is sufficient to resolve nearby supermassive black hole candidates on scales that correspond to their event horizons. The EHT team reports that the M87 bright emission disk subtends an angle of 42 ± 3 microarcseconds.
For comparison, the resolution of a human eye in visible light is about 60 arcseconds (1/60thof a degree; there are 3,600 arcseconds in one degree) and the 2.4-meter diameter Hubble Space Telescope has a resolution of about 0.05 arcseconds (50,000 microarcseconds).
You can read five open access papers on the first M87 Event Horizon Telescope results written by the EHT team and published on 10 April 2019 in the Astrophysical Journal Letters here:
Congratulations to the EHT Collaboration for their extraordinary success in creating the first-ever image of a black hole shadow.
7 April 2020 Update: EHT observations were complemented by multi-spectral (multi-messenger) observations by NASA spacecraft
On 10 April 2019, NASA reported on its use of several orbiting spacecraft to observe M87 in different wavelengths during the period of the EHT observation.
“To complement the EHT findings, several NASA spacecraft were part of a large effort, coordinated by the EHT’s Multiwavelength Working Group, to observe the black hole using different wavelengths of light. As part of this effort, NASA’s Chandra X-ray Observatory, Nuclear Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift Observatory space telescope missions, all attuned to different varieties of X-ray light, turned their gaze to the M87 black hole around the same time as the EHT in April 2017. NASA’s Fermi Gamma-ray Space Telescope was also watching for changes in gamma-ray light from M87 during the EHT observations.”
“NASA space telescopes have previously studied a jet extending more than 1,000 light-years away from the center of M87. The jet is made of particles traveling near the speed of light, shooting out at high energies from close to the event horizon. The EHT was designed in part to study the origin of this jet and others like it.”
NASA’s Neutron star Interior Composition Explorer (NICER) experiment on the International Space Station also contributed to the multi-spectral observations of M87, which were coordinated by EHT’s Multiwavelength Working Group.
On April 25, 2019, NASA released the following composite image showing the M87 galaxy, the position of the black hole and large relativistic jets of matter being ejected from the black hole. These infrared images were made by NASA’s orbiting Spitzer Space Telescope.
For more information:
See the following sources for more information on the EHT and imaging the M87 black hole: