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Transcript
Chapter 13: Gamma Ray Burst,
Neutron Stars, and Black Holes
An early gamma ray-burst
Vela satellite
A Gamma Ray Burst Sampler
Great debate: 1967-1997
Bepposax Satellite
GRBM:
40-600 keV
WFC: 2-30
keV
NFI: 2-10
keV
X-Ray Afterglow from GRB 971214
t=6.5 hrs
t=12.5 hrs
t=54 hrs
Optical Afterglow from GRB 971214
Keck
Images
2 days
2 months
Host
HST Image
Uh-oh
GRBs are a type of Supernova
“Hypernova”
●
Peak toward low end of gamma-ray, complex gamma-ray light curves
●
Often have bright afterglows
●
Evidence for a relativistic explosion
●
Energy required of ~ 1053 ergs (isotropic)
●
Associated with regions of star formation in distant galaxies (out to
edge of observable universe)
●
Sometimes obscured by dust
●
Plus …
Example Hypernova: 1998bw
Long GRBs clearly connected to Supernovae
Hjorth et al 2003
WR104 - Looking Down the Barrel
of a GRB system 8000 lt-years from us
Clicker Question:
All of the following atoms have a total of 4
nucleons (protons or neutrons). Which of the
following has the smallest mass?
A: 4 hydrogen atoms
B: 2 deuterium atoms
C: 1 tritium atom and 1 hydrogen atom
D: 1 Helium atom
E: None of the above, they all have the same total mass
Final States of a Star
1. White Dwarf
If initial star mass < 8 MSun or so.
(and remember: Maximum WD mass is 1.4 MSun , radius is
about that of the Earth)
2. Neutron Star
If initial mass > 8 MSun and < 25 MSun .
3. Black Hole
If initial mass > 25 MSun .
Pulsars
Discovery of LGM1 by Jocelyn Bell and Tony Hewish (Cambridge)
in 1967. Nobel Prize to Hewish in 1974.
Pulse periods observed from 0.001 sec to 10 seconds - DEMO
Explanation: "beamed" radiation from rapidly spinning neutron star.
Usually neutron stars are pulsars for 107 years after supernova.
The Crab Pulsar
The Crab Pulsar
Chandra
X-ray
image
On
Off
Neutron Stars
Leftover core from Type II supernova
- a tightly packed ball of neutrons.
Diameter: 20 km only!
Mass: 1.4 - 3(?) MSun
Density: 1014 g / cm3 !
1 teaspoon = 1000 great pyramids
Surface gravity: 1012 higher
Escape velocity: 0.5c
Rotation rate: few to many times
per second!!!
Magnetic field: 1012 x Earth's!
A neutron star over the Sandias?
General Relativistic deflection of light
More than half the
surface is visible at any
one time!
Each square is 30 degrees x 30 degrees
An Isolated Neutron Star
T ~ 2 million K
Size ~ 30 km
The Lighthouse model of a pulsar
Pulsars are incredibly accurate clocks!
Example: period of the first discovered "millisecond pulsar" is:
P = 0.00155780644887275 sec
It is slowing down at a rate of
1.051054 x 10 -19 sec/sec
The slowing-down rate is slowing down at a
rate of:
0.98 x 10 -31 /sec
Multi-wavelength observations of Pulsars
Pulsar Exotica
Binary pulsars: two pulsars in orbit around
each other.
Einstein predicted that binary orbits should
"decay", i.e. the masses would spiral in
towards each other, losing energy through
"gravitational radiation". Confirmed by
binary pulsar.
Curve: prediction of
decaying orbit. Points:
measurements.
Planets around pulsars: A pulsar was found in 1992 to
have three planets! Masses about 3 MEarth, 1 MEarth, and
1 MMoon !
year
Millisecond pulsars: periods of 1 to a few msec. Probably accreted
matter from a binary companion that made it spin faster.
Gamma-ray Bursts: some pulsars produce bursts of gamma-rays,
called Soft Gamma-Ray Repeaters or SGRs
Time history of the 4 confirmed SGRs:
Woods & Thompson 2004
Soft Gamma-Ray Repeaters
"
Eiso ~ a few1044 erg in gamma-rays
Where does this energy come
from?
X-ray image
- Accretion? No sign of a disk
- Rotation? Not enough energy available
- Magnetic fields? Yes
Clicker Question:
What is our basic model for a pulsar?
A: a rotating white dwarf
B: a rotating neutron star
C: a rotating black hole
D: an oscillating star
NS Merger Model for short GRBs
Mean redshift ~ 0.25 for short hard bursts (SHB)
No supernova association expected
SHBs often found at outskirts of galaxy (implies large
peculiar velocities)
SHBs found in
- Elliptical galaxies
- galaxies with low star
formation rates
Black Holes
A stellar mass black hole accreting material from a companion star
29