- 4066 - GAMMA RAY BURSTS - how first discovered? 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.
---------------- 4066 - GAMMA RAY BURSTS - how first discovered?
- 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 GRB 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.
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- One recent GRB 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.
-
- Launched in pairs
in 1963, 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 scientists determined
directions to 16 confirmed gamma-ray events well enough to rule out Earth and
the Sun as sources. They published a paper announcing the discovery in The
Astrophysical Journal on June 1, 1973.
-
- Using a detector aboard the “IMP 6 satellite”
intended to study solar flares quickly confirmed the Vela findings.
-
- 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
difficult. Without satellites, 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 BATSE team
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. Soon after, theorists associated long GRBs
with the collapse of massive stars and short ones with binary neutron star
mergers.
-
- The next step in
understanding came with watershed observations from the satellite
“BeppoSAX”. Although not specifically
designed as a GRB mission, its mix of instruments, including a gamma-ray
monitor and two wide-field X-ray cameras, proved a boon to the field.
-
- When a burst
occurred in the field of view of one of the 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.
-
- 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, the
“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.
-
- 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.
-
- We're building new
satellites with greater sensitivity to delve more deeply into this phenomenon,
so the future of GRB science is bright. “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 ,and slated for launch in early 2024; “MoonBEAM”, which
would orbit between Earth and the Moon.
-
- 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. 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.
-
-
June 24, 2023 GAMMA RAY
BURSTS - how first discovered? 4066
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