- 3575 - BLACKHOLES - how magnetic fields flip? Magnetic reversals are likely to be common events in the cosmos. The geologic record shows that Earth's field flips unpredictably, averaging a few reversals every million years in the recent past. Blackholes seem to experience a similar phenomena.
--------------------- 3575- BLACKHOLES - how magnetic fields flip?
- Astronomy’s understanding of blackholes is now central to our understanding of the cosmos. The next generation “Very Large Array” (ngVLA) will help astronomers study these mysterious objects.
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- Astronomers have long known that Einstein's theory of gravity allowed for an object to be so massive that light itself could not escape, but they initially doubted that black holes existed in the Universe.
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- Today, black holes are recognized as a standard result of the death of very massive stars. There is a supermassive black hole at the center of the Milky Way Galaxy, and the link between supermassive central black holes to the formation and evolution of their host galaxies is a topic of active research.
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- A century ago, astronomers thought that the Universe consisted mostly of stars. They shine with the colors of light that our human eyes can see, and to most of us, the picture of an astronomer includes a telescope turned to the heavens. Today, however, we now recognize that a variety of objects shine at wavelengths that our eyes cannot see, from long wavelength radio waves to extremely high-energy gamma rays.
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- We now know that there are a variety of other messengers carrying to us information about the Universe. Cosmic rays are energetic sub-atomic particles, with energies well above those that particle accelerators such as the Large Hadron Collider can produce.
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- In the most extreme cases, a sub-atomic particle can hit the Earth's atmosphere with as much energy as a fast-pitch baseball. Billions of neutrinos rain upon us every second. They are born from nuclear fusion in the Sun, from distant exploding stars, from the regions near supermassive black holes. And gravitational waves constantly wash over the Earth and the Solar System. These distortions of spacetime itself are generated by colliding black holes, and potentially by the expansion of the Universe.
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- Within our own Milky Way, the ngVLA will greatly expand our ability to detect black holes in binary systems, enabling probes of supernova explosions and black hole formation. It will also enable the detection of less massive black holes that dwell in the centers of dwarf galaxies throughout the local galactic cluster.
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- The detection of gravitational waves using the “ngVLA” will be able to resolve and observe the motion of mergers of supermassive black holes and neutron stars, both sources of gravitational waves.
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- New facilities can detect these merging stellar remnants in galaxies up to 600 million light-years away through the gravitational wave and neutrino events they produce, and the ngVLA will be able to detect the radio emission to the same distance, permitting us to determine the physical conditions at the location of neutrino production.
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- Astronomers still have much to learn from black holes. In the near future, the ngVLA provide astronomers with a central tool for understanding black holes and multi-messenger astronomy.
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- A mysterious outburst from a galaxy 236 million light-years away may have been sparked by a magnetic reversal, a spontaneous flip of the magnetic field surrounding its central black hole.
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- The eruption's unusual characteristics to changes in the black hole's environment likely would be triggered by such a magnetic switch. This event marks the first time astronomers have seen X-rays dropping out completely while the other wavelengths brighten.
The “Neil Gehrels Swift Observatory” and ESA's (European Space Agency) “XMM-Newton satellite” provided UV and X-ray measurements. Visible light observations came from Italy's 3.6-meter “Galileo National Telescope” and the 10.4-meter “Gran Telescopio Canarias“, both located on the island of La Palma in the Canary Islands, Spain. Radio measurements were acquired from the “Very Long Baseline Array“, a network of 10 radio telescopes located across the United States; the Very Large Array in New Mexico; and the European VLBI Network.
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- In early March 2018, the All-Sky Automated Survey for Supernovae alerted astronomers that a galaxy called “1ES 1927+654” had brightened by nearly 100 times in visible light. A search for earlier detections by the NASA-funded Asteroid Terrestrial-impact Last Alert System showed that the eruption had begun months earlier, at the end of 2017.
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- When Swift first examined the galaxy in May 2018, its UV emission was elevated by 12 times but steadily declining, indicating an earlier unobserved peak. Then, in June, the galaxy's higher-energy X-ray emission disappeared. A sudden reversal of the magnetic field around its million-solar-mass black hole may have triggered the outburst.
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- Most big galaxies, including our own Milky Way, host a supermassive black hole weighing millions to billions of times the Sun's mass. When matter falls toward one, it first collects into a vast, flattened structure called an accretion disk.
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- As the material slowly swirls inward, it heats up and emits visible, UV, and lower-energy X-ray light. Near the black hole, a cloud of extremely hot particles - called the corona - produces higher-energy X-rays. The brightness of these emissions depends on how much material streams toward the black hole.
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- An earlier interpretation of the eruption suggested that it was triggered by a star that passed so close to the black hole it was torn apart, disrupting the flow of gas. The unique disappearance of the X-ray emission provides astronomers with an important clue. They suspect the black hole's magnetic field creates and sustains the corona, so any magnetic change could impact its X-ray properties.
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- A magnetic reversal, where the north pole becomes south and vice versa, seems to best fit the observations to develop the magnetic model. The magnetic field initially weakens at the outskirts of the accretion disk, leading to greater heating and brightening in visible and UV light.
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- As the flip progresses, the field becomes so weak that it can no longer support the corona and the X-ray emission vanishes. The magnetic field then gradually strengthens in its new orientation.
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- In October, 2018, about 4 months after they disappeared, the X-rays came back, indicating that the corona had been fully restored. By summer 2021, the galaxy had completely returned to its pre-eruption state.
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- Magnetic reversals are likely to be common events in the cosmos. The geologic record shows that Earth's field flips unpredictably, averaging a few reversals every million years in the recent past.
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- The Sun, by contrast, undergoes a magnetic reversal as part of its normal cycle of activity, switching north and south poles roughly every 11 years. The same as the sunspot cycle.
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May 9, 2022 BLACKHOLES - how magnetic fields flip? 3575
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