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-------------------- 2593 - BLACKHOLE - new techniques to study?
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- When I first became interested in astronomy black holes existed only in theory. None had been identified in astronomy observations. Not even the massive black hole at the center of our Milky Way Galaxy was known.. Today hundreds of black holes have been found and are being studied.
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- A black hole sits in the middle of a galaxy about a billion light-years away. The supermassive object is surrounded by a swirling disk of million-degree matter with an x-ray corona having a temperature exceeding a billion degrees.
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- By charting how those x-rays are created gives us an extremely detailed map of the region around the black hole’s event horizon, the zone beyond which not even light can escape.
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- Black holes don’t give off any light themselves, so the only way we can study this is by watching what matter does as it falls onto it. The new measurements of the black hole helped scientists pin down its mass and spin, properties that can reveal vital clues about the black hole’s evolution. Understanding the spin distribution of black holes in many galaxies tells us about how we go from the early universe to the population we see today.
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- This is an active galaxy, meaning that its innermost region shines more brightly than can be explained by stars alone, and its x-ray brightness fluctuates by a factor of 50, sometimes over just a few hours.
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- An Earth-orbiting telescope that studies the cosmos in x-rays, XMM-Newton stared at the distant galaxy over the course of 16 orbits, totaling more than 550 hours, between 2011 and 2016. From those many hours’ worth of data scientists assembled a map of the supermassive black hole’s x-ray corona and its accretion disk, a ring of swirling matter that’s just outside the event horizon.
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- Some of the emitted x-rays head directly into the cosmos, but others slam into the accretion disk and take a little longer to exit the immediate environment. This extra path length causes a time delay between the x-rays that were produced originally in the corona. this echo time delay is called a “reverberation.”
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- Reverberation mapping, helped the scientists probe the gassy material around the black hole. The technique can be compared to the process to echolocation, in which animals such as bats bounce sound off objects to help them navigate in flight.
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- Reverberation mapping doesn't rely on spatial resolution , instead, it uses light echoes within the object to tell us about structures, even very small and very far away ones. The light echoes determine the precise geometry of the material surrounding the black hole, including the dimensions of its dynamic x-ray corona, which powers those echoes. This information is used to calculate the black hole’s mass and spin, two properties that do not fluctuate on human timescales.
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- Based on the new mapping, the team concluded that this supermassive black hole contains as much mass as two million suns, and that it is spinning nearly as fast as it possibly can without breaking the laws of physics.
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- Every large galaxy in the universe is likely anchored to a central supermassive black hole. Deciphering the ways in which those black holes form could offer clues to how they, and their host galaxies, originally formed and evolved over the age of the universe.
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- One way in which galaxies could form involves multiple small galaxies colliding and merging. As these galaxies merge, so do their central black holes. If those collisions are chaotic, they could not only contribute to the resulting bigger black hole’s mass, but also to the way it spins.
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- Another way in which black holes might bulk up is through a continual stream of inflowing gas.
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- Astronomers would like to use reverberation mapping to pin down the spins of hundreds of nearby supermassive black holes, in effect taking a census of these objects. Then, based on how far away those black holes are, scientists can look at how galaxies grew across the age of the universe.
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- The Event Horizon Telescope Collaboration released the first image of a black hole with observations of the massive, dark object at the center of Messier 87 in April, 2019. This black hole has a mass of about 6.5 billion times that of the Sun and is located about 55 million light years from Earth. The black hole has been called M87* by astronomers and has recently been given the Hawaiian name of "Powehi."
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- For years, astronomers have observed radiation from a jet of high energy particles powered by the black hole blasting out of the center of M87. They have studied the jet in radio, optical, and X-ray light, including with Chandra. By using Chandra observations, researchers have seen that sections of the jet are moving at nearly the speed of light.
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- When matter gets close enough to a black hole, it enters into a swirling pattern called an accretion disk. Some material from the inner part of the accretion disk falls onto the black hole and some of it is redirected away from the black hole in the form of narrow beams, or jets, of material along magnetic field lines. Because this infall process is irregular, the jets are made of clumps or knots that can sometimes be identified with Chandra and other telescopes.
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- The researchers used Chandra observations from 2012 and 2017 to track the motion of two X-ray knots located within the jet about 900 and 2,500 light years away from the black hole. The X-ray data show motion with apparent speeds of 6.3 times the speed of light for the X-ray knot closer to the black hole and 2.4 times the speed of light for the other.
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- Superluminal motion occurs when objects are traveling close to the speed of light along a direction that is close to our line of sight. The jet travels almost as quickly towards us as the light it generates, giving the illusion that the jet's motion is much more rapid than the speed of light. In the case of M87*, the jet is pointing close to our direction, resulting in these exotic apparent speeds.
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- The astronomers observed that the feature moving with an apparent speed of 6.3 times the speed of light also faded by over 70% between 2012 and 2017. This fading was likely caused by particles' loss of energy due to the radiation produced as they spiral around a magnetic field. For this to occur the team must be seeing X-rays from the same particles at both times, and not a moving wave.
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- The size of the ring around the black hole seen with the Event Horizon Telescope is about a hundred million times smaller than the size of the jet seen with Chandra. The EHT observed M87 over six days in April 2017, giving a recent snapshot of the black hole. The Chandra observations investigate ejected material within the jet that was launched from the black hole hundreds and thousands of years earlier.
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- Astronomers recently found another black hole so big that theory strains to explain it. A stellar-mass black hole was discovered that appears to be 68 times more massive than Earth's sun, nearly three times bigger than the heaviest such objects should exist, according to theory.
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- Calculations suggest that the Milky Way galaxy's stellar-mass black holes, which form after the violent deaths of giant stars, should top out at only 25 times the mass of the Sun. Although supermassive black holes, not stellar-mass blackholes, that lurk at the hearts of galaxies are much bigger, containing millions or billions of solar masses.
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- This huge black hole is also relatively close to Earth in cosmic terms. It sits at 13,800 light-years from our planet, a small fraction of the Milky Way's estimated diameter of 200,000 light-years. Black holes of such mass should not even exist in our galaxy.
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- Very massive stars with the chemical composition typical of our galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant.
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- Most black holes are found via their dramatic activity in X-rays or gamma rays, which are emitted as the black hole swallows up nearby gas and dust. Stars that are orbiting inactive black holes indicate their presence apparent only by their gravitational pull. A star called LB-1 is eight times the mass of the Sun and appears to orbit a black hole every 79 days, even though the black hole isn't visible.
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- The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave detectors have begun to catch ripples in space-time caused by collisions of black holes in distant galaxies. The black holes involved in such collisions are also much bigger than what was previously considered typical.
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- Astronomy has come a long way studying something we can not see. The trick is to use these new type of telescopes sensitive to much more of the electromagnetic spectrum. Also, get these instruments in orbit above our atmosphere that acts as a filter for much of the radiation. Lucky for us.
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- January 23, 2020 2593
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--------------------- Thursday, January 23, 2020 --------------------
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