Tag Archives: Very Long Baseline Interferometry

The Event Horizon Telescope Team has Produced the First Image Showing the Shadow of a Black Hole

The first image of a black hole was released today 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 first ever image showing the shadow of a black hole (M87), which remains unseen at the center of the dark circular region.
Source:The EHT Collaboration, et al.

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.  

The EHT array as used for the April 2017 observations.  
Source: The EHT Collaboration, et al.

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:

You’ll find more information on the Event Horizon Telescope website, here:  https://eventhorizontelescope.org/about

There are ongoing efforts to improve the performance of the EHT.  Key areas for improvement include:

  • Operating the telescopes in the array at a shorter wavelength of 0.87 mm (frequency of 345 GHz) will improve angular resolution by about 40%.  This is expected to start after 2020 and take 3 – 5 years to deploy.
  • Adding terrestrial telescopes to the array will improve image fidelity and lengthen some baselines to improve angular resolution.
  • Collecting data over wider bandwidths (several frequencies)
  • Enabling dynamic imaging of time-variable phenomena, which will be very useful for observing the dynamic black hole known as Sgr A* at the center of our Milky Way galaxy.
  • In the longer term, adding orbital radio telescopes to EHT will greatly expand the observation baselines and significantly improve angular resolution.

Congratulations to the EHT Collaboration for their extraordinary success in creating the first-ever image of a black hole shadow.

For more background information on the Event Horizon Telescope, see the following Lyncean post:

5 May 2019 Update:  Photo of the M87 galaxy released by NASA

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.  

The M87 galaxy, with two expanded views, first showing the location of the black hole and two plasma jets (orange) at the center of M87, and second, the closeup EHT image of the black hole’s shadow.  
Source: NASA/JPL-Caltech/IPAC/Event Horizon Telescope


The Event Horizon Telescope

The Event Horizon Telescope (EHT) is a huge synthetic array for Very Long Baseline Interferometry (VLBI), which is created through the collaboration of millimeter / submillimeter wave radio telescopes and arrays around the world. The goal of the EHT “is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon.”

The primary target for observation is Sagittarius A* (Sgr A*), which is the massive black hole at the center of our Milky Way galaxy. This target is of particular interest to the EHT team because it “presents the largest apparent event horizon size of any black hole candidate in the Universe.” The Sgr A* event horizon is estimated to have a Schwarzschild radius of 12 million kilometers (7.46 million miles) or a diameter of 24 million km (14.9 million miles). The galactic core (and hence Sgr A*) is estimated to be 7.6 to 8.7 kiloparsecs (about 25,000 to 28,000 lightyears, or 1.47 to 1.64e+17 miles) from Earth. At that distance, the Sgr A* black hole subtends an angle of about 2e-5 arcseconds (20 microarcseconds).

Another EHT target of interest is a much more distant black hole in the Messier 87 (M87) galaxy.

The member arrays and telescopes supporting EHT are:

  • Arizona Radio Observatory /Submillimeter Wave Telescope (ARO/SMT, Arizona, USA)
  • Atacama Pathfinder EXperiment (APEX, Chile)
  • Atacama Submillimeter Telescope Experiment (ASTE, Chile)
  • Combined Array for Research in Millimeter-wave Astronomy (CARMA, California, USA)
  • Caltech Submillimeter Observatory (Hawaii, USA)
  • Institute de Radioastronomie Millimetrique (IRAM, Spain)
  • James Clerk Maxwell Telescope (JCMT, Hawaii)
  • Large Millimeter Telescope Alfonso Serrano (LMT, Mexico)
  • The Submillimeter Array (Hawaii, USA)

The following arrays and telescopes are expected to join the EHT collaboration:

  • Atacama Large Millimeter / submillimeter Array (ALMA, Chile)
  • Northern Extended Millimeter Array (NOEMA, France)
  • South Pole Telescope (SPT, Antarctica)

Collectively, the arrays and telescopes forming the EHT provide a synthetic aperture that is almost equal to the diameter of the Earth (12,742 km, 7,918 miles).

EHT array sizeSource: graphics adapted by A. Cuadra / Science; data from Event Horizon Telescope

Technical improvements to the member telescopes and arrays are underway with the goal of systematically improving EHT performance. These improvements include development and deployment of:

  • Submillimeter dual-polarization receivers (energy content of cosmic radiation is split between two polarizations)
  • Highly stable frequency standards to enable VLBI at frequencies between 230 to 450 GHz (wavelengths of 1.3 mm – 0.6 mm).
  • Higher-bandwidth digital VLBI backends and recorders

In operations to date, EHT has been observing the Sgr A* and M87 black holes at 230 GHz (1.3 mm) with only some of the member arrays and telescopes participating. These observations have yielded angular resolutions of better than 60 microarcseconds. Significantly higher angular resolutions, up to about 15 microarcseconds, are expected from the mature EHT operating at higher observing frequencies and with longer baselines.

Coordinating observing time among all of the EHT members is a challenge, since participation in EHT is not a dedicated mission for any site. Site-specific weather also is a factor, since water in the atmosphere absorbs radiation in the EHT observing frequency bands. The next observing opportunity is scheduled between 5 – 14 April 2017. Processing the data from this observing run will take time, hence results are not expected to be known until later this year.

For more information on EHT, see the 2 March 2017 article by Daniel Clery entitled, ”This global telescope may finally see the event horizon of our galaxy’s giant black hole,” at the following link:

http://www.sciencemag.org/news/2017/03/global-telescope-may-finally-see-event-horizon-our-galaxys-giant-black-hole?utm_campaign=news_daily_2017-03-02&et_rid=215579562&et_cid=1194555

Much more information is available on the EHT website at the following link:

http://www.eventhorizontelescope.org

Radio telescope resolution

An article on the Las Cumbres Observatory (LCO) website explains how the angular resolution of radio telescopes, including VLBI arrays, is determined. In this article, the author, D. Stuart Lowe, states that “an array of radio telescopes of 217 km in diameter can produce an image with a resolution equivalent to the Hubble Space Telescope.” You’ll find this article here:

https://lco.global/spacebook/radio-telescopes/

The Hubble Space Telescope has an angular resolution of 1/10th of an arcsecond (1e-1 arcsecond).

A VLBI array with the diameter of the Earth (1.27e+6 meters) operating in the EHT’s millimeter / submillimeter wavelength band (1.3e-3 to 6.0e-4 meters) has a theoretical angular resolution of 2.5e-5 to 1.2e-5 arcseconds (25 to 12 microarcseconds).

EHT should be capable of meeting its goal of angular resolution comparable to a black hole’s event horizon.

X-ray observation of Sgr A*

Combining infrared images from the Hubble Space Telescope with images the Chandra X-ray Observatory, NASA created the following composite image showing the galactic core in the vicinity of Sgr A*. NASA reports:

“The large image contains X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards.”

This image gives you a perspective on the resolution of Sgr A* possible at X-ray frequencies with current equipment. EHT will have much higher resolution in its radio frequency bands.

NASA Sgr A* picSource: X-Ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

More details on this image are available at the following NASA link:

https://www.nasa.gov/mission_pages/chandra/multimedia/black-hole-SagittariusA.html

Animation of Sgr A* effects on nearby stars

See my 24 January 2017 post, “The Black Hole at our Galactic Center is Revealed Through Animations,” for more information on how teams of astronomers are developing a better understanding of the unseen Sgr A* black hole through long-term observations of the relative motions of nearby stars that are under the influence of this black hole.  These observations have been captured in a very interesting animation.