- 4345
- GAMMA RAY
BURSTS. - Gamma-ray bursts are universe's most
powerful explosions. Two neutron stars
begin to merge blasting jets of high-speed particles. These collision events
create short gamma-ray bursts. The most
powerful events in the known universe are gamma-ray bursts (GRBs). They are short-lived outbursts of the
highest-energy light.
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--------------------------------- 4345 -
GAMMA RAY BURSTS
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- Gamma Ray Bursts can erupt with a
quintillion (a 10 followed by 18 zeros) times the luminosity of our sun. Now
thought to announce the births of new black holes, they were discovered by
accident.
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- The backstory takes us to 1963, when the
U.S. Air Force launched the “Vela satellites” to detect gamma rays from banned
nuclear weapons tests. The United States had just signed a treaty with the
United Kingdom and the Soviet Union to prohibit tests within Earth's
atmosphere, and the Vela satellites ensured all parties' compliance. Instead,
the satellites stumbled upon 16 gamma-ray events.
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- By 1973, scientists could rule out that
both Earth and the sun were the sources of these brilliant eruptions. That's
when astronomers at Los Alamos National Laboratory published the first paper
announcing these bursts originate beyond our solar system.
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- Scientists at NASA's Goddard Space Flight
Center quickly confirmed the results through an X-ray detector on the “IMP 6
satellite”. It would take another two decades and contributions from the
Italian Space Agency's BeppoSax and NASA's Compton Gamma-Ray Observatory to
show that these outbursts occur far beyond our Milky Way galaxy, are evenly
distributed across the sky, and are extraordinarily powerful. The closest GRB
on record occurred more than 100 million light-years away.
-
- Though discovered by chance, GRBs have
proven invaluable for today's researchers. These flashes of light are rich with
insight on phenomena like the end of life of very massive stars or the
formation of black holes in distant galaxies.
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- In 2017, GRBs were first linked to
gravitational waves, ripples in the fabric of space-time, steering us toward a
better understanding of the how these events work.
-
- Astronomers separate GRBs into two main
classes: short (where the initial burst of gamma rays lasts less than two
seconds) and long events (lasting two seconds or longer). Shorter bursts also produce fewer gamma rays
overall, which lead researchers to hypothesize that the two classes originated
from different systems.
-
- Astronomers now associate short bursts with
the collision of either two neutron stars or a neutron star and a black hole,
resulting in a black hole and a short-lived explosion. “Short GRBs” are
sometimes followed by “kilonovae”, light produced by the radioactive decay of
chemical elements. That decay generates even heavier elements, like gold,
silver, and platinum.
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- Long bursts are linked to the explosive
deaths of massive stars. When a high-mass star runs out of nuclear fuel, its
core collapses and then rebounds, driving a shock wave outward through the
star. Astronomers see this explosion as a supernova. The core may form a either
a neutron star or a black hole.
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- In both classes, the newly born black hole
beams jets in opposite directions. The jets, made of particles accelerated to
near the speed of light, pierce through and eventually interact with the
surrounding material, emitting gamma rays when they do.
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- In August 2020, NASA's “Fermi Gamma-ray
Space Telescope” tracked down a second-long burst named GRB 200826A, more than
6 billion light-years away. It should have fallen within the short-burst class,
triggered by mergers of compact objects.
-
- However, other characteristics of this
event, like the supernova it created, suggested it originated from the collapse
of a massive star. Astronomers think this burst may have fizzled out before it
could reach the duration typical of long bursts.
-
- Swift Observatory captured its opposite
number, GRB 211211A in December 2021. Located a billion light-years away, the
burst lasted for about a minute. While this makes it a long GRB, it was
followed by a kilonova, which suggests it was triggered by a merger. Some
researchers attribute this burst's oddities to a neutron star merging with a
black hole partner.
-
- As astronomers discover more bursts lasting
several hours, there may still be a new class in the making: Ultra-long GRBs.
The energy created by the death of a high-mass star likely can't sustain a
burst for this long, so scientists must look to different origins.
-
- Some think ultra-long bursts occur from
newborn magnetars which are neutron stars with rapid rotation rates and
magnetic fields a thousand times stronger than average. Others say this new
class calls for the power of the universe's largest stellar residents, blue
supergiants. Researchers continue to explore ultra-long GRBs.
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- GRB afterglows occur when material in the
jets interact with surrounding gas.
Afterglows emit radio, infrared, optical, UV, X-ray, as well as
gamma-ray light, which provides more data about the original burst. Afterglows
also linger for hours to days (or even years) longer than their initial
explosion, creating more opportunities for discovery.
-
- Studying afterglows became key to deducing
the driving forces behind different bursts. In long bursts, as the afterglow
dims, scientists eventually see the source brighten again as the underlying
supernova becomes detectable.
-
- Although light is the universe's fastest
traveler, it can't reach us instantaneously. By the time we detect a burst,
millions to billions of years may have passed, allowing us to probe some of the
early universe through distant afterglows.
-
- Fermi and Swift discovered one of these
revolutionary events in 2022 with GRB 221009A, a burst so bright it temporarily
blinded most space-based gamma-ray instruments. A GRB of this magnitude is
predicted to occur once every 10,000 years, making it likely the
highest-luminosity event witnessed by human civilization. Astronomers
accordingly dubbed it the brightest of all time, or the “BOAT”.
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- GRBs also connect us to one of the
universe's most sought-after messengers. “Gravitational waves” are invisible
distortions of space-time, born from cataclysmic events like neutron-star
collisions. Think of space-time as the universe's all-encompassing blanket,
with gravitational waves as ripples wafting through the material.
-
- In 2017, Fermi spotted the gamma-ray flash
of a neutron-star merger just 1.7 seconds after gravitational waves were
detected from the same source. After traveling 130 million light-years, the
gravitational waves reached Earth narrowly before the gamma rays, proving
gravitational waves travel at the speed of light.
-
- James Webb telescope sees dozens of young
Quasars in the first billion years of the Universe. Within almost every galaxy is a supermassive
black hole. Millions, sometimes billions of solar masses locked within an event
horizon of space and time.
-
- They can power luminous quasars, drive star
formation, and change the evolution of a galaxy. Because of their size and
abundance, supermassive black holes must have formed early in cosmic history.
But how early is still an unanswered question.
-
- Since the luminosity of a black hole
depends in part on the size of the black hole, it can be used as a way to gauge
the mass of early supermassive black holes. Astronomers have identified 350
compact galaxies with a redshift greater than z = 6. The light from these
galaxies began their journey to Earth when the Universe was less than a billion
years old, making them among the earliest galaxies.
-
- Of these, 64 of them appeared to have
quasars, indicating the presence of an active supermassive black hole. They
then compared the luminosity and redshift to determine the age and mass of the
black holes.
-
- Astronomers found that statistically, these
early supermassive black holes were large compared to their galaxies, having a
mass of up to ten million solar masses, compared to a galactic mass of a few
billion Suns. This ratio is higher than that of the local Universe, which
suggests that the black holes form early when their galaxy is small, rather
than later when the galaxy is larger and more evolved. This supports the direct
collapse model of supermassive black holes rather than the idea that they grow
in mass through the merger of smaller black holes.
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- More deep sky surveys are in the pipeline,
and with more data we will have more early black holes to study. Right now we
know of dozens of early supermassive black holes. As that grows to hundreds we
should be able to understand the various origins of galactic black holes.
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February 7, 2023 GAMMA
RAY BURSTS 4345
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------ “Jim Detrick” -----------
--------------------- --- Friday, February 9,
2024
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