In April 2021, I posted a short article entitled, “Multi-messenger Astronomy Provides Extraordinary Views of Uranus,” which included two composite views of Uranus, created by combining near-infrared images taken by the Keck-1 telescope at an elevation of 4,145 meters (13,599 ft) on Maunakea, Hawaii, with X-ray images taken with the Advanced CCD Imaging Spectrometer (ACIS) aboard the orbiting Chandra X-Ray Observatory.
The Webb images of Neptune, taken on July 12, 2022, are reproduced below.
The Space Telescope Science Institute (STScI) has created a Resource Gallery of Webb Space Telescope images, which you can browse here: https://webbtelescope.org/resource-gallery/images. Currently there are 280 images in the Webb Resource Gallery. I think this is a website worth revisiting from time to time.
NASA’s Solar System Exploration website provides views of Neptune from several earlier sources, including the 1989 Voyager 2 deep space probe, the Hubble Space Telescope and the European Southern Observatory’s (ESO) Very Large Telescope (VLT). Check it out here: https://solarsystem.nasa.gov/planets/neptune/galleries/
2018: The following image was taken in July 2018 during the testing of the narrow-field, adaptive optics mode of the optical/infrared MUSE/GALACSI instrument on ESO’s VLT, which is located at an elevation of 2,635 m (8,645 ft) at Cerro Paranal, in the Atacama Desert of northern Chile.
1994: The more recent Webb Space Telescope and VLT images are much better than the Hubble Space Telescope optical-range images of Neptune taken more than two decades earlier, in 1994.
1989: In October 1989, the following whole planet view of Neptune was produced using images taken through the green and orange filters on the narrow angle camera during the Voyager 2 spacecraft flyby of the planet.
In the future, we can hopefully look forward to more detailed multi-messenger images of Neptune, combining the near-infrared images from Webb with images from other observatories that can view the planet in different spectral bands.
The first-ever direct image of a black hole was released on 10 April 2019 by the Event Horizon Telescope (EHT) team and the National Science Foundation (NSF). The target for their observation was the supermassive M87* black hole at the center of the distant Messier 87 (M87) galaxy, some 54 million light years away. The EHT team estimated that M87* has a mass of about 6.5 billion Solar-masses (6.5 billion times greater than the mass of our Sun), and the black hole consumes the equivalent of about 900 Earth-masses per day. One Solar mass is roughly equivalent to the weight of the Sun and about 333,000 times the mass of Earth. Gases orbiting around the giant M87* black hole take days to weeks to complete an orbit. For more information on the first M87* black hole image, see my 10 April 2019 article here: https://lynceans.org/all-posts/the-event-horizon-telescope-team-has-produced-the-first-image-showing-the-shadow-of-a-black-hole/
For decades, there has been mounting evidence that there is a massive black hole, known as Sagittarius A*, or Sgr A* for short, at the center of our Milky Way galaxy. Its presence has been inferred from the motions of visible stars that are orbiting under the gravitational influence of the black hole or are in the general vicinity of the black hole. Using observed data from more than 30 stars in the region around the galactic center, scientists developed high-resolution simulations that helped refine estimates of the location, mass and size of the Sgr A* black hole without having data from direct observations. For more information on this work, see my 24 January 2017 article here: https://lynceans.org/all-posts/the-black-hole-at-our-galactic-center-is-revealed-through-animations/
Even though it was much closer than M87*, getting an image of Sgr A* was much harder because the Sgr A* black hole had to be viewed through the densely populated central plane of our Milky Way. The Sgr A* radio frequency (millimeter wave) observations were made in 2017 at a wavelength of 1.3 mm (230 GHz), the same as the first image of M87*.
Details that have emerged so far from the Sgr A* observation include the following.
Sgr A* is about 27,000 light years away, at the heart of our own galaxy (about 2 thousand times closer than M87*, which is in a different galaxy).
Sgr A* has a mass is about 4 million times the mass of our Sun, which is just a small fraction (1/1,500th , or 0.07%) of the mass of M87*.
The glowing gas ring surrounding the Sgr A* black hole has an outer diameter of about 72 million miles (115 million km) across, which is approximately the diameter of Mercury’s orbit around the Sun in our solar system. The EHT team reported, “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity.” By comparison, M87* is vastly larger, with the inner black hole region measuring about 23.6 billion miles (38 billion km) across (about 330 times the diameter of the entire Sgr A* black hole, including the glowing gas ring), as shown in the following scale diagram.
The two black holes subtend approximately the same angle when viewed from Earth. The EHT team reported that the M87* bright emission disk subtends an angle of 42 ± 3 microarcseconds.
Gases orbiting around the Sgr A* black hole take mere minutes to an 1 hour to complete an orbit. The fast moving gases blur the image for an EHT observation typically lasting several hours. The released image of the Sgr A* black hole is an average of many different images the EHT team extracted from the data.
Sgr A* is far less active than M87*, and consumes only about 1/1,000th the mass per day (equivalent of about 1 Earth-mass per day).
The source of the three bright spots in the glowing gas ring are unknown at this time. They may be artifacts of the EHT observation process.
Follow-on EHT observations will benefit from additional telescopes joining the EHT network and significant technical improvements being made to the EHT telescopes and network systems. For example, 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%. More frequent observations and faster data processing would enable time-lapse movies to be created to show the dynamics of gas motion around the black hole. Details on planned improvements are discussed in my 9 April 2020 article here: https://lynceans.org/all-posts/working-toward-a-more-detailed-view-of-a-black-hole/
In most of the electromagnetic spectrum, a star composed of normal matter and a star composed of antimatter (anti-star) will look the same to an observer on Earth. Their visible spectra will be indistinguishable. A key difference in behavior may be observable in the gamma ray spectrum, where high-energy gamma rays characteristic of matter-antimatter annihilation (i.e., baryon-antibaryon reactions) may reveal the identity of an antimatter star within our galaxy or an antimatter star cluster outside our galaxy. Luigi Foschini provides a good introduction to this subject in his 2000 paper at the following link: https://cds.cern.ch/record/447091/files/0007180.pdf
NASA’s Alpha Magnetic Spectrometer (AMS) has developed into an important tool in the search for anti-stars. The prototype, AMS-01 flew on the STS-91 Space Shuttle mission from 2 to 12 June 1998 and was successfully tested in orbit. The full-scale AMS-2 was launched aboard the STS-134 Space Shuttle mission on 16 May 2011. Since it was installed on the International Space Station (ISS) and activated on 19 May 2011, this 18,739 pound (8,500 kg), 2,250 cu. ft (64 cu meter) instrument has collected and analyzed more than 165 billion cosmic ray events (as of April 2021), and identified 9 million of these as antimatter, including the possible detection of antihelium nuclei.
Another important source of data related to antimatter in our universe is NASA’s Fermi Gamma-ray Space Telescope, which was launched into a low Earth orbit on June 11, 2008. NASA’s website for the ongoing Fermi mission is here: https://fermi.gsfc.nasa.gov
In an 8 February 2021 article, astrophysicist Paul Sutter postulates the existence of antimatter star clusters that escaped the primordial matter-antimatter annihilations and now exist in relative isolation, for example, as an antimatter star cluster orbiting our Milky Way galaxy.
The antimatter stars in the cluster would continuously shed antimatter into the cosmos, leading to subsequent matter-antimatter interactions that produce high-energy particles that may be detectable from Earth.
Sutter commented, “…if astronomers are able to pinpoint a globular cluster as a particularly strong source of anti-particles, it would be like opening a time capsule, giving us a window into the physics that dominated the universe when it was only a second old.”
In a 20 April 2021 paper, authors Dupourqué, Tibaldo, and von Ballmoos report the possible detection of 14 anti-stars within our Milky Way galaxy. They used 10 years of data on 5,800 gamma-ray sources in Fermi’s data catalog to develop an estimate of the possible abundance of anti-stars. The authors report: “We identify in the catalog 14 anti-star candidates not associated with any objects belonging to established gamma-ray source classes and with a spectrum compatible with baryon-antibaryon annihilation.”
The 14 anti-star candidates await further analysis to confirm or refute their existence. If confirmed, they represent only a small fraction of the population of all gamma-ray sources observed by the Fermi Gamma-ray Space Telescope. Nonetheless, even one confirmed anti-star would be a remarkable achievement.
For more information:
Steve Naftilan, “How do we know that distant galaxies are composed of matter rather than anti-matter? If equal quantities of each were produced in the big bang, might not some parts of the universe contain primarily matter and other parts primarily anti-matter?” Scientific American, 21 October 1999: https://www.scientificamerican.com/article/how-do-we-know-that-dista/
Simon Dupourqué, Luigi Tibaldo, and Peter von Ballmoos, “Constraints on the antistar fraction in the Solar System neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog,” Phys. Rev. D 103, 083016, 20 April 2021 (abstract only without subscription): https://journals.aps.org/prd/abstract/10.1103/PhysRevD.103.083016
In March 2021, the National Aeronautics and Space Administration (NASA) announced that its orbiting Chandra X-ray Observatory had made the first ever detection of X-rays coming from the ice giant planet Uranus. Recent analysis of Chandra observations from 2002 and 2017 resulted in this discovery.
X-rays coming from other planets have been detected in the past. NASA reported, “Like Jupiter and Saturn, Uranus and its rings appear to mainly produce X-rays by scattering solar X-rays, but some may also come from auroras…… The X-rays from auroras on Jupiter come from two sources: electrons traveling down magnetic field lines, as on Earth, and positively charged atoms and molecules raining down at Jupiter’s polar regions. However, scientists are less certain about what causes auroras on Uranus.”
Another possible X-ray source could be from an interaction between Uranus’ rings and the near-space charged particle environment around the planet. This phenomenon has been observed at Saturn.
In connection with the discovery of X-rays coming from Uranus, NASA released two spectacular composite (multi-messenger) images of the planet created by combining images from two different parts of the electromagnetic spectrum: optical / near-infrared and X-ray.
Visible light has a wavelength in the range from about 350 to 750 nanometers (nm, 10-9meters) or 3,500 to 7,500 Angstroms. Near-infrared light is the part of the infrared spectrum that is closest to the visible light spectrum, but at a longer wavelength, from about 800 to 2,500 nm. X-rays have a much shorter wavelength, from about 20 to 0.001 nm. In the following chart, you can see the relative placement of visible and near-infrared light and X-rays in the electromagnetic spectrum.
The components of the first composite image are described below:
Near-infrared image: This was taken in July 2004 with the 10-meter (32-foot 10-inch) Keck-1 telescope located at an elevation of 4,145 meters (13,599 ft) on Maunakea, Hawaii. Image credit: Heidi B. Hammel, Space Science Institute; Lawrence Sromovsky, University of Wisconsin-Madison / W. M. Keck Observatory
The X-ray image: This was produced with 7 August 2002 data from the Advanced CCD Imaging Spectrometer (ACIS) aboard Chandra, which has a spatial resolution of 0.5” (seconds). The angular size of Uranus for the observation was 3.7”. The X-rays were in the 0.6 to 1.1 keV (2.1 to 1.1 nm) spectral range, which is consistent with X-ray emissions from Jupiter and Saturn. Image credit: NASA/CXO/University College London/W. Dunn et al.
The second 2021 composite image, shown below, was created from a Keck optical image and X-ray images made with Chandra’s High Resolution Camera (HRC) during observations on 11 and 12 November 2017. The HRC is sensitive to softer X-ray emissions (down to 0.06 keV, 20.7 nm) than ACIS, enabling it to collect more photons in the 0.1–1.2 keV (12.4 to 0.1 nm) range most important for planetary studies. The authors report, ”These fluxes exceed expectations from scattered solar emission alone, suggesting either a larger X-ray albedo than Jupiter/Saturn or the possibility of additional X-ray production processes at Uranus.”
The authors conclude by noting that, “Further, and longer, observations with Chandra would help to produce a map of X-ray emission across Uranus and to identify, with better signal-to-noise, the source locations for the X-rays, constraining possible contributions from the rings and aurora…… However, the current generation of X-ray observatories does not provide sufficient sensitivity to spectrally characterize the short interval temporal fluctuation observed in the November 12, 2017 observation.”
New space-based X-ray observational capabilities are being developed by NASA and the European Space Agency (ESA), but won’t be operational for a decade or more:
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 Atacama Cosmology Telescope (ACT) is a six-meter (19.7 foot) radio telescope designed to make high-resolution, microwave-wavelength surveys of the cosmic microwave background (CMB). It is located at a remote site in the Atacama Desert at an elevation of 5,190 meters (17,030 feet) in northern Chile.
ACT observes in three frequency bands (148, 218 and 277 GHz) and has a resolution of 1.3 arc minutes at 148 GHz, near the peak of the CMB spectrum. This is significantly higher than the 5-10 arc minute resolution of the Planck spacecraft, which observed the CMB from 2009 to 2013 in the frequency range from 30 to 857 GHz. You’ll find a detailed description of the Atacama Cosmology Telescope (ACT) at the following link: https://www.cosmos.esa.int/documents/387566/387653/Ferrara_Dec3_09h20_Devlin_ACT.pdf
New results from the ACT survey, reported in December 2020, affirm the Planck CMB survey results.
The universe is isotropic
The estimate of the age of the universe was refined to 13.77 billion years old ± 0.04 billion years, overlapping uncertainty bands with the 2015 Planck estimate of 13.813 ± 0.038 billion years
The value of the Hubble constant was refined to 67.6 kilometers / second / megaparsec, up slightly from the 2018 Planck estimate of 67.4 kilometers / second / megaparsec. The significant difference from the value derived from astrophysical measurements, 73.5 km / second / megaparsec, remains unexplained.
Japan’s Hayabusa2 (Japanese for Peregrine falcon 2) spacecraft returned from its six-year mission to asteroid 162173 Ryugu for a high-speed fly-by of Earth on 5 December 2020, during which it released a reentry capsule containing the material collected during two separate sampling visits to the asteroid’s surface. The capsule successfully reentered Earth’s atmosphere, landed in the planned target area in Australia’s Woomera Range and was recovered intact. The sample return capsule is known as the “tamatebako” (treasure box).
The first asteroid sample return mission was Japan’s Hayabusa1, which was launched on 9 May 2003 and rendezvoused with S-type asteroid 25143 Itokawa in mid-September 2005. A small sample was retrieved from the surface on 25 November 2005. The sample, comprised of tiny grains of asteroidal material, was returned to Earth on 13 June 2010, with a landing in the Woomera Range.
Japan’s Hayabusa2 and the US OSIRIS-Rex asteroid sample return missions overlap, with Hayabusa2 launching about two years earlier and returning its surface samples almost three years earlier. Both spacecraft were orbiting their respective asteroids from 31 December 2018 to 12 November 2019.
You’ll find a great deal of information and current news on the Hayabusa2 and OSIRIS-REx asteroid sample return missions on their respective project website:
An extended mission to explore additional asteroids was made possible by the excellent health of the Hayabusa2 spacecraft and the economic use of fuel during the basic mission. Hayabusa2 still has 30 kg (66 lb) of xenon propellant for its ion engines, about half of its initial load of 66 kg (146 lb).
As of September 2020, JAXA’s plans are is to target the Hayabusa2 spacecraft for the following two asteroid encounters:
Conduct a high-speed fly-by of L-type asteroid (98943) 2001 CC21 in July 2026. This asteroid has a diameter between 3.47 to 15.52 kilometers (2.2 to 9.6 miles).
Continue on a rendezvous with asteroid 1998 KY26 in July 2031. This is a 30-meter (98-foot) diameter asteroid, potentially X-type (metallic), and rotating rapidly with a period of only 10.7 minutes.
The Arecibo Observatory (AO) on Puerto Rico has been out of service since 10 August 2020, when a three-inch auxiliary support cable slipped out of its socket and fell onto the fragile radio telescope dish below. Three months later, on 6 November 2020, a second cable associated with the same support tower broke, damaging nearby cables, causing more damage to the reflector dish, and leaving the radio telescope’s support structure in a weakened and uncertain state.
On 19 November 2020, the National Science Foundation (NSF) announced it has begun planning for decommissioning the 57-year old Arecibo Observatory’s (AO) 1,000-foot (305-meter) radio telescope due to safety concerns after the two support wires broke and seriously damaged the antenna. You can read NSF News Release 20-010 at the following link: https://www.nsf.gov/news/news_summ.jsp?cntn_id=301674
Not included in the NSF timeline is the 1974 first-ever broadcast into deep space of a powerful signal that could alert other intelligent life to our technical civilization on Earth. The 1,679 bit “Arecibo Message” was directed toward the globular star cluster M13, which is 22,180 light years away. The message will be in transit for another 22,134 years.
A key capability lost is AO’s planetary radar capability that enabled the large dish to function as a high-resolution, active imaging radar. You’ll find examples of AO’s radar images of the Moon, planets, Jupiter’s satellites, Saturn’s rings, asteroids and comets on the NSF website here: https://www.naic.edu/~pradar/radarpage.html
More impressive than the still images were animations created from a sequence of AO radar images, particularly of passing asteroids. The animations defined the motion of the object as it flew near Earth. As an example, you can watch the following short (1:07 minutes) video, “Big asteroid 1998 OR2 seen in radar imagery ahead of fly-by”:
The US still has a reduced capability for planetary radar imaging with NASA’s Deep-Space Network’s Uplink Array.
The 19 November 2020 NSF news release stated, “After the telescope decommissioning, NSF would intend to restore operations at assets such as the Arecibo Observatory LIDAR facility — a valuable geospace research tool — as well as at the visitor center and offsite Culebra facility, which analyzes cloud cover and precipitation data.”
Adieu to radio astronomy at Arecibo.
Update 1 December 2020: Arecibo radio telescope collapsed.
NPR reported, “The Arecibo Observatory in Puerto Rico has collapsed, after weeks of concern from scientists over the fate of what was once the world’s largest single-dish radio telescope. Arecibo’s 900-ton equipment platform, suspended 500 feet above the dish, fell overnight after the last of its healthy support cables failed to keep it in place. No injuries were reported, according to the National Science Foundation, which oversees the renowned research facility.”
Update 8 December 2020: National Science Foundation video shows the moment of collapse.
Update 19 October 2022: No NSF funding
On 18 October 2022, Science magazine reported on NSF’s plans to convert the iconic observatory in Puerto Rico into a center for education and outreach in science, technology, engineering, and math (STEM). The limited funding available for this purpose “does not include support for remaining instruments at the site, including a 12-meter radio telescope, a radio spectrometer, and a suite of optical laser instruments for studying the upper atmosphere.”
On 20 April 2020, the U.S. Geological Survey (USGS) released the first-ever comprehensive digital geologic map of the Moon. The USGS described this high-resolution map as follows:
“The lunar map, called the ‘Unified Geologic Map of the Moon,’ will serve as the definitive blueprint of the moon’s surface geology for future human missions and will be invaluable for the international scientific community, educators and the public-at-large.”
This remarkable mapping product is the culmination of a decades-long project that started with the synthesis of six Apollo-era (late 1960s – 1970s) regional geologic maps that had been individually digitized and released in 2013 but not integrated into a single, consistent lunar map.
This intermediate mapping product was updated based on data from the following more recent lunar satellite missions:
The Lunar Reconnaissance Orbiter Camera (LROC) is a system of three cameras that capture high resolution black and white images and moderate resolution multi-spectral images of the lunar surface: http://lroc.sese.asu.edu
Topography for the north and south poles was supplemented with Lunar Orbiter Laser Altimeter (LOLA) data: https://lola.gsfc.nasa.gov
The final product is a seamless, globally consistent map that is available in several formats: geographic information system (GIS) format at 1:5,000,000-scale, PDF format at 1:10,000,000-scale, and jpeg format.
At the following link, you can download a large zip file (310 Mb) that contains a jpeg file (>24 Mb) with a Mercator projection of the lunar surface between 57°N and 57°S latitude, two polar stereographic projections of the polar regions from 55°N and 55°S latitudes to the poles, and a description of the symbols and color coding used in the maps.
These high-resolution maps are great for exploring the lunar surface in detail. A low-resolution copy (not suitable for browsing) is reproduced below.
For more information on the Unified Geologic Map of the Moon, refer to the paper by C. M. Fortezzo, et al., “Release of the digital Unified Global Geologic Map of the Moon at 1:5,000,000-scale,” which is available here: https://www.hou.usra.edu/meetings/lpsc2020/pdf/2760.pdf
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).”
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: