Transcript Gamma Ray Bursts (GRB)

Gamma Ray Bursts (GRB)
Compton Gamma
Ray Observatory
(CGRO)
Gamma Ray Bursts (GRB)
Gamma Rays – the highest energy photons in the electromagnetic
spectrum
Gamma-ray bursts - short-lived bursts of gamma ray photons, the
most energetic form of light. At least some of them are associated with
a special type of supernova, the explosions marking the deaths of
especially massive stars.
Gamma Ray Bursts (GRB)
Lasting anywhere from a few milliseconds to several minutes, gamma-ray
bursts shine hundreds of times brighter than a typical supernova and about a
million trillion times as bright as the Sun, making them briefly the brightest
source of cosmic gamma-ray photons in the observable universe. GRBs are
detected roughly once per day from wholly random directions of the sky.
Gamma Ray Bursts (GRB)
No two gammaray bursts are the
same, as can be
seen from this
sample of light
curves.
Gamma Ray Bursts (GRB)
An Interesting Start
Gamma Ray bursts were discovered serendipitously in the late 1960s by
U.S. military satellites which were on the look out for Soviet nuclear testing
in violation of the atmospheric nuclear test ban treaty. These satellites
carried gamma ray detectors since a nuclear explosion produces gamma
rays.
Gamma Ray Bursts (GRB)
As recently as the early 1990s, astronomers didn't
even know if GRBs originated at the edge of our
solar system, in our Milky Way Galaxy or incredibly
far away near the edge of the observable Universe.
Gamma Ray Bursts (GRB)
Once discovered, GRBs were
observed to be distributed
uniformly over the sky. If they
occurred within the galaxy, they
should primarily occur in the
Milky Way disk (where most of
the stars in our galaxy are
found).
Either
GRBs occurred outside of the
galaxy
or
there is a “halo” around the
Milky Way.
Gamma Ray Bursts (GRB)
If the GRBs were far away, they would have large
Doppler shifts. However, with a Gamma Ray
wavelength of on the order of 10-15, shifts are too
small to measure
Then…
Gamma Ray Bursts (GRB)
In 1997 the Dutch-Italian gamma-ray satellite Bepposax monitored a gamma-ray
burst. Soon after, scientists could look with the Keck-telescope at the found
spot. And thus the first spectrum of a fast-fading optical counterpart was
obtained.
The redshift of this spectrum places the gamma-ray
burst at a distance of a few Gigaparsecs.
This is in galaxies far, far away….
Gamma Ray Bursts (GRB)
The first optical images, taken with
the Herschel-telescope, of the
gamma-ray burst that was
discovered at february 28 of 1997.
At the top picture one can see the
gamma-ray burst. At the bottom
picture, taken eight days later, one
can see the burst is fading away.
(Gamma-ray bursts; G.J.Fishman
D.H.Hartmann; Scientific
American; March 1998)
Gamma Ray Bursts (GRB)
Long or Short Duration?
Gamma Ray Bursts are separated into two classes: long-duration bursts and shortduration bursts.
Long duration bursts – Gamma Ray Bursts that last more than 2 seconds.
Short-duration bursts – Gamma Ray Bursts that last less than 2 seconds.
Short duration bursts range from a few milliseconds to 2 seconds with an
average duration time of about 0.3 seconds (300 milliseconds). The longduration bursts last anywhere from 2 seconds to a few hundreds of seconds
(several minutes) with an average duration time of about 30 seconds.
Astronomers think that long and short duration GRBs are created by fundamentally
different physical properties. And whereas they now are fairly confident of what
drives the long GRBs, there are only theories when it comes to what drives shortduration bursts.
Gamma Ray Bursts (GRB)
Model for Long Duration Bursts
Collapsar Model – The core of a massive star rapidly collapses into a
black hole. In doing so, a shock wave is created in the same way as in
a supernova, but the amount of gravitational energy is very large near
the event horizon. An especially violent supernova occurs, shedding
high energy photons (gamma rays).
Hypernova – the especially violent supernova associated with gamma
bursts.
Gamma Ray Bursts (GRB)
Model for Short Duration Bursts
Binary Neutron Star Model – Gamma bursts created by the collision
and coalescence of two neutron stars. Energy in the form of gamma
rays is created for relatively short time periods in the collision process.
Gamma Ray Bursts (GRB)
Swift GRB Mission
Latest NASA mission launched in November 2004. It is
studying GRBs with more sensitivity than other
instruments.
Gamma Ray Bursts
Sprites – Curious (non astronomical gamma rays)
Sprites are created
during lightning storms
on earth. The are
vertical ejections from
the atmosphere, and
have been detected by
satellites.
Sprites are generators
or gamma rays
Gamma Ray Bursts (GRB)
Gamma Ray Bursts (GRB)