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3859 - GAMMA
RAYS - the hunt for the
source? - New research shows 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.
-------------- 3859 - GAMMA RAYS - the hunt for the source?
- Gamma rays from a dwarf galaxy appear as a
glowing blob known as "the cocoon,"
Inside is one of the enormous gamma-ray emanations from the center of
our galaxy dubbed the "Fermi bubbles," was discovered in 2012.
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- Life on Earth is protected as 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. They are more powerful than x-rays.
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- Because our ground-level view is obscured,
scientists had no idea of the richness of the gamma-ray sky until instruments
were lofted into space. These gamma rays were first discoveried by the Vela
satellites put into orbit in the 1960s to monitor the Nuclear Test Ban.
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- 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. Fermi's ability to resolve fine detail and
detect faint sources has uncovered a number of surprises about our Milky Way
and the wider cosmos.
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- One of these 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
"Fermi bubbles" cover fully 10% of the sky.
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- A prime suspect for the source of the
bubbles is the galaxy's center, supermassive black hole. This black hole, 4
million times more massive than the sun, lurks in the galactic nucleus, the
region from which the bubbles emanate.
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- Most galaxies host such giant black holes in
their centers. In some, these black holes are actively gulping down matter.
Thus fed and simultaneously spew out giant, outflowing "jets" visible
across the electromagnetic spectrum.
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- Inside each bubble was a thin gamma-ray
jet pointing back towards the galactic center.
With further data, this picture became muddied. While the jet-like
feature in one of the bubbles was confirmed, the apparent jet in the other
seemed to evaporate.
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- The bubbles looked strangely lopsided: one
contained an elongated bright spot , the "cocoon", with no
counterpart in the other bubble.
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- The cocoon astronmers found has nothing to
do with the Fermi bubbles or, the
galaxy's supermassive black hole.
Rather, the cocoon is actually
something else entirely. These gamma
rays come from the Sagittarius dwarf galaxy, which happens to be behind the
southern bubble as seen from the position of Earth.
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- The Sagittarius dwarf, 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. Indeed, stars pulled out
of the Sagittarius dwarf can be found in "tails" that wrap around the
entire sky.
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- 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 force that pulled away so
many of its stars.
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- So where do the gamma rays come from? There are several possibilities, including
the exciting prospect they are a signature of dark matter, the invisible
substance known only by its gravitational effects which astronomers believe
makes up much of the universe. Unfortunately, the shape of the cocoon closely
matches the distribution of visible stars, which rules out dark matter as the
origin.
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- One way or another, the stars were
responsible for the gamma rays. And yet: the stars of the Saggitarius dwarf are
old and quiescent. What type of source amongst such a population produces gamma
rays?
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- There is is only one other 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.
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- 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.
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- 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.
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- New findings shed light on millisecond
pulsars as sources of gamma rays in other old stellar systems. At the same time, they also cast a pall over
efforts to find evidence for dark matter via observations of other satellite
galaxies of the Milky Way; unfortunately, there is a stronger
"background" of gamma rays from millisecond pulsars in these systems
than previously realized.
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- Thus, any signal they produce might not be
unambiguously interpreted as due to dark matter. The hunt for dark matter signals goes on.
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February 2, 2023 GAMMA RAYS -
the hunt for the source?
3856
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--------------------- --- Thursday, February 2, 2023 ---------------------------
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