- 3888 - GAMMA RAY BURSTS - what causes them? What are those elusive gamma ray bursts? Gamma-ray bursts are really bright and all you need is a relatively small detector to spot them. Gamma-ray bursts are the highest-energy explosions known to take place in the universe.
------------- 3888 - GAMMA RAY BURSTS - what causes them?
- Astronomers
believe that only the Big Bang produced more energy than these mysterious
flashes of super-energetic photons that come from distant galaxies.
-
-
Accidentally discovered in the 1960s by American satellites keeping an
eye on Russia's testing of nuclear weapons (which too produce the dangerous,
penetrating gamma radiation), gamma-ray bursts had long puzzled astronomers.
While some last a fraction of a second, others can brighten up the sky for
several minutes.
-
- It took
until the 1990s for astronomers to find that short gamma-ray bursts are likely
caused by collisions of neutron stars, super-dense remnants of giant stars that
prior to their death were over ten times more massive than our sun.
-
- The
long-lasting bursts occur when even larger stars explode into supernovas at the
end of their lives and then turn into black holes. Both of these events emit
jets of super energetic material that illuminate the surrounding universe like
the beam of a flashlight.
-
- Satellites
orbiting Earth only detect a gamma-ray burst when this flashlight is directed
toward our part of the cosmos, but detecting a gamma-ray burst is not a rare
event. Almost every day, one flashes briefly at our planet from somewhere in
the universe. Many more are believed to take place throughout the cosmos that
go undetected because the "flashlight" is not aimed at us.
-
- But because
gamma-ray bursts are so fleeting, astronomers don't always manage to locate
their source. Only about 30% of detected
gamma-ray bursts get tracked to their origins, a problem cubesats could solve. 15-year-old “Fermi” are the current flagship
gamma-ray burst spotters. Optimized to detect super high energy gamma
radiation, which is billions of times more energetic than what human eyes can
see, these two spacecraft together detect the vast majority of gamma-ray bursts
aimed at Earth.
-
- Spacecraft,
“Swift”, just a year and a half short of its 20th birthday, is equipped to find
the source of gamma-ray bursts. Swift, however, only sees about one ninth of
the sky. And since gamma-ray bursts are distributed evenly all over the
universe, Swift only detects a small fraction of them. It misses 8 out of 9 gamma-ray bursts.
-
- Beyond a
single cubesa, a whole constellation
could make up for the shortcomings of the existing, aging gamma-ray
burst-detecting fleet. The second
advantage is that you can measure the difference in time between when the
different cubesats detect the gamma-ray burst and you can triangulate the
position of the gamma-ray burst on the sky.
-
- By
2018, a new detector that would fit on a
1U cubesat, the smallest type of cubesat measuring only 3.9 by 3.9 by 3.9
inches in size. The smallest
astrophysical observatory in the world, GRBAlpha has been orbiting Earth 340
miles above the planet's surface. The innovative detector bolted to the tiny
satellite's surface has been exceeding the team's expectations from the start.
-
- The first
burst was detected in August 2022.
Since then, “GRBAlpha” has detected 22 gamma-ray bursts. Three times larger than GRBAlpha, the Czech
Republic's VZLUSAT-2 has been orbiting Earth since January , 2022, having
scored 12 gamma-ray bursts since then.
-
- In addition
to these powerful flashes of light, colliding neutron stars (and possibly
supernova explosions too) produce another fascinating phenomenon: Gravitational
waves.
-
- First
predicted by the iconic physicist Albert Einstein in 1916, gravitational waves
are ripples in spacetime that arise from the interplay of gravitational forces
of two or more supermassive objects, such as neutron stars and black holes.
-
- Across the
universe, these objects frequently get pulled into each other's sphere of
gravitational influence and start orbiting each other. Gradually, they spiral
closer and closer to each other and eventually collide, the collision producing
a gravitational tsunami that can be detected from Earth by gravitational wave
detectors such as the American LIGO (The Laser Interferometer
Gravitational-Wave Observatory) and the European Gravitational Observatory
Virgo.
-
- For these
further runs of the gravitational wave detectors, we need a gamma-ray
burst-monitoring system that will allow us to see whether these gravitational
wave events do produce observable counterparts in gamma-ray bursts. Especially in the neutron star mergers, it is
important to see whether there was a gamma-ray burst or not because only a
small fraction of those jets that produce the gamma-ray burst should be aimed
at us.
-
- An Italian
project called “HERMES” won European Union funding in 2018 to build and launch
a constellation of six gamma-ray burst-detecting 3U cubesats (three times
larger than the tiny, pioneering GRBAlpha).
-
- The first
of these spacecraft is expected to launch in the second half of 2024. Prior to that, the HERMES project team will
fly a hosted payload on a cubesat built by the University of Melbourne,
Australia, which is set for launch at the end of 2023.
-
- The HERMES
satellites feature a detector sensitive
to gamma-rays but also the slightly less energetic X-rays. The satellites are
also equipped with a set of high-tech sensors including GPS receivers and
accelerometers that will be able to report the spacecraft's positions with an
accuracy of a few meters. And since there will be six of these satellites
astronomers will be able, for the first time, to use the minuscule time
difference in the arrival of the gamma-ray burst at the individual satellites,
to calculate the location of the burst's source.
-
- The temporal
resolution is of about 300 nanoseconds.
That is three to seven times better than any instrument flown so far for
gamma-ray burst science.
-
- NASA is also
working on a gamma-ray burst-detecting cubesat. The BurstCube, twice as big as
the HERMES satellites, is expected to launch by the end of 2023.
-
- A glowing
blob known as "the cocoon," which appears to be inside one of the
enormous gamma-ray emanations from the center of our galaxy dubbed the
"Fermi bubbles," has puzzled astronomers since it was discovered in
2012.
-
- The cocoon
is caused by gamma rays emitted by fast-spinning extreme stars called
"millisecond pulsars" located in the Sagittarius dwarf galaxy, which
orbits the Milky Way.
-
- Thankfully
for life on Earth, our atmosphere blocks gamma rays. These are particles of
light with energies more than a million times higher than the photons we detect
with our eyes.
-
- The
state-of-the-art gamma-ray instrument operating today is the Fermi Gamma Ray
Space Telescope, a large NASA mission in orbit for more than a decade. One of the surprises emerged in 2010, soon
after Fermi's launch: something in the Milky Way's center is blowing what look
like a pair of giant, gamma-ray-emitting bubbles. These completely
unanticipated "Fermi bubbles" cover fully 10% of the sky.
-
- A prime
suspect for the source of the bubbles is the galaxy's resident supermassive
black hole. This behemoth, 4 million times more massive than the sun, lurks in
the galactic nucleus, the region from which the bubbles emanate.
-
- Most
galaxies host such giant black holes in their centers. In some, these black
holes are actively gulping down matter. They simultaneously spew out giant,
outflowing "jets" visible across the electromagnetic spectrum.
-
- With time
and further data, this picture became muddied, however. While the jet-like feature
in one of the bubbles was confirmed, the apparent jet in the other seemed to
evaporate under scrutiny. The bubbles
looked strangely lopsided: one contained an elongated bright spot—the
"cocoon"—with no counterpart in the other bubble.
-
- They found
this structure has nothing to do with the Fermi bubbles or, indeed, the
galaxy's supermassive black hole.
Rather, they found the cocoon is actually something else entirely: gamma
rays from the Sagittarius dwarf galaxy, which happens to be behind the southern
bubble as seen from the position of Earth.
-
- The
Sagittarius dwarf, so called because its sky position is in the constellation
of Sagittarius, is a "satellite" galaxy orbiting the Milky Way. It is
the remnant of a much larger galaxy that the Milky Way's strong gravitational
field has literally ripped apart. Stars
pulled out of the Sagittarius dwarf can be found in "tails" that wrap
around the entire sky.
-
- What's
making the gamma rays? In the Milky Way,
the main source of gamma rays is when high-energy particles, called cosmic
rays, collide with the very tenuous gas between the stars.
-
- However,
this process cannot explain the gamma rays emitted from the Sagittarius dwarf.
It long ago lost its gas to the same gravitational that pulled away so many of
its stars.
-
- So where do
the gamma rays come from? What type of
source amongst such a population produces gamma rays? There is only one possibility: rapidly
spinning objects called "millisecond pulsars." These are the remnants
of particular stars, significantly more massive than the sun, that are also
closely orbiting another star.
-
- Under just
the right circumstances, such binary systems produce a neutron star, an object
about as heavy as the sun but only about 20 km across, that rotates hundreds of
times per second.
-
- Because of
their rapid rotation and strong magnetic field, these neutron stars act as
natural particle accelerators: they launch particles at extremely high energy
into space.
-
- These
particles then emit gamma rays. Millisecond pulsars in the Sagittarius dwarf
were the ultimate source of the mysterious cocoon. These findings shed new light on millisecond
pulsars as sources of gamma rays in other old stellar systems.
-
February 25, 2023 GAMMA RAY BURSTS - what causes them? 3888
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