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  • richardmitnick 12:31 pm on January 22, 2019 Permalink | Reply
    Tags: , , , , , GMVA-Global mm-VLBI Array, Our Galaxy's Supermassive Black Hole Could Be Pointing a Relativistic Jet Right at Us, ,   

    From Science Alert: “Our Galaxy’s Supermassive Black Hole Could Be Pointing a Relativistic Jet Right at Us” 


    From Science Alert

    22 JAN 2019

    A black hole simulation (Bronzwaer/Davelaar/Moscibrodzka/Falcke/Radboud University)

    Things are officially getting exciting. New science has just come in from the collaboration to photograph Sagittarius A*, the supermassive black hole at the centre of the Milky Way, and it’s ponying up the secrets at our galaxy’s dusty heart.

    SGR A and SGR A* from Penn State and NASA/Chandra

    The image below is the best picture yet of Sgr A* (don’t worry, there’s more to come from the Event Horizon Telescope), and while it may look like just a weird blob of light to you, astrophysicists studying the radio data can learn a lot from what they’re looking at – and they think they’ve identified a relativistic jet angled towards Earth.

    EHT map

    Because the image taken of the region is the highest resolution yet – twice as high as the previous best – the researchers were able to precisely map the properties of the light around the black hole as scattered by the cloud.

    “The galactic centre is full of matter around the black hole, which acts like frosted glass that we have to look through,” astrophysicist Eduardo Ros of the Max Planck Institute for Radio Astronomy in Germany told New Scientist.

    Using very long baseline interferometry to take observations at a wavelength of 3.5 millimetres (86 GHz frequency), a team of astronomers has used computer modelling to simulate what’s inside the thick cloud of plasma, dust and gas surrounding the black hole.

    Above: The bottom right image shows Sgr A* as seen in the data. The top images are simulations, while the bottom left is Sgr A* with the scattering removed.
    (S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA)

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

    GMVA The Global VLBI Array

    It revealed that Sgr A*’s radio emission comes from a smaller region than previously thought.

    Most of it is coming from an area just 300 milllionth of a degree of the night sky, with a symmetrical shape. And, since black holes don’t emit detectable radiation on their own, the source is most likely one of two things.

    “This may indicate that the radio emission is produced in a disk of infalling gas rather than by a radio jet,” said astrophysicist Sara Issaoun of Radboud University in The Netherlands.

    “However, that would make Sgr A* an exception compared to other radio emitting black holes. The alternative could be that the radio jet is pointing almost at us.”

    Active black holes are surrounded by a swirling cloud of material that’s falling into it like water down a drain. As this material is swallowed by the black hole, it emits jets of particles from its rotational poles at velocities approaching light speed.

    We’re not quite sure how this happens, but astronomers believe that material from the inner part of the accretion disc is channelled towards and launched from the poles via magnetic field lines.

    Since Earth is in the galactic plane, having a jet pointed in our direction would mean that the black hole is oriented quite strangely, as if it’s lying on its side. (Nearby galaxy Centaurus A, for instance, has jets shooting perpendicular to the galactic plane.)

    But this orientation has been hinted at before. Last year the GRAVITY Collaboration described flares around Sgr A* consistent with something orbiting it face-on from our perspective – like looking at the Solar System from above.

    This means the long-awaited picture of the shadow of a black hole will – hopefully – be breathtakingly detailed.

    Meanwhile, studying data such as these help build a comprehensive picture of how these mysterious cosmic objects work.

    “Understanding how black holes work … takes more than the picture of its shadow (although incredible in its own right),” Issaoun wrote on Facebook. “It takes observations at many different wavelengths (radio, X-ray, infrared etc) to piece together the entire story, so every piece counts!”

    The team’s paper has been published in The Astrophysical Journal..

    So “Maybe this is true after all,” said Radboud University astronomer Heino Falcke, “and we are looking at this beast from a very special vantage point.”

    Hopefully, when the Event Horizon Telescope releases the first images of Sgr A*’s event horizon – something we are expecting very soon – they will reveal more. And, in case you were starting to get worried, the 1.4-millimetre wavelength (230 GHz) will reduce the light scattering by a factor of 8.

    See the full article here .

    See also here .


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  • richardmitnick 8:30 pm on July 7, 2017 Permalink | Reply
    Tags: , , , , , , GMVA-Global mm-VLBI Array, SagA*   

    From ESO: “5. How to build an Earth-sized radio telescope” 

    ESO 50 Large

    European Southern Observatory

    A black hole is an extraordinary object with extremely strong gravity, but when observed from Earth it would look like a tiny dot. To capture an image of a black hole, a telescope with extraordinarily high resolution is required. But how can such a revolutionary telescope be created?

    As we discovered in the previous post, the larger the diameter of the telescope, the better the resolution. This is true both for optical and radio telescopes, which means that a tremendously large telescope is needed to observe a small object that can barely be seen from Earth — like a black hole.

    The Atacama Large Millimeter/submillimeter Array (ALMA), which is operated in Chile by a global partnership, combines multiple antennas spread over distances from 150 metres to 16 kilometres. This allows it to simulate a single giant telescope much larger than any individual dish that could be built, achieving a resolution equivalent to up to a 16-km diameter telescope. ALMA’s resolution reaches 1/100 of 1/3600 of a degree angle — 5000 times better than the human eye!

    And yet even with such an exceptionally good “eyesight”, ALMA would need to improve its resolution 100 fold in order to capture a black hole at the centre of the Milky Way galaxy.

    To simulate a telescope with 100 times the resolution of ALMA, telescopes must be spread over a much larger area — far beyond the Chilean Andes, beyond even South America and extending to North America and Europe. Using the Very-Long-Baseline Interferometry (VLBI) technique, telescopes of several thousand kilometres in diameter can be simulated. The Event Horizon Telescope (EHT) and Global mm-VLBI Array (GMVA) both form Earth-sized telescopes that combine the observing power and the data collected by a range of telescopes across the world, including ALMA. The participating telescopes are listed below.

    Telescopes contributing to the EHT and GMVA observations of Sagittarius A*. The connected telescopes simulate a telescope equivalent to the dimensions of the whole western hemisphere of the Earth. Credit: ESO/O. Furtak

    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)

    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, Chile

    Future Array/Telescopes

    Plateau de Bure interferometer
    Plateau de Bure interferometer

    South Pole Telescope SPTPOL
    South Pole Telescope SPTPOL

    Global mm-VLBI Array

    GMVA ALMA, VLBA (eight locations in U.S.), GBT 100m (West Virginia, U.S.), IRAM 30m (Spain), OAN 40m (Spain), Max Planck Institute for Radio Astronomy 100m (Germany)

    But how does VLBI work? With ALMA, it’s relatively straightforward: each antenna receives signals from a target object and then sends them via optical fibres to a central location on site, where they are processed and combined by a dedicated supercomputer. However, when telescopes are located thousands of kilometres apart — halfway across the world from each other — it’s impossible to connect them via optical fibres to a central location and transmit such enormous volumes of data. VLBI therefore uses a different technique: the data are first recorded at each individual telescope and stored on recording devices on site. These devices are then shipped or flown back to one place and played back all together in a computer for data synthesis.

    A schematic diagram of the VLBI mechanism. Each antenna, spread out over vast distances, has an extremely precise atomic clock. Analogue signals collected by the antenna are converted to digital signals and stored on hard drives together with the time signals provided by the atomic clock. The hard drives are then shipped to a central location to be synchronised. An astronomical observation image is obtained by processing the data gathered from multiple locations. Credit: ALMA (ESO/NAOJ/NRAO), J.Pinto & N.Lira.

    A key component of the VLBI technique are the clocks — and these are no ordinary clocks. To synthesise the data gathered simultaneously by contributing telescopes around the world, each telescope requires a clock set to the accurate time with astonishing precision. These clocks measure small differences in the arrival time of the radio waves coming from the target object to each antenna of the array. Every telescope taking part in VLBI is equipped with an extremely precise, specially-developed atomic clock — so accurate that over a period of 100 million years, each clock would be off by less than 1 second!

    Hydrogen maser atomic clock installed at the ALMA Array Operations Site (AOS), along with the technicians who installed it. Credit: ALMA (ESO/NAOJ/NRAO), C. Padilla.

    Another key element in VLBI is the device used to record the data. The first VLBI experiments were carried out in the 1960s and used magnetic tapes to record observations, but since entering the 21st century, more and more VLBI observations have been recorded on hard drives because of their larger storage capacity and lower prices. The hard drives used in the EHT and GMVA observations are based on magnetic-disk technology and incorporate primarily low-cost PC components.

    One important aspect of such a recording device is the speed with which it records data. The faster the data recording speed, the more extensive the range of frequency signals that can be recorded, which improves the overall sensitivity of the observations. Some of the hard drives used in the EHT observations can record data at up to a total rate of 16 gigabits per second! Of course, the storage capacity of the hard drives is also important. The capacity of the hard drives ALMA used for the EHT/GMVA observations exceeds 1 petabyte (1 million gigabytes) in total.

    The dedicated supercomputer to process the recorded data is called a “correlator”. The EHT correlator was developed by the Massachusetts Institute of Technology in the US, while the GMVA correlator was developed by the Max Planck Institute for Radio Astronomy located in Bonn, Germany. The enormous amount of data obtained is firstly recorded at the telescopes around the world, and then sent to these two locations where it is read from each disk to the correlator at up to 4096 MB per second. The data is then processed by the correlator to form an astronomical image.

    The VLBI technique can use these cutting-edge technologies to form an Earth-sized telescope capable of achieving extremely high resolution. This raises the question: Can any type of celestial object be revealed in detail by VLBI observations? Unfortunately, the answer is no. Some objects will be a good target of the VLBI observations, while others will not.

    If you’ve ever used a microscope to observe a small object, you might be familiar with the experience of raising the magnification — only to see a dimmer view. The same thing happens with telescopes. Increasing the resolution means seeing an object in a view that is divided into many smaller parts. This inevitably results in decreasing the amount of light received from each part of the target object. Eventually, as the resolution increases, fainter objects becomes invisible.

    For this reason, the extraordinarily high resolution of VLBI is mainly suitable for observing bright objects. VLBI is often used to observe objects that emit intense radio waves, such as astrophysical masers (similar to the kind of lasers that we are familiar with, but at microwave wavelengths) occurring around young and old stars, and high-speed jets of gas ejected from supermassive black holes. Since a high-temperature gas disk around a supermassive black hole is thought to emit strong radio waves, the EHT and GMVA observations aim to use the VLBI technique to capture the elusive black hole at the centre of the galaxy.

    This is the fifth post of a blog series following the Event Horizon Telescope and the Global mm-VLBI Array projects. The next topic will focus on the supermassive black hole Sagittarius A* located at the centre of the Milky Way, a high-priority target object of the EHT/GMVA observations.

    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

    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 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 to be built at Cerro Armazones at 3,060 m

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

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