- 4032 - GAMMA RAY BURSTS - deep space phenomena? - As gravitational and gamma-ray facilities both improve their reach, a new chapter of the GRB story will open. 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.
------ 4032 - GAMMA RAY BURSTS - deep space phenomena?
- Fifty years ago,
on June 1, 1973, astronomers around the world were introduced to a powerful and
perplexing new phenomenon, 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. With GRBs, just about everything is extreme.
They 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.
-
- 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.
-
- Over a 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.
-
- The GRB story
begins in October 1963, when a treaty signed by the United States, the United
Kingdom, and the Soviet Union prohibiting the testing of nuclear weapons in the
atmosphere, under water, or in space went into effect. To ensure compliance,
the U.S. Air Force had been managing an unclassified research and development
effort to detect nuclear tests from space. A week after the treaty went into
effect, the first two of these satellites, called Vela (from the Spanish
"to watch"), began their work.
-
- Launched in pairs,
the Vela satellites carried detectors designed to sense the initial flash of
X-rays and gamma rays from nuclear explosions. Sometimes they triggered on
events that clearly were not nuclear tests, and scientists collected and
studied these observations.
-
- With improved
instruments on the four Vela 5 and 6 satellites, they determined directions to
16 confirmed gamma-ray events well enough to rule out Earth and the Sun as
sources.
-
-Using a detector aboard the IMP 6 satellite intended to
study solar flares, they quickly confirmed the Vela findings. Theorists proposed 100 models in an effort
to explain GRBs—most involving neutron stars in our own galaxy, but
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.
-
- 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.
-
- 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.
-
- The astronomers
found that 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.
-
- When a burst
occurred in the field of view of one of the X-ray cameras, the new 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.
-
- 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.
-
- 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.
-
- Following up on
GRBs detected by these missions confirmed that long bursts were associated with
the star-forming regions of galaxies and were often accompanied by supernovae.
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.
-
- In 2008, NASA's
Fermi Gamma-ray Space Telescope joined Swift in hunting GRBs and has observed
about 3,500 by today, 2023. 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.
-
- 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.
-
- New satellites
with greater sensitivity to delve more deeply into this phenomea. “StarBurst”,
is a small satellite designed to explore GRBs from neutron star mergers. Other
missions include “Glowbug”, part of an experiment package launched to the
International Space Station in March, 2023.
-
- Scheduled 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.
-
- The earliest 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.
-
- June 2, 2023 GAMMA RAY BURSTS - deep space phenomena 4032
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Friday, June 2, 2023
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