Far out cosmology: gamma ray bursts more understood (Introduction)

by David Turell , Wednesday, June 30, 2021, 21:30 (1029 days ago) @ David Turell

They come from far, far away and are not dangerous to us even though they are massive bursts of energy:

https://www.quantamagazine.org/brighter-than-a-billion-billion-suns-gamma-ray-bursts-co...

"...in 1997, an Italian and Dutch satellite called BeppoSAX confirmed that gamma-ray bursts were extragalactic, in some cases originating many billions of light-years away.

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"A gamma-ray burst will emit the same amount of energy as a supernova, caused when a star collapses and explodes, but in seconds or minutes rather than weeks. Their peak luminosities can be 100 billion billion times that of our sun, and a billion times more than even the brightest supernovas.

"It turned out to be fortunate that they were so far away. “If there was a gamma-ray burst in our galaxy with a jet pointed at us, the best thing you could hope for is a quick extinction,” said Zhu. “You would hope that the radiation smashes through the ozone and immediately fries everything to death. Because the worst scenario is if it’s farther away, it could cause some of the nitrogen and oxygen in the atmosphere to turn into nitrous dioxide. The atmosphere would turn brown. It would be a slow death.”

"Gamma-ray bursts come in two flavors, long and short. The former, which can last up to several minutes or so, are thought to result from stars more than 20 times the mass of our sun collapsing into black holes and exploding as supernovas. The latter, which last only up to about a second, are caused by two merging neutron stars (or perhaps a neutron star merging with a black hole), which was confirmed in 2017 when gravitational-wave observatories detected a neutron star merger and NASA’s Fermi Gamma-ray Space Telescope caught the associated gamma-ray burst.

"In each instance, the gamma-ray burst does not come from the explosion itself. Rather it comes from a jet moving at a fraction below the speed of light that gets fired out from the explosion in opposite directions. (The exact mechanism that powers the jet remains a “very fundamental question,” said Zhu.)

“'It is that combination of the speed at high energy and the focusing into a jet that makes them extremely luminous,” said Nial Tanvir, an astronomer at the University of Leicester in England. “That means we can see them very far away.” On average, there is thought to be one observable gamma-ray burst in the visible universe every day.

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Until recently, the only way to study gamma-ray bursts was to observe them from space, as "Earth’s ozone layer blocks gamma rays from reaching the surface. But as gamma rays enter our atmosphere, they bump into other particles. These particles get pushed faster than the speed of light in air, which leads them to emit a blue glow known as Cherenkov radiation. Scientists can then scan for these blue bursts of light.

"Because our atmosphere has a much larger collecting area than a single telescope, this search strategy gives astrophysicists a greater chance of finding the highest-energy gamma-ray bursts, which are rare and hard to spot.

"The first observation of such an ultrahigh-energy burst was made in July 2018 by an array of antennas in Namibia called the High Energy Stereoscopic System (HESS). The radiation came not from the initial gamma-ray burst itself, but from an effect called the afterglow. In this case, the gamma-ray burst’s jet collided with material thrown off from the star as it went supernova. The collision accelerated particles to high speeds, producing electromagnetic radiation that then made its way to Earth.

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"Gamma-ray bursts and their afterglows can play an important role in our understanding of the universe too. Supernovas and neutron star mergers are thought to produce the universe’s heavy elements, such as gold and platinum. Since bursts give a window into the wreckage following these events, scientists can use them to track how the chemical composition of the universe has changed over cosmic time."

Comment: Another example of violent cosmos activity not directly affecting us now.