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---------------------- 2330 - Gamma-Rays Bursts - The Brightest Flash in Our History
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- Monday, December 27, 2004, the brightest flash of light in our history passed by Earth. It originated over 50,000 years ago when, here on Earth, only 50 northern Europeans survived the ice age on Earth from which all Europeans are descendents.
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- When the Barringer crater was formed in Arizona by an asteroid impact. Cro-Magnon man was learning stone working. It took that long for the flash of radiation to reach us.
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- The flash occurred on a neutron star in the constellation Sagittarius 50,000 light years away. If the flash were visible light it would have been as bright as a full moon coming from 50,000 lightyears away. However, most all the electromagnetic radiation was in the gamma ray and X-ray frequencies not detectable by our eyes
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- As the radiation stormed through our solar system it blitzed 15 spacecraft and satellites knocking their instruments off scale. One satellite recovered quickly. The RHESSI telescope, Reuvan Ramaty High Energy Solar Spectroscopic Imager (RHESSI), saturated for 0.5 seconds and recovered to measure the tail of X-rays that followed the gamma ray burst.
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- The X-rays stormed through our ionosphere for the next six minutes ripping atoms apart, expanding the ionizing layer deeper than the biggest solar flares have done.
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- The source was a neutron star called a magnetar, named 1806-20, located on the other side of our Milky Way Galaxy almost exactly the same distance as our Solar System is located on this side of the galaxy center.
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- 50,000 years ago a starquake occurred on this neutron star’s surface creating a tremendous release of energy that flared out into the galaxy. The star is called a magnetar because the neutron star spins rapidly and has an enormous magnetic field spinning with it.
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- The strength of the magnetism is 10^12 Gauss, compared to the average neutron star at 10^9 Gauss and the planet Earth at 0.5 Gauss. The starquake on the surface caused a break and reconnect of the magnetic field which unleashed a flare of energy exceeding 10^40 watts. For a tenth of a second the flare released more energy than our Sun will emit for the next 150,000 years(1).
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- The X-rays that were measured after the Gamma Ray Burst went past us lasted for 6 minutes and had an oscillation in their brightness of 7.56 seconds. This happens to be the exact rotation rate of the Neutron star 1806-20.
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- The flare’s tail remained attached to the surface of the rotating star modulating the strength of the radiation. The electrons and positrons trapped in the magnetic field collided and annihilated each other in a burst of gamma rays that flared away from the spinning star.
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- Neutron stars are the remnants of giant stars that have ended their lives in a supernova explosion. These giant stars start out at 30 to 40 times the mass of our Sun. They live short lives of only 10 million years before they have burned all their nuclear fuel down to an iron core.
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- When the iron core collapses it creates a tremendous supernova explosion. Only a core of pure neutrons is left after the supernova debris is scattered throughout the Universe. The neutron star that is left behind is only 20 kilometers in diameter. The neutrons in the star are a super fluid, that flows without friction.
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- Since all friction is fundamentally created by the electric force, the neutrons are a neutral charge and therefore there is no friction between them. Because the star is spinning rapidly it acts like a giant electric generator. Its rotating magnetic field creates an intense electric field which acts on protons and electrons that are near the star’s surface.
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- The electrons and protons become cosmic rays that get accelerated and shot out of the magnetic poles in both directions. If the rotational axis is not aligned with the magnetic axis the beam of charged particles and radiation sweep out like a lighthouse beacon. If the Earth happens to be in the path of the beam we see a Pulsar, a pulse of radiation that flashes by with every rotation.
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- There are millions of neutron stars in the Milky Way Galaxy. Of these 1,500 known pulsars have been identified and 12 are known magnetars. They are thought to be the same beast except the magnetars started out as 30 to 40 solar mass supergiant stars and the pulsars at 8 to 20 solar mass giant stars.
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- Because magnetars and pulsars are giving off energy they are slowing down in their speed of rotation. After 10,000 years a magnetar will slow down enough to turn off its
X-ray flash. Pulsars spin at several times per second but they do not have the inertia of the magnetars to create the enormous magnetic fields. Pulsars emissions are in the radio frequency range. Magnetars rotate once every 10 seconds emitting X-rays and even flashes of gamma rays.
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- The rotating neutron stars get their inertia from the conservation of angular momentum. Much like an ice skater that brings in her arms to spin faster, when a supernova collapses into a neutron star the reduced radius of rotation cause the star to spin faster.
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- With a supernova explosion a supergiant star reduces its core from 435,000 mile sphere to a 12 mile diameter sphere (700,000 to 10 kilometers). The period of rotation for the supergiant is 57 days (Compared to our Sun which has a period of rotation of 25 days).
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- The angular momentum must be preserved. So if the radius is reduced and the mass is the same the velocity of rotation must increase accordingly.
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---------------- Angular momentum of Supergiant = Angular momentum of Neutron star
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---------------- Angular momentum = mass * velocity * radius = m*v*r
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---------------- Assuming all the mass of the stellar core is retained during the collapse, the mass remains constant.
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----------------------------- M*V*R = M*v*r
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---------------- Velocity of rotation = angular velocity * radius
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---------------------------- W = V*R
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---------------------------- Therefore: M*W*R^2 = M*w*r^2
--------------------------- And, since the mass is the same:
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--------------------------- W*R^2 = w*r^2
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--------------------------- Angular velocity = 2*pi / Period
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--------------------------- W = 2*pi / P
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--------------------------- W = 2*pi / 57 days = 2*pi / 4.9248*10^6 seconds
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--------------------------- W = 1.276*10^-6 radians / second
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- Substituting back into the conservation of angular momentum formula:
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------------------------- 1.276*10^-6 radians / second * (7*10^5 km)^2 = w * (10 km)^2
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------------------------- w = 651 radians / second
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------------------------- w = 2* pi / period
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- Period = 0.001 seconds / rotation
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- The neutron star is rotating 1000 times every second.
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- This magnetar gamma ray burster was 50,000 lightyears away. Had it been 10 to 20 lightyears away it would have exterminated life on Earth. The nearest star is 4.2 lightyears away. Fortunately, all know magnetars are 1000’s of lightyears away. Earth’s atmosphere and magnetosphere should be able to protect us , as long as the gamma ray bursts do not get any bigger or any closer.
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- The life of the magnetar is only about 10,000 years because it looses so much energy supporting its intense magnetic fields. In 10,000 years it would slow its rotation to once per few seconds and it would turn off its X-ray flash capability.
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- But, in the meantime, the rotation and magnetic forces put immense stress on the 1.3 kilometer crust that makes up the surface of the neutron star. Breaks in the crust and resulting starquakes occur causing the enormous flashes that we experienced on December 27, 2004. When a magnetar that far away can reach across the galaxy and tap you on the shoulder like this it reminds you how closely our lives are tied to the cosmos.
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- Footnote:
-------------- The luminosity of the Sun is 3.9 * 10^26 joules/second, or watts.
-------------- The neutron flare was 10^40 watts.
-------------- There are 3.156*10^7 seconds in a year.
------------- In one year the Sun would emit 12.3 *10^33 joules.
-------------- To get to 10^40 joules it would have to shine another 800,000 years according to my calculations. But, the article said 150,000 years. So my calculation education is incomplete.
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- April 9, 2019 542
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-------------------------- Tuesday, April 9, 2019 --------------------------
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