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  1. #1211
    Moderator at Work ilan's Avatar
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    Milky Way’s black hole appears to be getting hungrier
    EarthSky in SPACE | September 15, 2019

    “We have never seen anything like this in the 24 years we have studied the supermassive black hole. It’s usually a pretty quiet, wimpy black hole on a diet. We don’t know what is driving this big feast.”


    Artist’s concept of an object called S0-2 orbiting our Milky Way’s supermassive black hole. Astronomers tracked this object for years, hoping to catch it falling over the hole’s event horizon. It did not fall in, but its close approach in 2018 might be one reason for the black hole’s growing appetite now. Image via Nicolle Fuller/National Science Foundation.
    UCLA astronomers announced on September 11, 2019, that, last May, they caught the supermassive black hole at the center of our Milky Way galaxy having an unusually large meal of interstellar gas and dust. They caught the feast on May 13 (although of course it happened some 25,000 years ago earlier, since the center of the galaxy is about 25,000 light-years away). What they saw was this. The black hole – called Sagittarius A*, pronounced Sagittarius A-star – became extremely bright in May 2019, growing 75 times as bright for a few hours. Yet, as of now, the researchers don’t yet understand why. Why did the area just outside the black hole’s event horizon – its point of no return – suddenly become brighter? What did it ingest, and why at that time?

    Astronomer Tuan Do is lead author of new research describing this event, published September 11 in Astrophysical Journal Letters. He also produced the timelapse in the tweet below, which depicts the brightness changes at Sgr A*. Andrea Ghez, of the UCLA Galactic Center Group, is co-senior author on the new paper. She said:

    We have never seen anything like this in the 24 years we have studied the supermassive black hole. It’s usually a pretty quiet, wimpy black hole on a diet. We don’t know what is driving this big feast.

    In a statement, the researchers also said they:

    … analyzed more than 13,000 observations of the black hole from 133 nights since 2003. The images were gathered by the W. M. Keck Observatory in Hawaii and the European Southern Observatory’s Very Large Telescope in Chile. The team found that on May 13, the area just outside the black hole’s ‘point of no return’ (so called because once matter enters, it can never escape) was twice as bright as the next-brightest observation.

    They observed large changes on two other nights this year; all three of those changes were ‘unprecedented,’ Ghez said.

    They said the brightness surrounding the black hole always varies somewhat, but the extreme variations in brightness observed this year left them “stunned.”

    So what is going on?

    In an absolute sense, the increased brightness on a few nights in 2019 can be explained by radiation from gas and dust falling into the black hole. One hypothesis about the increased activity is that when a star called S0-2 made its closest approach to the black hole during the summer 2018, it launched a large quantity of gas that reached the black hole this year. Tuan Do, the study’s lead author, said:

    The first image I saw that night, the black hole was so bright I initially mistook it for the star S0-2, because I had never seen Sagittarius A* that bright. But it quickly became clear the source had to be the black hole, which was really exciting.

    Another possibility involves a bizarre object known as G2, which is most likely a pair of binary stars, which made its closest approach to the black hole in 2014. It’s possible the black hole could have stripped off the outer layer of G2, Ghez said, which could help explain the increased brightness just outside the black hole.

    Morris said another possibility is that the brightening corresponds to the demise of large asteroids that have been drawn in to the black hole.

    The question for astronomers is, what does this activity mean? Is it simply an extraordinary singular event, or is it a precursor to significantly increased activity for Sgr A*? Mark Morris, UCLA professor of physics and astronomy, is another author on the paper. He said:

    The big question is whether the black hole is entering a new phase – for example if the spigot has been turned up and the rate of gas falling down the black hole ‘drain’ has increased for an extended period – or whether we have just seen the fireworks from a few unusual blobs of gas falling in.

    The team has continued to observe the area. They say they’ll try to settle the question based on what they see from new images. Ghez said:

    We want to know how black holes grow and affect the evolution of galaxies and the universe. We want to know why the supermassive hole gets brighter and how it gets brighter.

    By the way, these astronomers commented:

    The black hole is some 26,000 light-years away and poses no danger to our planet. Do said the radiation would have to be 10 billion times as bright as what the astronomers detected to affect life on Earth.

    Astrophysical Journal Letters also published a second article by the researchers, describing speckle holography, the technique that enabled them to extract and use very faint information from 24 years of data they recorded from near the black hole.

    Ghez’s research team reported July 25 in the journal Science the most comprehensive test of Einstein’s iconic general theory of relativity near the black hole. Their conclusion that Einstein’s theory passed the test and is correct, at least for now, was based on their study of S0-2 as it made a complete orbit around the black hole.

    Ghez’s team said it:

    … studies more than 3,000 stars that orbit the supermassive black hole. Since 2004, the scientists have used a powerful technology that Ghez helped pioneer, called adaptive optics, which corrects the distorting effects of the Earth’s atmosphere in real time. But speckle holography enabled the researchers to improve the data from the decade before adaptive optics came into play. Reanalyzing data from those years helped the team conclude that they had not seen that level of brightness near the black hole in 24 years.

    Ghez said:

    It was like doing LASIK surgery on our early images. We collected the data to answer one question and serendipitously unveiled other exciting scientific discoveries that we didn’t anticipate.

    ______________________________

    It's hard to believe all of this is happening at the center of our quiet little town. It does suggest one thing, though: If you tease a black hole that is on a diet, it is liable to forget the diet and pig out! - ilan
    Last edited by ilan; 09-15-2019 at 01:11 PM.
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  2. #1212
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    wow ......!!!
    pepisee

  3. #1213
    Moderator at Work ilan's Avatar
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    Sagittarius A* can be a little glutton when it wants to bulk up...
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  4. #1214
    Moderator at Work ilan's Avatar
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    By the way, a monster asteroid (2000 QW7) zipped safely by us yesterday. It was roughly the size of a huge skyscraper (over 2300 feet in diameter) and was traveling at 14,000 mph. It came within 3 million miles of us, which is a stone's throw in astronomical terms.
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  5. #1215
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    Mystery green blob appears and disappears in distant galaxy
    Paul Scott Anderson in SPACE | September 16, 2019

    hat is ULX-4 – a mystery green blob of X-ray light that appeared in the Fireworks Galaxy – and then soon disappeared again? A black hole or neutron star are 2 possibilities.

    This visible-light image of the Fireworks Galaxy (NGC 6946) comes from the Sloan Digital Sky Survey. It’s overlaid with data from NASA’s NuSTAR X-ray telescope (in blue and green). The mystery green blob is toward the middle of the galaxy, apparently coincident with one of its spiral arms. Image via NASA/JPL-Caltech.
    The universe is full of mysteries, and now there’s a new one for astronomers to puzzle over. A space-based X-ray observatory – NASA’s NuSTAR – saw what astronomers described as a green blob in the galaxy known as NGC 6946, aka the Fireworks Galaxy. The blob appeared within a 10-day period and then disappeared again just as quickly.

    A new peer-reviewed paper discussing the intriguing finding was published in The Astronomical Journal on August 9, 2019.

    It’s really unusual for a celestial object to appear and then vanish again over such a short time period. The object was given the label ULX-4, as it was the fourth ULX – ultraluminous X-ray source – found in that galaxy.

    NuSTAR’s primary mission at the time had been to study a supernova in the galaxy, which appears as a bright blue-green spot in the upper right of the image above. The mystery green blob is closer to the galaxy’s center in the image, apparently coincident with one of its spiral arms.

    The mystery object probably isn’t a supernova, however, because it was only detected in X-ray but not visible light images. It had first been seen by NuSTAR, and then later was found to have disappeared by NASA’s Chandra X-ray Observatory.

    One of the main questions right now is how the object appeared and disappeared so quickly. According to Hannah Earnshaw, a postdoctoral researcher at Caltech:

    Ten days is a really short amount of time for such a bright object to appear. Usually with NuSTAR, we observe more gradual changes over time, and we don’t often observe a source multiple times in quick succession. In this instance, we were fortunate to catch a source changing extremely quickly, which is very exciting.

    So if the object isn’t a supernova, then what is it?

    One possibility is that it is a black hole. According to that theory, the greenish light came from a black hole that was consuming another object, perhaps a star. We tend to think of black holes as “sucking in” material, but the immense gravity from the black hole could also rip the object apart, with the resulting debris going into orbit around the black hole. That debris can be heated to millions of degrees, emitting X-rays. So even though light can’t escape from inside a black hole, the light emitted by super-heated debris orbiting the black hole can easily be seen.

    Usually, ULXs are long-lived, since black holes will feed for a long time on the stars they destroy. But if, say, a small star is destroyed more quickly, then that could explain short-term events like ULX-4.

    ULX-4 could also possibly be a neutron star, an extremely dense object that forms when a star explodes, but isn’t massive enough to create a black hole. They can also have disks of debris orbiting them, creating slow-feeding ultraluminous X-ray sources. Material is channeled down to the surface in “columns” by powerful magnetic fields. But material can be blocked by those magnetic fields from reaching the surface if the neutron star is spinning too fast.

    The only times you would then see the neutron star as a bright source of X-rays would be at the moment when the magnetic field barrier wavers a bit. This could explain the sudden appearance and disappearance of the light coming from ULX-4. According to Earnshaw:

    It would kind of be like trying to jump onto a carousel that’s spinning at thousands of miles per hour. This result is a step towards understanding some of the rarer and more extreme cases in which matter accretes onto black holes or neutron stars.

    Further observations will be needed to determine which of these theories – if either of them – is the right one. There may still turn out to be a different explanation altogether. Space is full of weird and mysterious phenomena, and “the case of the appearing and disappearing green blob” is certainly an odd one!
    Last edited by ilan; Yesterday at 10:48 PM.
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