Photographing a Black Hole?
« Thread started on: Mar 2nd, 2017, 4:23pm »
This global telescope may finally see the event horizon of our galaxy's giant black hole
By Daniel Clery Mar. 2, 2017
Last year researchers "heard" black holes for the first time, when they detected the gravitational waves unleashed as two of them crashed together and merged. Now, they want to see a black hole, or at least its silhouette. Next month, astronomers will harness radio telescopes across the globe to create the equivalent of a single Earth-spanning dish—an instrument powerful enough, they hope, to image black holes backlit by the incandescent gas swirling around them. Their targets are the supermassive black hole at the heart of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), and an even bigger one in the neighboring galaxy M87.
Earlier observations using this Event Horizon Telescope (EHT) without its full roster of dishes yielded tantalizing results, but in images the two black holes remained featureless blobs. This year, for the first time, the EHT will add dishes in Chile and Antarctica, sharpening its resolution and raising expectations. Astronomers now hope to see how the black holes whip the hot gas around them into accretion disks and spawn matter-spewing jets. They also hope to chart the size and shape of the event horizon—the boundary of the black hole—to test whether Albert Einstein's theory of gravity, general relativity, still works under such extreme conditions.
"It's a very bold and gutsy experiment," says theoretical astrophysicist Roger Blandford of Stanford University in Palo Alto, California, who is not involved in the project. Blandford believes the EHT may not only show how black holes work, but also deliver a more fundamental message. "It will validate this remarkable proposition: that black holes are common in the universe. Seeing is believing."
The EHT takes aim just once a year, when good weather is likely, when both black holes are visible in the sky, and when it's possible to get time at all the observatories around the globe. This year, the team will observe for 5 nights during a 10-night window from 5 to 14 April. Then, an intensive data processing effort begins, and it may be a year before they know whether they've succeeded. "It's an exercise in delayed gratification. Delayed gratification squared," says EHT director Shep Doeleman at the Massachusetts Institute of Technology's Haystack Observatory in Westford.
Imaging black holes is a formidable challenge, and not just because their intense gravity prevents even light from escaping. They are also surprisingly small. Sgr A* is calculated to contain the mass of 4 million suns, based on the nervy, high-speed orbits of stars in its gravitational grip. But its event horizon, the point of no return for anything approaching a black hole, is 24 million kilometers across, just 17 times wider than the sun. To see something so small from 26,000 light-years away requires a telescope dish of global dimensions.
At optical wavelengths, Sgr A* is hidden by the shroud of dust and gas obscuring the galaxy's heart. Radio waves can pass through more easily, but ordinary radio dishes are still hampered by ionized gas clouds and low resolution. Best are telescopes sensitive to the shortest radio waves—millimeter waves—but the dishes, detectors, and data processing technology for this part of the spectrum were developed only in the past few decades. "There is only a tiny window where we can see the event horizon," says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands, and chair of the EHT science council. "The Milky Way is like a milky glass."
A shot in the dark The Event Horizon Telescope now combines eight millimeter-wave radio observatories into a global telescope. The farther apart they are, the better the resolution.
The Event Horizon Telescope now combines eight millimeter-wave radio observatories into a global telescope. The farther apart they are, the better the resolution. (GRAPHIC) ADAPTED BY A. CUADRA/SCIENCE; (DATA) EVENT HORIZON TELESCOPE
Early this decade, Doeleman and other EHT researchers began testing the idea with millimeter-sensitive dishes in Hawaii, California, and Arizona. Later, they extended the array to include the Large Millimeter Telescope in Mexico. Along the way, they got a good enough image of the black hole in M87 to see the base of its matter-spewing jets—data that are helping them understand how the jets are created. In 2015, they glimpsed the magnetic field around Sgr A*, which may help explain how black holes heat up the material around them. But to see the event horizon itself, the EHT had to grow even larger. Over the years, it has evolved from a loose, poorly funded group to a worldwide collaboration involving 30 institutions in 12 countries. Next month it will include farflung additions, including the IRAM dish in Spain, the South Pole Telescope, and the Atacama Large Millimeter/submillimeter Array (ALMA), a large international observatory comprising 66 dishes in northern Chile. With its huge dish area, ALMA is the big catch because it will boost the EHT's sensitivity by an order of magnitude. "That's the key for us," Doeleman says.
Adding new instruments isn't simple. The technique for combining signals from distant dishes is known as very long baseline interferometry, and most millimeter-wave telescopes are not equipped to take part. EHT researchers had to visit each facility to tinker with its hardware and install new digital signal processors and data recorders. In the case of ALMA, that took some persuading. "We had to go into the bowels of ALMA and rewire it," Doeleman says. "It required political buy-in at all levels."
The campaign next month will be a nervous time for the EHT team. All eight observatories need clear skies and no technical glitches to get the best possible observations. "The first time, things can go wrong," Falcke says. Data volumes will be so large that they have to be recorded on hard drives and shipped back to the Haystack Observatory and the Max Planck Institute for Radio Astronomy in Bonn, Germany, for processing. There, devices known as correlators, made from clusters of PCs but with the power of supercomputers, will spend months crunching through the data, combining the signals from separate dishes as if they came from a single dish as wide as Earth. Adding further delay, data from the South Pole Telescope won't arrive until September or October, when planes can retrieve the hard drives after the Antarctic winter.
When the data finally all come together sometime next year, the team hopes to see a bright ring of light from photons orbiting close to the event horizon, with a dark disk in its center. The ring should be brighter on one side, where the rotation of the black hole gives photons a boost, although the images on this first attempt may not be as crisp as the team's simulations. "It'll probably be a crappy image, but scientifically it will be very interesting," Falcke says.
Doeleman hopes to see structure in the matter swirling around the event horizon and watch, movielike, as gas falls into it and vanishes. Such observations might help explain why some black holes gorge on matter and shine brightly, whereas others—like Sgr A*—seem to be on a starvation diet. Falcke has a simpler wish. "The event horizon is the defining thing about a black hole," he says. "I hope to see it; to literally see it."
Re: Photographing a Black Hole?
« Reply #1 on: Apr 20th, 2017, 10:11pm »
This from Chandra : Not our galaxy, but.....
NGC 4696: The Arrhythmic Beating of a Black Hole Heart
At the center of the Centaurus galaxy cluster, there is a large elliptical galaxy called NGC 4696. Deeper still, there is a supermassive black hole buried within the core of this galaxy.
New data from NASA's Chandra X-ray Observatory and other telescopes has revealed details about this giant black hole, located some 145 million light years from Earth. Although the black hole itself is undetected, astronomers are learning about the impact it has on the galaxy it inhabits and the larger cluster around it.
In some ways, this black hole resembles a beating heart that pumps blood outward into the body via the arteries. Likewise, a black hole can inject material and energy into its host galaxy and beyond.
By examining the details of the X-ray data from Chandra, scientists have found evidence for repeated bursts of energetic particles in jets generated by the supermassive black hole at the center of NGC 4696. These bursts create vast cavities in the hot gas that fills the space between the galaxies in the cluster. The bursts also create shock waves, akin to sonic booms produced by high-speed airplanes, which travel tens of thousands of light years across the cluster.
This composite image contains X-ray data from Chandra (red) that reveals the hot gas in the cluster, and radio data from the NSF's Karl G. Jansky Very Large Array (blue) that shows high-energy particles produced by the black hole-powered jets. Visible light data from the Hubble Space Telescope (green) show galaxies in the cluster as well as galaxies and stars outside the cluster.
Quote : "The features in the Centaurus Cluster are similar to the ripples seen in the Perseus cluster of galaxies. The pitch of the sound in Centaurus is extremely deep, corresponding to a discordant sound about 56 octaves below the notes near middle C. This corresponds to a slightly higher (by about one octave) pitch than the sound in Perseus. Alternative explanations for these curved features include the effects of turbulence or magnetic fields."
"In some ways, this black hole resembles a beating heart that pumps blood outward into the body via the arteries. Likewise, a black hole can inject material and energy into its host galaxy and beyond."
"The features in the Centaurus Cluster are similar to the ripples seen in the Perseus cluster of galaxies. The pitch of the sound in Centaurus is extremely deep, corresponding to a discordant sound about 56 octaves below the notes near middle C. This corresponds to a slightly higher (by about one octave) pitch than the sound in Perseus. Alternative explanations for these curved features include the effects of turbulence or magnetic fields."
Re: Photographing a Black Hole?
« Reply #5 on: Aug 25th, 2017, 3:55pm »
Pictor A: Blast from Black Hole in a Galaxy Far, Far Away
The Star Wars franchise has featured the fictitious "Death Star," which can shoot powerful beams of radiation across space. The Universe, however, produces phenomena that often surpass what science fiction can conjure.
The Pictor A galaxy is one such impressive object. This galaxy, located nearly 500 million light years from Earth, contains a supermassive black hole at its center. A huge amount of gravitational energy is released as material swirls towards the event horizon, the point of no return for infalling material. This energy produces an enormous beam, or jet, of particles traveling at nearly the speed of light into intergalactic space.
To obtain images of this jet, scientists used NASA's Chandra X-ray Observatory at various times over 15 years. Chandra's X-ray data (blue) have been combined with radio data from the Australia Telescope Compact Array (red) in this new composite image.
By studying the details of the structure seen in both X-rays and radio waves, scientists seek to gain a deeper understanding of these huge collimated blasts.
The jet [to the right] in Pictor A is the one that is closest to us. It displays continuous X-ray emission over a distance of 570,000 light years. By comparison, the entire Milky Way is about 100,000 light years in diameter. Because of its relative proximity and Chandra's ability to make detailed X-ray images, scientists can look at detailed features in the jet and test ideas of how the X-ray emission is produced.
In addition to the prominent jet seen pointing to the right in the image, researchers report evidence for another jet pointing in the opposite direction, known as a "counterjet". While tentative evidence for this counterjet had been previously reported, these new Chandra data confirm its existence. The relative faintness of the counterjet compared to the jet is likely due to the motion of the counterjet away from the line of sight to the Earth.
A giant jet spanning continuously for over 570,000 light years is seen blasting out of the galaxy Pictor A.
A new composite image shows this jet in X-rays (blue) and radio waves (red).
In addition to the main jet, there is evidence for a jet moving in the opposite direction.
Chandra observations at various times over a 15-year period provide new details of this impressive system.