- 4067 - GAMMA RAY BURSTS - how to explain them? Fifty years ago, on June 1, 1973, astronomers around the world were introduced to a powerful and perplexing new phenomenon called GRBs (gamma-ray bursts). Today sensors on orbiting satellites like NASA's Swift and Fermi missions detect a GRB somewhere in the sky about once a day on average. Astronomers think the bursts arise from catastrophic occurrences involving stars in distant galaxies, events thought to produce new black holes.
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4067 - GAMMA RAY
BURSTS - how to explain them?
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- GRBs occur so far
beyond our galaxy that even the closest-known burst exploded more than 100
million light-years away. Each burst produces an initial pulse of gamma rays,
the highest-energy form of light, that typically lasts from milliseconds to
minutes. This emission comes from a jet of particles moving close to the speed
of light launched in our direction, and the closer we are to looking straight down
the barrel, the brighter it appears.
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- Following this
prompt emission is a fading afterglow of gamma rays, X-rays, ultraviolet,
visible, infrared, and radio light that astronomers may be able to track for
hours to months.
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- Even half a century
on, GRBs offer up surprises. One recent burst was so bright it temporarily
blinded most of the gamma-ray detectors in space. Nicknamed the BOAT (for
brightest of all time), the 7-minute blast may have been the brightest GRB in
the past 10,000 years. It also showed that scientists' most promising models of
these events are nowhere near complete.
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- While theorists
proposed 100 models in an effort to explain GRBs, most involving neutron stars
in our own galaxy, observational progress was slow despite the growing number
of detections by different spacecraft. Gamma rays can't be focused like visible
light or X-rays, making precise localizations quite difficult. Without them, it
was impossible to search for GRB counterparts in other wavelengths using larger
telescopes in space or on the ground.
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- In 1991, NASA
launched the Compton Gamma Ray Observatory, which included an instrument named
BATSE (Burst and Transient Science Experiment) dedicated to exploring
GRBs. BATSE was about 10 times more
sensitive than previous GRB detectors. Over Compton's nine-year mission, BATSE
detected 2,704 bursts, which gave astronomers a rich set of observations made
with the same instrument.
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- In its first year,
BATSE data showed that bursts were distributed all over the sky instead of in a
pattern that reflected the structure of our Milky Way galaxy. This suggested that they were coming from
distant galaxies, and that meant they were more energetic than most scientists
thought possible.
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- Burst durations
clustered into two broad groups,one lasting less than two seconds, the other
lasting longer than two seconds, and that short bursts produced higher-energy
gamma rays than long ones.
-
- So both temporal
and spectral properties agreed in identifying two separate groups of GRBs:
short and long. Theorists associated
long GRBs with the collapse of massive stars and short ones with binary neutron
star mergers.
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- When a burst
occurred in the field of view X-ray cameras, the spacecraft could locate it well
enough over a couple of hours that additional instruments could be brought to
bear. Whenever BeppoSAX turned to a GRB's position, its instruments found a
rapidly fading and previously unknown high-energy source,the X-ray afterglow
theorists had predicted.
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- These positions
enabled large ground-based observatories to discover long GRB afterglows in
visible light and radio waves, and also permitted the first distance
measurements, confirming that GRBs were truly far-away events.
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- Although most GRBs
originate from exploding massive stars or neutron-star mergers, the researchers
concluded that “GRB 191019A” instead came from the collision of stars or
stellar remnants in the jam-packed environment surrounding a supermassive black
hole at the core of an ancient galaxy.
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- This study will be
published June 22, 2023, in the journal Nature Astronomy. This remarkable
discovery grants astronomers a tantalizing glimpse into the intricate dynamics
at work within these cosmic environments, establishing them as factories of
events that would otherwise be deemed impossible.
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- Most stars die,
according to their mass, in one of three predictable ways. When relatively
low-mass stars like our sun reach old age, they shed their outer layers,
eventually fading to become white dwarf stars. More massive stars, on the other
hand, burn brighter and explode faster in cataclysmic supernovae explosions,
creating ultra-dense objects like neutron stars and black holes. The third
scenario occurs when two such stellar remnants form a binary system and
eventually collide.
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- But the new study
finds there might be a fourth option.
Stars can meet their demise in some of the densest regions of the
universe, where they can be driven to collide. Long past their star-forming prime,
ancient galaxies have few, if any, remaining massive stars. Their cores,
however, teem with stars and a menagerie of ultra-dense stellar remnants, such
as white dwarfs, neutron stars and black holes.
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- Astronomers have
long suspected that in the turbulent beehive of activity surrounding a
supermassive black hole, it only would be a matter of time before two stellar
objects collided to produce a GRB. But evidence for that type of merger has
remained elusive.
-
- On Oct. 19, 2019,
astronomers glimpsed the first hints of such an event when NASA’s Neil Gehrels
Swift Observatory detected a bright flash of gamma rays that lasted a little
over one minute. Any GRB lasting longer than two seconds is considered “long.”
Such bursts typically come from the collapse of stars at least 10 times the
mass of our sun.
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- Making long-term
observations of the GRB’s fading afterglow enabled the astronomers to pinpoint
the location of the GRB to a region less than 100 light-years from the nucleus
of an ancient galaxy, very near the galaxy’s supermassive black hole.
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- The lack of a
supernova accompanying the long GRB 191019A tells us that this burst is not a
typical massive star collapse. The
location of GRB 191019A, embedded in the nucleus of the host galaxy, teases a
predicted but not yet evidenced theory for how gravitational-wave emitting
sources might form.
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- In typical
galactic environments, the production of long GRBs from colliding stellar
remnants, such as neutron stars and black holes, is incredibly rare. The cores
of ancient galaxies, however, are anything but typical, and there may be a
million or more stars crammed into a region just a few light-years across.
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- Such extreme
population density may be great enough that occasional stellar collisions can
occur, especially under the titanic gravitational influence of a supermassive
black hole, which would perturb the motions of stars and send them careening in
random directions. Eventually, these wayward stars would intersect and merge, triggering
a titanic explosion that could be observed from vast cosmic distances.
-
- It is possible
that such events occur routinely in similarly crowded regions across the
universe but have gone unnoticed until this point. A possible reason for their
obscurity is that galactic centers are brimming with dust and gas, which could
obscure both the initial flash of the GRB and the resulting afterglow. GRB
191019A may be a rare exception, allowing astronomers to detect the burst and
study its aftereffects.
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- While this event is
the first of its kind to be discovered, it’s possible there are more out there
that are hidden by the large amounts of dust close to their galaxies. If this long-duration event came from merging
compact objects, it contributes to the growing population of GRBs that defies
our traditional classifications.
-
- In 2000, NASA
launched HETE 2, a small satellite designed to detect and localize GRBs. It was
the first mission to compute accurate positions onboard and quickly—in tens of
seconds—communicate them to the ground so other observatories could study early
afterglow phases. The burst it discovered on March 29, 2003, also exhibited
definitive supernova characteristics, confirming a suspected relationship
between the two phenomena.
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- In May 2005, Swift
was able to pinpoint the first afterglow of a short GRB, showing that these
blasts occur in regions with little star formation. This bolstered the model of
short bursts as mergers of neutron stars, which can travel far from their birth
place over the many millions of years it takes for them to crash together.
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- In 2008, NASA's
Fermi Gamma-ray Space Telescope joined Swift in hunting GRBs and has observed
about 3,500 to date. Its GBM (Gamma-ray Burst Monitor) and Large Area Telescope
allow the detection and follow-up of bursts from X-rays to the highest-energy
gamma rays detected in space, an energy span of 100 million times. This has
enabled the discovery of afterglow gamma rays with billions of times the energy
of visible light.
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- In 2017, Fermi and
the European INTEGRAL satellite linked a short GRB to a source of gravitational
waves, ripples in space-time produced as orbiting neutron stars spiraled inward
and merged. This was an important first that connected two different cosmic
"messengers," gravity and light. While astronomers haven't seen
another "gravity and light" burst since, they hope more will turn up
in current and future observing runs of gravitational wave observatories.
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- “StarBurst” is a
small satellite designed to explore GRBs from neutron star mergers. Other
missions include “Glowbug”. “BurstCube”
slated for launch in early 2024; “MoonBEAM”, which would orbit between Earth
and the Moon and “LEAP”, designed to study GRB jets from the space station.
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- What will
completely revolutionize our understanding of GRBs will be the ability to track
them back to when the universe was most intensely forming stars, around 10
billion years ago.
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- This part of the
universe will be probed by the next generation of gravitational wave
detectors—10 times more sensitive than what we currently have—and by future
gamma-ray missions that can ensure continuity with the fantastic science Swift
and Fermi have enabled.
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June 25, 2023 GAMMA RAY BURSTS - how
to explain them? 4067
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