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Christmas Gamma Ray Burst:
Naughty or Nice?
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Last
Christmas, NASA’s SWIFT satellite observed an unusual and spectacular
cosmic explosion from the direction of the constellation Andromeda.
Everyone agrees this was a gamma ray burst with peculiar
characteristics. Two competing explanations have emerged, both
involving a neutron star**. In one, a very distant neutron star smashed
into another star creating a black hole and a horrific explosion —
we’ll call that “naughty.” In the other, a nearby, innocent neutron
star was hit by a comet-sized body, releasing a modest flash — we’ll
call that “nice.”
SWIFT has dramatically increased our understanding of Gamma Ray Bursts
(GRBs.) Launched seven years ago, SWIFT rapidly scans the sky for
energetic bursts and then hones in on the source with its X-ray,
ultraviolet, and visible light telescopes. SWIFT can determine the
source location to arc-second precision and follow the decaying burst
across a wide spectrum of light frequencies.
GRBs are our universe’s most spectacular explosions and we believe
they’re related to collapsed stars, either neutron stars or black
holes. GRBs are conventionally divided into two types, long and short,
depending on their duration. Long GRBs last 20 to 40 seconds and are
attributed to the collapse of massive stars. By contrast, most short
GRBs come and go in under 1/5th of a second. A phenomenon that fast can
only originate from something less than 1/5th of a light-second across,
comparable to the planet Uranus. Merging neutron stars or black holes
are the leading candidates.
The Christmas Burst, technically named GRB 101225A, was remarkable
because it lasted 28 minutes, about 100 times longer than some long
GRBs.
The leading proponent of the “naughty” Xmas Burst story is Christina
Thoene of the Institute of Astrophysics of Andalusia in beautiful
Granada, Spain. Christina proposes a two-star system—a neutron star and
a normal star orbiting one another, held together by their mutual
gravity. When a normal star comes to the end of its life, it expands
enormously into a red giant. If its neutron star partner is close
enough, it may become enveloped by the extended, diffuse outer layers
of the red giant. This would cause a drag that would force the stars to
spiral together, creating a black hole with an immensely energetic
explosion. Based on the amount of energy reaching Earth, Christina’s
team estimates the Xmas Burst occurred 5.5 billion light-years away (33
billion, trillion miles). Indeed, her team has identified what seems
like a faint galaxy at the expected location; perhaps this galaxy
hosted this very naughty outburst.
In a field where so much remains undiscovered, there’s often another
viewpoint. Sergio Campana leads a group at Brera Observatory in Merate,
Italy that proposes a “nice” alternative. (Since I’ve been to Granada
and seen the Alhambra, perhaps I should go to Merate, just to be fair.)
Sergio proposes a much smaller explosion that is much closer to Earth.
If a large comet or small dwarf planet crashed into a neutron star only
10,000 light-years from Earth, Sergio says the blast seen here would
look the same as Christina’s immense explosion 550,000 times farther
away. Sergio suggests searching for an X-ray point source with NASA’s
Chandra satellite and searching for a pulsar with a radio telescope.
Either would support the nice neutron star version and contradict the
naughty version with the new black hole.
We said GRB’s are spectacularly energetic. Just how energetic is an
interesting question. Originally, some observers took the amount of GRB
energy they saw and assumed it was spread equally in all directions.
That made the total energy phenomenal—about 100 million, million,
million times brighter than our Sun, and equivalent to converting the
entire mass of an average star into radiation energy in just a few
seconds, with this equivalence based on Einstein’s equation E=mc2.
Instead, astrophysicists now believe the total energy in a GRB is
equivalent to “only” a thousandth of a star’s mass but that its energy
is concentrated in two narrow jets that shoot out in opposite
directions. We only see the GRBs that happen to shoot our way, and miss
the vast majority that are aimed elsewhere. When we do see a GRB, we
observe the energy in the highly concentrated jet.
 The
brightest known GRB was GRB 080319B, seen on March 19, 2008, brighter
than 10 million galaxies and exploded 7.5 billion light-years away
(shown at left). It occurred when our universe was less than half its
current age; its light took that long to reach us. For about one
minute, this object was visible to the naked eye in the constellation
Bootes.
Fortunately GRBs haven’t happened close to Earth, at least not
recently. A GRB that originated in our galaxy and was aimed our way
could kill every form of life on Earth by its combination of gamma ray,
X-ray, and UV radiation, and by depleting Earth’s ozone layer that
shields us from the Sun’s deadly rays. More distant GRBs would be much
less deadly. Some scientists estimate that GRBs harmful to life have
hit Earth about once every 5 million years, or about 1000 times since
Earth formed. They even postulate that a major extinction event 450
million years ago could have been due to a GRB that was too close for
comfort. Might want to consider lead-lined long-johns.
**Lastly,
here’s some background on neutron stars. Stars are powered by nuclear
fusion—fusing small nuclei into larger ones. When a star exhausts its
nuclear fuel, gravity crushes its core, forming a white dwarf, a
neutron star, or a black hole. The least massive stars become white
dwarfs; the most massive become black holes; and middling stars become
neutron stars. The self-gravity of a neutron star is strong enough to
convert electrons and protons into neutrons and neutrinos. The
long-wavelength electrons disappear, leaving only short-wavelength
neutrons, which pack together at an immense density. Neutron stars can
pack more than the mass of our Sun into an object only six miles
across—a teaspoon full of neutron would contain 3 billion tons of mass.
Best Regards,
Robert
Correction: in my last newsletter, I erroneously said "Caltech Professor Kip Throne", his name is Kip Thorne. Sorry Kip.
Next Newsletter: "Is the Higgs boson the God Particle or the Goddamn Particle?"
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Dr Robert
Piccioni,
Author of "Everyone's
Guide to Atoms, Einstein, and the Universe"
and " Can Life Be
Merely An Accident?"
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