четверг, 25 апреля 2019 г.

The Giant In Our Backyard

Artist impression of the heart of galaxy NGC 1068, which harbors an actively feeding supermassive black hole.

Credit: NRAO/AUI/NSF; D. Berry / Skyworks

The center of our Milky Way Galaxy is only clearly visible to radio telescopes. The supermassive black hole in its core is glaring in radio waves, surrounded by the smoke rings of supernova remnants and the arcs of material caught in the core’s strong magnetic fields. This gigantic image was pieced together by multiple observations taken by the Very Large Array (VLA). Credit: NRAO/AUI/NSF. Hi-Res File

Top left: Simulation of Sgr A* at 86 GHz. Top right: Simulation with added effects of scattering. Bottom right: Scattered image from the observations, this is how we see Sgr A* on the sky. Bottom left: The unscattered image, after removing the effects of scattering along our line of sight, this is how Sgr A* really looks like. Credit: S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA. Hi-Res File

This infographic details the locations of the participating telescopes of the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA). Their goal is to image, for the very first time, the shadow of the event horizon of the supermassive black hole at the center of the Milky Way, as well as to study the properties of the accretion and outflow around the Galactic Centre. Credit: ESO/O. Furtak. Hi-Res File

Synopsis: Recently, a collection of radio observatories combined to form the GMVA, a powerful tool that probed the region near our galaxy’s supermassive black hole. This produced curious images of this region, glowing brightly in millimeter-wavelength radio light. These observations, which involved three U.S. radio telescopes – VLA, VLBA, and GBT – are an important step toward observing the event horizon of a supermassive black hole. Here is this story of this quest so far:

There is a giant in our backyard. We know it’s there, but no one has ever seen it. It’s a supermassive black hole , and it lurks in the center of our galaxy.

In 1931, engineer Karl Jansky first observed a strong cosmic radio signal emanating from the constellation Sagittarius, which lies in the direction to the center of our galaxy. Jansky assumed that the radio signals originated from the center of our galaxy, but he had no idea what that source could be and his telescope was incapable pinpointing the location of the exact source. That happened in 1974, when Bruce Balick and Robert Brown used three radio dishes at Green Bank Observatory and a fourth smaller dish about 35 km away to form a vastly more precise radio telescope called an interferometer.

An interferometer is a way to use multiple radio telescopes or antennas as a single virtual telescope. When two antenna dishes are pointed at the same object in the sky they receive the same signal, but the signals are out of sync because it takes a bit longer to reach one antenna than the other. The time difference depends upon the direction of the antennas and their spacing apart from each other. By correlating the two signals you can determine the location of the source very precisely. With the Green Bank Interferometer, Balick and Brown confirmed the radio source as a small region near the galactic center. Brown later named the source Sagittarius A*, or Sgr A* for short.

The Green Bank Interferometer was a precursor to NRAO’s Very Large Array (VLA). The VLA has an array of 28 antennas capable of both widely separated and closely spaced configurations, making it the perfect tool for studying Sgr A*. In 1983, a team led by Ron Ekers used the VLA to make the first radio image of the Galactic Center, which revealed a mini-spiral of hot gas. Later observations showed not only the spiral of gas, but also a distinct and bright radio source at the exact center of the Milky Way.

By this time it was strongly suspected that this radio source was a massive black hole. From 1982 to 1998, Don Backer and Dick Sramek at the VLA measured the position of Sgr A* and found that it had almost no apparent motion. This meant it must be extremely massive since the gravitational tugs of nearby stars weren’t moving it about. They estimated it must have a mass at least two million times larger than the Sun. Long-term observations of stars orbiting the Galactic Center

have found Sgr A* to be about 3.6 million solar masses, and precise radio imaging has confirmed it can be no larger than the orbit of Mercury. We now know it is indeed a supermassive black hole.

Knowing a black hole is there isn’t the same as seeing it directly. Astronomers have long dreamed of directly observing a black hole, and perhaps even glimpsing its event horizon . Sagittarius A* is the closest supermassive black hole to Earth, so there have been various efforts to observe it directly. But there are two big challenges to be overcome. The first is that the center of our galaxy is surrounded by dense gas and dust. Almost all the visible light from the region is obscured, so we can’t observe the black hole with an optical telescope. Fortunately, the gas and dust are relatively transparent to radio light, so radio telescopes can see to the heart of our galaxy. But this leads to the second major challenge: resolution.

Although the Sgr A* black hole is massive, it is only about the size of a large star. According to Einstein’s theory of general relativity, a black hole of 3.6 million solar masses would have an event horizon only 15 times wider than the Sun. Since the Galactic Center is about 26,000 light years away from Earth, the black hole appears very small in the sky, about the same apparent size as a baseball sitting on the surface of the Moon. To see a radio object that small, you’d need a telescope the size of Earth itself.

Obviously, we can’t build a radio telescope the size of our planet, but with radio interferometry we can build a virtual Earth-sized telescope. NRAO observatories are currently working with two projects trying to observe a black hole, the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA). The Atacama Large Millimeter/submillimeter Array (ALMA) is participating in both projects, while the Green Bank Telescope (GBT) and the Very Long Baseline Array (VLBA)

are part of GMVA. Just like the Very Large Array, these projects combine signals from multiple antennas. Since the antennas are located all over the world, this virtual telescope is about the size of the Earth. But unlike the VLA antennas, they all have different sizes and sensitivities. This diversity of antennas makes it more difficult to combine signals, but it also gives the projects a big advantage.

In the VLA, for example, all the antennas of the array are identical. Each antenna contributes equally, and the sensitivity of the array depends upon the size of a single antenna. But when telescopes, or antennas of different sizes, are combined, the sensitivity of the larger antennas helps boost the sensitivity of the smaller ones. The Green Bank Telescope, for example, has a diameter of 100 meters. When combined with smaller telescopes in a large interferometer, the total sensitivity depends upon the average size of all the antennas. This makes the ALMA array — connected to the EHT and the GMVA — and the GBT — linked to the GMVA — much more sensitive to signals from the Milky Way’s black hole, and we’ll need all the sensitivity we can get to capture the image of a black hole.

In January of 2019, GMVA captured an image of Sagittarius A* at 3mm wavelengths, but the scattering of 3mm light by the plasma between us and Sgr A* made it impossible to see the shadow of its event horizon. The first clear image of a black hole was announced by the Event Horizon Telescope in April 2019. It was an image of the black hole in the galaxy M87. While M87 is more than 2,000 times more distant than the black hole in the center of our galaxy, its black hole is also 1,500 times more massive. It’s a very active black hole, and not obscured by the gas and dust in our galaxy, making it easier to observe. Observing our smaller, quieter black hole is a bigger challenge. But by working with observatories all over the world, ALMA and the GBT will soon catch the first clear glimpse of the giant in our backyard.


Brian Koberlein

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