Transcript Gamma-Ray Bursts

Gamma-Ray Bursts
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Which wavelength range
probes the most violent
events in the Universe?
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Radio Waves
Gamma-Rays
X-Rays
Visible Light
Infrared Light
Gamma-Ray Bursts
Short (~0.1 – 100 seconds) flashes of gamma-rays
from random directions in the sky
Frequency, Energy
Wavelength l
Mysterious Flashes of
Gamma-Rays
Discovered in 1967:
Brighter than any other gammaray source in the sky, for a few
seconds - minutes
Gamma-Ray Burst Missions
Burst and Transient
Search Experiment
(BATSE) on the Compton
Gamma-Ray Observatory:
1991 - 2000
Two Types of Gamma-Ray Bursts
Long GRBs
(duration > 2 s)
Short GRBs
(duration < 1 s)
Brightness
GRB Light Curves
Time
Long GRBs (duration > 2 s)
Short GRBs (duration < 1 s)
Two different types of GRBs: Long and short bursts
Galactic Latitude
General Properties of GRBs
Galactic Plane
Galactic Longitude
• Random distribution in the sky
• Approx. 1 GRB per day observed
• No repeating: Only one single burst, and it’s gone!
BeppoSAX
(1996 – 2003)
First
identification of
X-ray and
optical
counterparts
(afterglows) of
gamma-ray
bursts in 1997
Afterglows of GRBs
On the day of the GRB
3 days after the GRB
X-ray afterglow of GRB 970228
(GRBs are named by their date: Feb. 28, 1997)
Most GRBs have gradually decaying afterglows in X-rays,
some also in optical and radio.
1 day after GRB
2 days after GRB
Optical afterglow of GRB 990510 (May 10, 1999)
Optical afterglows of GRBs are
extremely difficult to localize:
Very faint (~ 18 – 20 mag.);
decaying within a few days.
A GRB Afterglow Observed at the
MDM Observatory
If an optical afterglow of a GRB has an
apparent visual magnitude of 19, how
many times fainter is it than the faintest
object visible to the bare eye?
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13
13*2.512 ≈ 37
(2.512)13 ≈ 158,583
(2.512)19 ≈ 39,844,930
10013 = 1026
Optical Afterglows of GRBs
Host Galaxy
Optical Afterglow
Optical afterglow of GRB 990123 (Hubble Space
Telescope)
GRBs are found in
spiral arms of
distant host galaxies
(half-way across the
Universe!)
Energy Output of GRBs
Observed brightness
combined with large
distance implies huge
energy output of GRBs
10,000,000,000,000,000,000,000,000,000
(1028) Hydrogen bombs!
Energy equivalent to the
entire mass of the sun
(E = mc2), converted
into gamma-rays in just
a few seconds!
Beaming
GRBs are most
likely not
emitting
isotropically
(i.e. with the
same intensity
in all
directions), but
are beamed.
If GRBs are actually beamed, then the
total energy that they emit is …
compared to isotropic emission (i.e.
equal energy output in all directions).
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smaller
equal
larger
Depends on the duration of the GRB.
Depends on the photon frequency at which
most of the energy is radiated away.
GRBs are actually emitting less energy
than implied for isotropic emission.
If we assume isotropic
emission, the GRB
would have to emit the
same energy in all
directions.
For beamed GRBs, it’s
only along the beams
=> Less total energy
If GRBs are actually beamed, then the total number
of GRBs occurring in the Universe is … compared
to what we would infer for isotropic emission.
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smaller
equal
larger
Depends on the duration of the GRB.
Depends on the photon frequency at
which most of the energy is radiated
away.
There must be more GRB sources than
implied for isotropic emission.
If GRBs emitted
isotropically, we would
basically observe all
GRBs, irrespective of
their orientation.
For beamed GRBs, we
only observe those
GRBs which are
beamed towards us
=> There must be more
sources that we don’t
see as GRBs.
Current Gamma-Ray Burst Missions
Swift Gamma-Ray Burst Explorer:
Launched 2004
• Gamma-Ray Burst Monitor
• X-ray Telescope
• Optical/UV telescope
The Swift Gamma-Ray Burst Mission
Current Gamma-Ray Burst Missions
Fermi Gamma-Ray
Space Telescope
(Launched August 2008):
GLAST Burst monitor
(GBM)
Multi-Wavelength View of
Gamma-Ray Bursts
Gamma-Rays
Visible
X-Rays
How do massive stars (> 8 solar
masses) end their lives?
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The core collapses to a white dwarf, and the rest of the
star explodes in a supernova explosion.
The core collapses into a neutron star, and the rest of
the star explodes in a supernova explosion.
The core collapses into a white dwarf, and the rest of
the star explodes to form a planetary nebula.
The core collapses into a neutron star, and the rest of
the star explodes to form a planetary nebula.
The gradually use up their Hydrogen fuel and never
ignite Helium burning.
Origin of GRBs
There’s no consensus about what causes GRBs.
Several models have been suggested, e.g.:
Hypernova:
Supernova explosion of a
very massive (> 25 Msun)
star
Iron core collapse
forming a black hole;
Material from the outer
shells accreting onto
the black hole
Accretion disk =>
Jets => GRB!
Origin of GRBs
Black-hole – neutron-star merger:
Black hole and neutron star (or
2 neutron stars) orbiting each
other in a binary system
Neutron star will be
destroyed by tidal effects;
neutron star matter accretes
onto black hole
=> Accretion disk
=> Jets => GRB!
Model works probably
only for short GRBs.
Origin of Short GRBs
Black-hole – neutron-star merger
Origin of Short GRBs
Neutron-star – neutron-star merger