- 3830 - GAMMA RAYS - where and what causes them? Astronomers hope 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.
--------- 3830 - GAMMA RAYS - where and what causes them?
- 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. While our results
clear up the mystery of the cocoon, they also cast a pall over attempts to
search for dark matter in any gamma-ray glow it may emit.
-
- 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.
-
- 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. But, starting with the discoveries made by the “Vela
satellites” (put into orbit in the 1960s to monitor the Nuclear Test Ban), more
and more of this richness has been revealed.
-
- 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.
-
- 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
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 black hole is 4
million times more massive than the sun, lurking 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.
Thus fed, they simultaneously spew out giant, outflowing "jets"
visible across the electromagnetic spectrum.
-
- The bubbles looked strangely lopsided: one
contained an elongated bright spot—the "cocoon"—with no counterpart
in the other bubble. Astronmers 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.
-
- 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 harassment that pulled away
so many of its stars.
-
- Are they 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.
-
- 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?
-
- We are satisfied 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.
-
- The hunt for dark matter signals goes
on. Astronomy gives a solution and three
more problems to solve to replace it.
-
January 17, 2022 GAMMA RAYS
- where and what causes them? 3830
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--------------------- --- Wednesday, January 18, 2023 ---------------------------
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