Transcript Gamma Ray Bursts

Gamma Ray Bursts
A High Energy Mystery
By Tessa Vernstrom
Ast 4001, Fall 2007
Briefly: What are They?
Gamma Ray Bursts, or GRBs, are short lived bursts of high
energy gamma photons. They have wavelengths ranging
from .03nm to .003nm.There are two different categories
of GRBs: long duration and short duration. It is believed
these different types come from different sources.
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Discovery
GRBs were discovered by the US Vela
Nuclear Test Detection Satellites in the
1960s. These were built to detect gamma
rays coming from nuclear weapon tests in
space. The satellites did detect flashes of
radiation that were not from nuclear
explosions seeming to come from random
directions in space. As recent as the
1990’s scientists didn’t know where these
flashes were coming from or what was
causing them .
Long Duration Bursts
Last anywhere from 2 seconds
to a few hundred seconds, the
average is about 30 seconds.
Come from stars billions of
light years away.
Evenly distributed all over the
sky.
 Believed to come from a
hypernova, and Wolf-Rayet
stars.
•They can vary greatly in duration, counts per
second, and other properties.
Wolf-Rayet Stars
 Mass at least 20 solar Masses
Temperature around 45,000 degrees
Fahrenheit.
Expel heated gas in bursts and have very
strong winds.
 Very short life spans, typically about a few
million years.
During its death throws can undergo uneven
gravitational collapse, causing gamma rays
bursts.
Turns into a black hole.
Example is Eta Carinea
Hypernova
 The death of a super massive star
 Has mass 50 to 150 solar masses
 The core of the hypernova collapses directly into a black hole
and two extremely energetic jets of plasma are emitted from its
rotational poles at nearly the speed of light. These jets emit
intense gamma rays, and are a candidate explanation for
gamma ray bursts.
 About 1 out of every 100 supernova is a
hypernova
Here are some animations to illustrate these points
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After Glow
 The emission of the jets from the
collapsing star excite nearby gas and
dust.
 This gives off photons and allows us
to see the phenomenon in other
wavelengths such as x-rays, visible
light, and radio waves.
 Afterglow can last for days or weeks.
 Gives credence to the super or
“hyper” nova hypothesis.
Short GRBs
 A GRB lasting less than 2 seconds,
with an average of 300
milliseconds.
 Significantly dimmer than long
GRBs and have higher energies.
 Explanation not as well understood.
Several possible explanations.
 Until recently were “dark” GRBs,
not giving off a detectable
afterglow.
Short GRBs Cont
 Scientists think it could come from the
collisions of two very dense objects, such as
neutron stars in a binary.
 Also, some think, they could be from the
same process as the long GRB but were not
directly along the axis of the emission, i.e.
not seeing it face on.
Why Study Them
 Tells us about the evolution/death of
high mass star or exotic stars.
 Tells us about the early universe
 Because so much is still unknown may
discover new stellar properties
 Mass extinctions
Still To know
 The means by which gamma-ray bursts convert energy into
radiation remains poorly understood, and as of 2007 there
is still no generally accepted model for how this process
occurs.
 A successful model of GRBs must explain not only the
energy source, but also the physical process for generating
an emission of gamma rays which matches the durations,
light spectra, and other characteristics of observed GRBs.
References
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http://imagine.gsfc.nasa.gov/docs/science/know_l1/bursts.html
http://en.wikipedia.org/wiki/Gamma_ray_burst
http://www.astro.psu.edu/users/nnp/grbphys.html
http://grb.sonoma.edu/
http://science.hq.nasa.gov/kids/imagers/ems/gamma.html
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http://cfa-www.harvard.edu/~pberlind/atlas/htmls/wrstars.html
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Kouveliotou, C. et al. (1993). "Identification of two classes of
gamma-ray bursts". Astrophysical Journal 413: L101
 Sari,Re'em; Piran,Tsvi; Halpern,JP. ”Jets in Gamma Ray Bursts”.
Astrophysical Journal, Volume 519, Issue 1, pp. L17-L20.