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  • richardmitnick 1:57 pm on January 3, 2018 Permalink | Reply
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    From Ethan Siegel: “2018 Will Be The Year Humanity Directly ‘Sees’ Our First Black Hole” 

    Ethan Siegel
    Jan 3, 2018

    1
    The black hole, as illustrated in the movie Interstellar, shows an event horizon fairly accurately for a very specific class of rotating black holes. Image credit: Interstellar / R. Hurt / Caltech.

    The Event Horizon Telescope has come online and taken its data. Now, we wait for the results.

    Black holes are some of the most incredible objects in the Universe. There are places where so much mass has gathered in such a tiny volume that the individual matter particles cannot remain as they normally are, and instead collapse down to a singularity. Surrounding this singularity is a sphere-like region known as the event horizon, from inside which nothing can escape, even if it moves at the Universe’s maximum speed: the speed of light. While we know three separate ways to form black holes, and have discovered evidence for thousands of them, we’ve never imaged one directly. Despite all that we’ve discovered, we’ve never seen a black hole’s event horizon, or even confirmed that they truly had one. Next year, that’s all about to change, as the first results from the Event Horizon Telescope will be revealed, answering one of the longest-standing questions in astrophysics.

    Event Horizon Telescope Array

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    NSF CfA Greenland telescope

    Greenland Telescope

    The idea of a black hole is nothing new, as scientists have realized for centuries that as you gather more mass into a given volume, you have to move at faster and faster speeds to escape from the gravitational well that it creates. Since there’s a maximum speed that any signal can travel at — the speed of light — you’ll reach a point where anything from inside that region is trapped. The matter inside will try to support itself against gravitational collapse, but any force-carrying particles it attempts to emit get bent towards the central singularity; there is no way to exert an outward push. As a result, a singularity is inevitable, surrounded by an event horizon. Anything that falls into the event horizon? Also trapped; from inside the event horizon, all paths lead towards the central singularity.

    2
    An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets, may describe the black hole at the center of our galaxy in many regards. Image credit: Mark A. Garlick.

    Practically, there are three mechanisms that we know of for creating real, astrophysical black holes.

    1.When a massive enough star burns through its fuel and goes supernova, the central core can implode, converting a substantial fragment of the pre-supernova star into a black hole.
    2.When two neutron stars merge, if their combined post-merger mass is more than about 2.5-to-2.75 solar masses, it will result in the production of a black hole.
    3.And if either a massive star or a cloud of gas can undergo direct collapse, it, too, will produce a black hole, where 100% of the initial mass goes into the final black hole.

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    Artwork illustrating a simple black circle, perhaps with a ring around it, is an oversimplified picture of what an event horizon looks like. Image credit: Victor de Schwanberg.

    Over time, black holes can continue to devour matter, growing in both mass and size commensurately. If you double the mass of your black hole, its radius doubles as well. If you increase it tenfold, the radius goes up by a factor of ten, also. This means that as you go up in mass — as your black hole grows — its event horizon gets larger and larger. Since nothing can escape from it, the event horizon should appear as a black “hole” in space, blocking the light from all objects behind it, compounded by the gravitational bending of light due to the predictions of General Relativity. All told, we expect the event horizon to appear, from our point of view, 250% as large as the mass predictions would imply.

    4
    A black hole isn’t just a mass superimposed over an isolated background, but will exhibit gravitational effects that stretch, magnify and distort background light due to gravitational lensing. Image credit: Ute Kraus, Physics education group Kraus, Universität Hildesheim; Axel Mellinger (background).

    Taking all of this into account, we can look at all the known black holes, including their masses and how far away they are, and compute which one should appear the largest from Earth. The winner? Sagittarius A*, the black hole at the center of our galaxy. Its combined properties of being “only” 27,000 light years distant while still reaching a spectacularly large mass that’s 4,000,000 times that of the Sun makes it #1. Interestingly, the black hole that hits #2 is the central black hole of M87: the largest galaxy in the Virgo cluster. Although it’s over 6 billion solar masses, it lies some 50–60 million light years away. If you want to see an event horizon, our own galactic center is the place to look.

    SGR A* , the supermassive black hole at the center of the Milky Way. NASA’s Chandra X-Ray Observatory

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    Some of the possible profile signals of the black hole’s event horizon as simulations of the Event Horizon Telescope indicate. Image credit: High-Angular-Resolution and High-Sensitivity Science Enabled by Beamformed ALMA, V. Fish et al., arXiv:1309.3519.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    If you had a telescope the size of Earth, and nothing in between us and the black hole to block the light, you’d be able to see it, no problem. Some wavelengths are relatively transparent to the intervening galactic matter, so if you look at long-wavelength light, like radio waves, you could potentially see the event horizon itself. Now, we don’t have a telescope the size of Earth, but we do have an array of radio telescopes all across the globe, and the techniques of combining this data to produce a single image. The Event Horizon Telescope brings the best of our current technology together, and should enable us to see our very first black hole.

    Instead of a single telescope, 15-to-20 radio telescopes are arrayed across the globe, observing the same target simultaneously. With up to 12,000 kilometers separating the most distant telescopes, objects as small as 15 microarcseconds (μas) can be resolved: the size of a fly on the Moon. Given the mass and distance of Sagittarius A*, we expect that to appear more than twice as large as that figure: 37 μas. At radio frequencies, we should see lots of charged particles accelerated by the black hole, but there should be a “void” where the event horizon itself lies. If we can combine the data correctly, we should be able to construct a picture of a black hole for the very first time.

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    Five different simulations in general relativity, using a magnetohydrodynamic model of the black hole’s accretion disk, and how the radio signal will look as a result. Note the clear signature of the event horizon in all the expected results. Image credit: GRMHD simulations of visibility amplitude variability for Event Horizon Telescope images of Sgr A*, L. Medeiros et al., arXiv:1601.06799.

    The telescopes comprising the Event Horizon Telescope took their very first shot at observing Sagittarius A* simultaneously last year. The data has been brought together, and it’s presently being prepared and analyzed. If everything operates as designed, we’ll have our first image in 2018. Will it appear as General Relativity predicts? There are some incredible things to test:

    -whether the black hole has the right size as predicted by general relativity,
    -whether the event horizon is circular (as predicted), or oblate or prolate instead,
    -whether the radio emissions extend farther than we thought, or
    -whether there are any other deviations from the expected behavior.

    7
    The orientation of the accretion disk as either face-on (left two panels) or edge-on (right two panels) can vastly alter how the black hole appears to us. Image credit: ‘Toward the event horizon — the supermassive black hole in the Galactic Center’, Class. Quantum Grav., Falcke & Markoff (2013).

    Whatever we do (or don’t) wind up discovering, we’re poised to make an incredible breakthrough simply by constructing our first-ever image of a black hole. No longer will we need to rely on simulations or artist’s conceptions; we’ll have our very first actual, data-based picture to work with. If it’s successful, it paves the way for even longer baseline studies; with an array of radio telescopes in space, we could extend our reach from a single black hole to many hundreds of them. If 2016 was the year of the gravitational wave and 2017 was the year of the neutron star merger, then 2018 is set up to be the year of the event horizon. For any fan of astrophysics, black holes, and General Relativity, we’re living in the golden age. What was once deemed “untestable” has suddenly become real.

    See the full article here .

    Please help promote STEM in your local schools.

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 11:28 am on October 17, 2017 Permalink | Reply
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    From astrobites: ” Is S0-2 a Binary Star?” 

    Astrobites bloc

    astrobites

    Title: Investigating the Binarity of S0-2: Implications for its Origins and Robustness as a Probe of the Laws of Gravity around a Supermassive Black Hole
    Authors: D. S. Chu, T. Do, A. Hees, A. Ghez, et al.
    First Author’s Institution: University of California, Los Angeles

    Status: Submitted to ApJ, open access

    The most exciting discoveries in astronomy all have something in common: they let us marvel at the fact that nature obeys laws of physics. The star S0-2 is one of these exciting discoveries. S0-2 (also known as S2) is a fast-moving star that has been observed to follow a full elliptical, 16-year orbit around the Milky Way’s central supermassive black hole, precisely according to Kepler’s laws of planetary motion.

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    S0-2

    SGR A* NASA’s Chandra X-Ray Observatory

    Serving as a test-particle probe of the gravitational potential, S0-2 provides some of the best constraints on the black hole’s mass and distance yet. S0-2 is the brightest of the S-stars, a group of young main-sequence stars concentrated within the inner 1” (0.13 ly) of the nuclear star cluster.

    The next time S0-2 reaches its closest approach to the black hole, in 2018, there will exist a unique opportunity to detect a deviation from Keplerian motion — namely the relativistic redshift of S0-2’s radial (line-of-sight) velocity — in a direct measurement. In anticipation of this event, the authors of today’s paper investigate possible consequences of S0-2 being not a single star, but a spectroscopic binary, which would complicate this measurement.

    3
    Figure 1: Top: Radial velocity measurements of S0-2 over time. Bottom: Residual velocities after subtraction of the best-fit model for the orbital motion. [Chu et al. 2017]

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 7:20 pm on September 6, 2017 Permalink | Reply
    Tags: , SgrA*, ,   

    From Universe Today: “Supermassive Black Holes or Their Galaxies? Which Came First?” 

    universe-today

    Universe Today

    6 Sep , 2017
    Fraser Cain

    There’s a supermassive black hole at the center of almost every galaxy in the Universe. How did they get there? What’s the relationship between these monster black holes and the galaxies that surround them?

    Every time astronomers look farther out in the Universe, they discover new mysteries. These mysteries require all new tools and techniques to understand. These mysteries lead to more mysteries. What I’m saying is that it’s mystery turtles all the way down.

    One of the most fascinating is the discovery of quasars, understanding what they are, and the unveiling of an even deeper mystery, where do they come from?

    As always, I’m getting ahead of myself, so first, let’s go back and talk about the discovery of quasars.

    Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a class of bizarre objects in the distant Universe. They were very bright, and incredibly far away; hundreds of millions or even billion of light-years away. The first ones were discovered in the radio spectrum, but over time, astronomers found even more blazing in the visible spectrum.

    In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation. It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the “excited atoms” perspective.

    SGR A* NASA’s Chandra X-Ray Observatory

    See the full article here .

    Please help promote STEM in your local schools.

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    Stem Education Coalition

     
  • richardmitnick 11:14 am on August 23, 2017 Permalink | Reply
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    From astrobites: “Students on the Hunt for Black Holes” 

    Astrobites bloc

    Astrobites

    Aug 22, 2017
    Ana Torres Campos
    Crossposts, Personal Experiences

    Inside the Large Millimeter Telescope Alfonso Serrano (LMT), at an altitude of 4500 meters above sea level, Queen’s Don’t Stop Me Now can be heard in the background while a group of people cheer and shake hands after successfully concluding an observation run for one of the most important astronomical projects in the last years: The Event Horizon Telescope (EHT).

    Event Horizon Telescope Array
    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    This project aims to obtain, for the first time ever, the image of the projected shadow of the supermassive black hole at the center of our galaxy, as well as the one in Messier 87.

    SGR A* NASA’s Chandra X-Ray Observatory

    1
    Messier 87, sources of images posted in graphic.

    In this post I share my personal experience as a guest of the team responsible for the observations of the EHT that were taken with the LMT in April of this year.

    2
    Figure 1 The LMT control room onApril 11th, 2017 at the end of the EHT 2017 observing run. Left to right: Antonio Hernández, Sergio Dzib, Emir Moreno, Edgar Castillo, Gopal Narayanan, Katie Bouman and Sandra Bustamante. Credit: Ana Torres Campos.

    Why should you care about the Event Horizon Telescope?
    The New York Times and National Geographic, among others, have written articles about the EHT, so don’t be surprised if one of these days your relatives or friends ask you about the project. Its greatness lies in not only being the first opportunity to observe an unknown event, or proving Einstein’s general relativity at never- imagined scales, but in demonstrating that, in times of discord, a close collaboration among a large number of nations is essential for scientific advancement.

    The Event Horizon Telescope is a network of eight [with ALMA it is now nine] millimeter-wave radio observatories located on four continents and representing over 20 nations. These observatories work together as a single Earth-sized telescope using the Very Large Baseline Interferometry (VLBI) technique. One of these facilities is the LMT, a 32-meter single-dish millimeter-wavelength telescope led by the Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE, Mexico) and the University of Massachusetts Amherst (UMass, U.S.A.). Each night, the LMT synchronized (with a precision higher than 10-12 seconds) with the other telescopes using a hydrogen maser and recorded approximately 30 Terabytes of data which were stored in Mark 6 data systems developed by the Massachusetts Institute of Technology (MIT) Haystack Observatory.

    4
    Massachusetts Institute of Technology (MIT) Haystack Observatory.

    The observing run at the LMT

    The observing run was set to begin on April 4 with a 10-day window to accomplish five observing sessions. Some of the EHT team members got to the telescope site days before to check on the instruments (a 1.3-mm receiver, the backend system and the hydrogen maser) and get acclimated to the high altitude and time change (since the observations were to be performed at night time). My adventure began on May 31 when I met the EHT-LMT observing team, led by Dr. Gopal Narayanan (UMass researcher and the developer of the 1.3-mm receiver). I was really surprised that the crew consisted mainly of PhD students; Sandy, Aleks, and Michael are in their first years and Katie is a Computer Science PhD candidate. The first thing that came to mind was: how come young students are in charge of such an important task?! I got the answer to my question after spending a few days with them. Not only they are outstandingly capable but also they know how to work as a team.

    Since the observing sessions were to be ~17 hours long, the team split into two groups: Group 1 (David, Lindy, Aleks and Michael) began the observing session at 5:30 pm and ended at 2 am, and Group 2 (Edgar,Gopal, Katie and Sandy) started to observe at 1 am and finished at 10 am. This schedule allowed all of the team members to sleep for at least 6 hours, join with the Command Center (and the other observatories) at the 2:30 pm video conference for the go/no-go decision (given the weather conditions at the different sites), and having a one hour overlap in between observing groups’ shifts.

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    (Top) Left to Right: the 1.3-mm receiver, Sandra Bustamante, Aleksandar Popstefanija and Gopal Narayanan. (Bottom-Left) Sergio Dzib, Antonio Hernández and Gopal Narayanan, at the back stands the backend system with the Mark 6. (Bottom-Right) Gopal Narayanan checking the hydrogen maser.

    Both groups had an expert telescope operator (Edgar or David), but in one group was the backend system expert (Lindy) while in the other was the receiver expert (Gopal). This made the students a little nervous at first because if any problem arose then they would have had to face it alone before calling the expert (whom would very likely be sleeping at Base Camp).

    The first target of the observing run was a binary black hole called OJ 287.

    6
    http://www.as.up.krakow.pl/sz/oj287.html

    This object is scientifically interesting all on its own, but because it is a deeply studied object, it will instead be used to calibrate the observations of the project’s primary observing targets. These are Sagittarius A* (Sgt A*), the supermassive black hole at the center of our galaxy, and the supermassive black hole in M87, the most important object of the first night since Sgt A* observations were planned for the following days.

    This object is scientifically interesting all on its own, but because it is a deeply studied object, it will instead be used to calibrate the observations of the project’s primary observing targets. These are Sagittarius A* (Sgr A*), the supermassive black hole at the center of our galaxy, and the supermassive black hole in Messier 87, the most important object of the first night since Sgr A* observations were planned for the following days.

    7
    (Left) Members of the EHT project infront of the LMT. From left to right: Aleksandar Popstefanija, Michael Janssen, Sandra Bustamante, Lindy Blackburn, Katie Bouman, Gopal Narayanan and Edgar Castillo. (Middle) Lindy explains the data recording instructions to the students. (Right) Telescope operators David Sánchez and Edgar Castillo. Credit: Ana Torres Campos.

    Along with the observations came the uncomfortable situations that nobody talks about but that every observational astronomer has suffered from: power failure, difficulty to perfectly focus the telescope, and coffee shortage, the last one being the most stressful of them all. The good thing was that the exceptional skills of the telescope staff (including the operators) managed to quickly fix these inconveniences and halfway through the observing run reinforcement arrived: Antonio (a PhD student at IRyA/Université Toulouse III – Paul Sabatier) and Sergio (a postdoc at MPIFRA). Nevertheless, tiredness increased every day, but the 24-hour interactions among the team members helped them feel relaxed, increasing the moments of laughter and jokes.

    In my personal opinion, one of the keys to the EHT success is the excellent communication between the project team members, based not only on frequent videoconferences, emails and chats on Slack, but also a very well organized web or “wiki” where you can find the manuals of the instruments, tutorials on the observing run procedures, and even contact telephone numbers.

    What I learned from the EHT-LMT team
    1. An observing run will only be successful if the team works efficiently.
    2. It is necessary to be capable of occupying different roles on a team.
    3. Being assertive when listening and giving instructions will save you time.
    4. Relaxing and fun moments will improve the job performance of the team.
    Finally, I would like to thank Gopal, Katie, Michael, Sandy, Lindy, Aleks, David, Edgar, Antonio, Michael and Sergio for sharing with me such an incredible experience and to the LMT site and Base Camp crew for the outstanding job they do.

    See the full article here .

    Please help promote STEM in your local schools.

    STEM Icon

    Stem Education Coalition

    What do we do?

    Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
    Why read Astrobites?

    Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.
    Our goal is to solve this problem, one paper at a time. In 5 minutes a day reading Astrobites, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in a new area of astronomy.

     
  • richardmitnick 1:56 pm on August 15, 2017 Permalink | Reply
    Tags: Black hole imaging, , , , SgrA*   

    From ESO: “Taking the First Picture of a Black Hole” 

    ESO 50 Large

    European Southern Observatory

    1.8.2017 Challenges in Obtaining an Image of a Supermassive Black Hole

    “Seeing a black hole” has been a long-cherished desire for many astronomers, but now, thanks to the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) projects, it may no longer be just a dream.

    Event Horizon Telescope Array
    Event Horizon Telescope map

    The locations of the radio dishes that will be part of the Event Horizon Telescope array. Image credit: Event Horizon Telescope sites, via University of Arizona at https://www.as.arizona.edu/event-horizon-telescope.

    Arizona Radio Observatory
    Arizona Radio Observatory/Submillimeter-wave Astronomy (ARO/SMT)

    ESO/APEX
    Atacama Pathfinder EXperiment (APEX)

    CARMA Array no longer in service
    Combined Array for Research in Millimeter-wave Astronomy (CARMA)

    Atacama Submillimeter Telescope Experiment (ASTE)
    Atacama Submillimeter Telescope Experiment (ASTE)

    Caltech Submillimeter Observatory
    Caltech Submillimeter Observatory (CSO)

    IRAM NOEMA interferometer
    Institut de Radioastronomie Millimetrique (IRAM) 30m

    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA
    James Clerk Maxwell Telescope interior, Mauna Kea, Hawaii, USA

    Large Millimeter Telescope Alfonso Serrano
    Large Millimeter Telescope Alfonso Serrano

    CfA Submillimeter Array Hawaii SAO
    Submillimeter Array Hawaii SAO

    ESO/NRAO/NAOJ ALMA Array
    ESO/NRAO/NAOJ ALMA Array, Chile

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Global mm-VLBI Array

    To make it possible to image the shadow of the event horizon of Sagittarius A*, many researchers and cutting-edge technologies have been mobilised — because obtaining an image of a black hole is not as easy as snapping a photo with an ordinary camera.

    Sagittarius A* has a mass of approximately four million times that of the Sun, but it only looks like a tiny dot from Earth, 26 000 light-years away. To capture its image, incredibly high resolution is needed. As explained in the fifth post of this blog series, the key is to use Very-Long-Baseline Interferometry (VLBI), a technique that combines the observing power of and the data from telescopes around the world to create a virtual giant radio telescope.

    The resolution of a telescope can be calculated from the radio wavelength the telescope is observing at and the size of the telescope — or in VLBI, the distance between the antennas. However, while actually observing, several kinds of noise and errors interfere with the telescope’s performance and affect the resolution.

    In VLBI, each antenna is equipped with an extremely precise atomic clock to record the time at which radio signals from the target object were received. The gathered data are synthesised using the times as a reference, so that the arrival time of the radio waves to each antenna can be accurately adjusted.

    But this process isn’t always straightforward because the Earth’s atmosphere blocks a certain range of wavelengths. Several kinds of molecules such as water vapour absorb a fraction of radio waves that pass through the atmosphere, with shorter wavelengths more susceptible to absorption. To minimise the effect of atmospheric absorption, radio telescopes are built at high and dry sites, but even then they are still not completely immune from the effect.

    The tricky part of this absorption effect is that the direction of a radio wave is slightly changed when it passes through the atmosphere containing water vapour. This means that the radio waves arrive at different times at each antenna, making it difficult to synthesise the data later using the time signal as a reference. And even worse: since VLBI utilises antennas located thousands of kilometres apart, it has to take into account the differences in the amount of water vapour in the sky above each site, as well as the large fluctuations of water vapour content during the observation period. In optical observations, these fluctuations make the light of a star flicker and lower the resolution. Radio observations have similar problems.

    “We have only a few ways to reduce this effect in VLBI observations,” explains Satoki Matsushita at the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) of Taiwan. “If there is a compact object emitting intense radiation near the target object, we can remove most of the effect of refraction of radio waves by water vapour by using such an intense radiation source as a reference. However, no such intense reference source has been found near Sagittarius A* so far. And even if there is a reference source, there are still necessary conditions that must be satisfied: the telescopes need to have the ability to observe the target object and reference object at the same time; or the telescopes need to have the high-speed drive mechanism to quickly switch the observation between the target object and the reference object. Unfortunately, not all telescopes participating in the EHT/GMVA observations have this capability. One of the methods to remove the effect is to equip each antenna with an instrument to measure the amount of water vapour, but ALMA is the only telescope that has adopted this method at this point.”

    Another major challenge in imaging a black hole is obtaining a high-quality image. By combining the data collected by antennas thousands of kilometres apart, VLBI achieves a resolution equivalent to a radio telescope several thousands of kilometres in diameter. However, VLBI also has a lot of large blank areas that are not covered by any of the antennas. These missing parts make it difficult for VLBI to reproduce a high-fidelity image of a target object from the synthesised data. This is a common problem for all radio interferometers, including ALMA, but it can be more serious in VLBI where the antennas are located very far apart.

    It might be natural to think that a higher resolution means a higher image quality, as is the case with an ordinary digital camera, but in radio observations the resolution and image quality are quite different things. The resolution of a telescope determines how close two objects can be to each other and yet still be resolved as separate objects, while the image quality defines the fidelity in reproducing the image of the structure of the observed object. For example, imagine a leaf, which has a variety of veins. The resolution is the ability to see thinner vein patterns, while the image quality is the ability to capture the overall spread of the leaf. In normal human experience, it would seem bizarre if you could see the very thin veins of a leaf but couldn’t grasp a complete view of the leaf — but such things happen in VLBI, since some portions of data are inevitably missing.

    2
    This infographic illustrates how ALMA contributes to the EHT observations. With its shorter baseline, ALMA is sensitive to larger scales than the EHT and so ALMA can fill in the lower-resolution, larger-scale structures that the EHT misses. Credit: NRAO.

    Researchers have been studying data processing methods to improve image quality for almost as long as the history of the radio interferometer itself, so there are some established methods that are already widely used, while others are still in an experimental phase. In the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) projects, which are both aiming to capture the shadow of a black hole’s event horizon for the first time, researchers began to develop effective image analysis methods using simulation data well before the start of the observations.

    4
    A simulated image of the supermassive black hole Sagittarius A*, which is likely to be obtained in the most recent EHT observations. The dark gap at the centre is the shadow of the black hole. Credit: Kazunori Akiyama (MIT Haystack Observatory).

    The observations with the EHT and the GMVA were completed in April 2017. The data collected by the antennas around the world has been sent to the US and Germany, where data processing will be conducted with dedicated data-processing computers called correlators. The data from the South Pole Telescope, one of the participating telescopes in the EHT, will arrive at the end of 2017, and then data calibration and data synthesis will begin in order to produce an image, if possible. This process might take several months to achieve the goal of obtaining the first image of a black hole, which is eagerly awaited by black hole researchers and the general astronomical community worldwide.

    This lengthy time span between observations and results is normal in astronomy, as the reduction and analysis of the data is a careful, time-consuming process. Right now, all we can do is wait patiently for success to come — for a long-held dream of astronomers to be transformed into a reality.

    Until then, this is the last post in our blog series about the EHT and GMVA projects. When the results become available in early 2018, we’ll be back with what will hopefully be exciting new information about our turbulent and fascinating galactic centre.

    See the full article here .

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    ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

    ESO LaSilla
    ESO/Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.

    ESO VLT
    VLT at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.

    ESO Vista Telescope
    ESO/Vista Telescope at Cerro Paranal, with an elevation of 2,635 metres (8,645 ft) above sea level.

    ESO NTT
    ESO/NTT at Cerro LaSilla 600 km north of Santiago de Chile at an altitude of 2400 metres.

    ESO VLT Survey telescope
    VLT Survey Telescope at Cerro Paranal with an elevation of 2,635 metres (8,645 ft) above sea level.

    ALMA Array
    ALMA on the Chajnantor plateau at 5,000 metres.

    ESO E-ELT
    ESO/E-ELT to be built at Cerro Armazones at 3,060 m.

    ESO APEX
    APEX Atacama Pathfinder 5,100 meters above sea level, at the Llano de Chajnantor Observatory in the Atacama desert.

    Leiden MASCARA instrument, La Silla, located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

    Leiden MASCARA cabinet at ESO Cerro la Silla located in the southern Atacama Desert 600 kilometres (370 mi) north of Santiago de Chile at an altitude of 2,400 metres (7,900 ft)

     
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