- 2986 - GAMMA RAYS - magnetars and neutron stars. If the neutron star is spinning very fast it can create a rotating magnetic field that is a ‘pulsar’ or a ‘magnetar” depending on how strong the magnetic field is. Our newest telescopes and instruments are beginning to measure the characteristics of these extreme stars.
------------------ 2986 - GAMMA RAYS - magnetars and neutron stars.
- When stars get really big. They get too big and the atoms in the stars get crushed by gravity. When that happens the mass of the giant star collapses into the center creating a gigantic bounce. The electrons orbiting the protons get crushed into the nucleus and become neutrons.
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- The crush rebounds into exploding material becoming a supernova spreading atomic elements into space to be collected by gravity and creating another star millions of years later. What is left behind is a “neutron star” that is only a dozen kilometers in diameter.
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- By combining several gamma-ray telescopes into one giant virtual instrument, scientists can measure the diameters of individual stars hundreds of light-years away. The astronomers used the four “VERITAS telescopes” (Very Energetic Radiation Imaging Telescope Array System) in the US as one combined instrument.
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- The combined telescopes were able to determine the size of Beta Canis Majoris, a blue giant star located 500 light-years from the Sun, and, Epsilon Orionis, a blue supergiant star located 2,000 light-years from the Sun.
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- The “Stellar Intensity Interferometry” technique, demonstrated for the first time nearly 50 years ago, could be a secondary use for other gamma-ray observatories as well, including the upcoming “Cherenkov Telescope Array”.
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- Interferometry has been widely successful in achieving the angular resolution needed to spatially resolve stars and we've demonstrated the capability to perform optical intensity interferometry measurements with an array of many telescopes that in turn will help to improve our understanding of stellar systems.
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- The “VERITAS telescopes” are used to monitor the sky for faint blue flashes of Cherenkov light that are produced when gamma rays from the cosmos hit Earth's atmosphere. However, these observations are limited to dark moonless hours.
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- Modern electronics allow astronomers to computationally combine light signals from each telescope. The resulting instrument has the optical resolution of a football-field-sized reflector.
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- The measurements resulted in angular diameters of 0.523 milliarcseconds for Beta Canis Majoris and 0.631 milliarcseconds for Epsilon Orionis. A milliarcsecond is about the size of a nickel coin atop the Eiffel Tower in Paris as seen from New York.
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- The scientists have proven that dozens of telescopes could be combined using modern electronics. This could prove an interesting option for the future Cherenkov Telescope Array. It will be the world's largest “gamma-ray observatory“.
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- The “Cherenkov Telescope Array” will feature gamma-ray telescopes in three size classes, “DESY” is responsible for the medium-sized telescopes. “CTA” will employ up to 99 telescopes with kilometer baseline in the southern hemisphere and 19 telescopes with several hundred-meter baselines in the Northern hemisphere.
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- “Intensity interferometry” could not only enable scientists to determine the diameters of stars, but also to image stellar surfaces, and to measure the properties of systems like interacting binary stars, rapidly rotating stars, or the pulsation of Cepheid variables.
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- On March 12, 2020, the space telescope, “Swift” detected a burst of radiation from halfway across the Milky Way. Within a week, the newly discovered X-ray source, named “Swift J1818.0–1607“, was found to be a “magnetar“, a rare type of slowly rotating neutron star with one of the most powerful magnetic fields in the universe.
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- Spinning once every 1.4 seconds, it's the fastest spinning magnetar known, and possibly one of the youngest neutron stars in the Milky Way. It also emits radio pulses like those seen from “pulsars“, another type of rotating neutron star. Only four other radio-pulse-emitting magnetars were known.
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- The pulses from the magnetar become significantly fainter when going from low to high radio frequencies: Rthat is it has a steep radio spectrum. Its radio emission is not only steeper than the four other radio magnetars, but also steeper than 90% of all pulsars. They found that the magnetar had become over 10 times brighter in only two weeks.
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- Comparatively, the other four radio magnetars have almost constant brightness across radio frequencies. These observations were made using the ultra wideband-low receiver system installed on the “Parkes radio telescope“, “The Dish“.
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- Another pulsar underwent a magnetar-like outburst back in 2016, where it experienced a rapid increase in brightness and developed a steep radio spectrum. If the outburst of this pulsar and Swift J1818.0–1607 share the same power source, then slowly over time, the magnetar's spectrum should begin to look like other observed radio magnetars.
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- The age of the young magnetar, between 240 and 320 years, was measured from both its rotation period and how quickly it slows down over time. The spin-down rates of magnetars are highly variable on year-long timescales, particularly after outbursts, and can lead to incorrect age estimates.
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- Swift J1818.0–1607 may have started out life as a more ordinary radio pulsar that obtained the rotational properties of a magnetar over time. This can happen if the magnetic and rotational poles of a neutron star rapidly become aligned, or if supernova material fell back onto the neutron star and buried its magnetic field.
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- The buried magnetic field would then slowly emerge back to the surface over thousands of years. Continued observations of Swift J1818.0–1607, over many months to years, are needed to test these theories.
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- The origin of high-energy cosmic neutrinos observed by the “IceCube Neutrino Observatory“, whose detector is buried deep in the Antarctic ice, is a mystery that has perplexed physicists and astronomers.
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- A new model could help explain the unexpectedly large flux of some of these neutrinos inferred by recent neutrino and gamma-ray data. The theories point to the supermassive blackholes found at the cores of active galaxies as the sources of these mysterious neutrinos.
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- Neutrinos are subatomic particles so tiny that their mass is nearly zero and they rarely interact with other matter. High-energy cosmic neutrinos are created by energetic cosmic-ray accelerators in the universe, which may be extreme astrophysical objects such as blackholes and neutron stars.
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- The neutrinos must be accompanied by gamma rays or electromagnetic waves at lower energies, and even sometimes gravitational waves. Astronomers expect the levels of these various “cosmic messengers” to be related.
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- The IceCube data have indicated an excess emission of neutrinos with energies below 100 tera-electron volt (TeV), compared to the level of corresponding high-energy gamma rays seen by the “Fermi Gamma-ray Space Telescope."
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- Scientists combine information from all of these cosmic messengers to learn about events in the universe and to reconstruct its evolution in the burgeoning field of "multimessenger astrophysics."
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- For cosmic neutrinos above 100 TeV it is possible to be in accord with high-energy gamma rays and ultra-high-energy cosmic rays which fits with a multimessenger picture. However, there is growing evidence for an excess of neutrinos below 100 TeV, which cannot be explained.
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- Very recently in 2020, the “IceCube Neutrino Observatory” reported another excess of high-energy neutrinos in the direction of one of the brightest active galaxies, known as “NGC 1068“, in the northern sky.
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- We know that the sources of high-energy neutrinos must also create gamma rays, so the question is: Where are these missing gamma rays? The sources are somehow hidden from our view in high-energy gamma rays, and the energy budget of neutrinos released into the universe is surprisingly large.
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- The best candidates for this type of source have dense environments, where gamma rays would be blocked by their interactions with radiation and matter but neutrinos can readily escape. Supermassive blackhole systems are promising sites to explain the neutrinos below 100 TeV with modest energetic requirements.
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- The new model suggests that the corona, the aura of superhot plasma that surrounds stars and other celestial bodies, around supermassive black holes found at the core of galaxies, could be such a source.
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- Blackholes could have a corona above the rotating disk of material, known as an accretion disk, that forms around the black hole through its gravitational influence. This corona is extremely hot, with a temperature of about one billion degrees Kelvin, magnetized, and turbulent.
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- In this environment, particles can be accelerated, which leads to particle collisions that would create neutrinos and gamma rays, but the environment is dense enough to prevent the escape of high-energy gamma rays.
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- The model also predicts electromagnetic counterparts of the neutrino sources in `soft' gamma-rays instead of high-energy gamma rays. High-energy gamma rays would be blocked initially. They would eventually be cascaded down to lower energies and released as `soft' gamma rays in the megaelectron volt range, but most of the existing gamma-ray detectors, like the Fermi Gamma-ray Space Telescope, are not tuned to detect them.
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- New gamma-ray and neutrino detectors will enable deeper searches for multimessenger emission from supermassive black hole coronae. This will make it possible to critically examine if these sources are responsible for the large flux of mid-energy level neutrinos observed by IceCube.
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January 21, 2021 GAMMA RAYS - magnetars, neutron stars. 2986
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