Transcript PHYS3380_111815_bw - The University of Texas at Dallas

PHYS 3380 - Astronomy
I will not be here the week of Nov 30.
Dr Russell Stoneback will be teaching the class
PHYS 3380 - Astronomy
Pulsar Periods
Pulsar energy generated by
rotation - as it blows away
pulsar wind and blasts
radiation outward, it slows
down.
So, over time, pulsars lose
energy and angular
momentum Pulsar rotation gradually
slows down
Oldest about 10 million years
Glitches consequences of angular momentum transfer between a solid crust,
which rotates at the measured pulsar periodicity, and a more rapidly rotating
"loose' component of the neutron star interior. Possibly caused by
“starquakes” or vortices in fluid (neutron) interior.
PHYS 3380 - Astronomy
The Crab Pulsar
Pulsar wind + jets
Remnant of a supernova observed in A.D. 1054
PHYS 3380 - Astronomy
Pulsar Wind
Combination of rapid rotating and strong
magnetic field generate jets of matter and
anti-matter moving away from the north
and south poles and an intense wind
flowing out in the equatorial direction carry 99.9% of energy released from
slowing down of pulsar rotation rate.
Chandra X-ray Image of Crab Nebula
Inner X-ray ring thought to be shock wave
marking boundary between surrounding
nebula and the pulsar wind. Energetic
electrons and positrons move outward
from this ring to brighten the outer ring
and produce an extended X-ray glow.
Fingers, loops, and bays indicate that magnetic field of the nebula and filaments of
cooler matter are controlling the motion of the electrons and positrons. The particles
can move rapidly along the magnetic field and travel several light years before
radiating away their energy - move much more slowly perpendicular to the magnetic
field, and travel only a short distance before losing their energy.
This effect can explain the long, thin, fingers and loops, as well as the sharp
boundaries of the bays. The conspicuous dark bays on the lower right and left are
likely due to the effects of a toroidal magnetic field - a relic of the progenitor star.
PHYS 3380 - Astronomy
Composite X-ray (Chandra - left) and visible (Hubble) movie
PHYS 3380 - Astronomy
Proper Motion of Neutron Stars
Some neutron stars are moving
rapidly through interstellar space
- might be a result of anisotropies
during the supernova explosion
forming the neutron star
Composite X-ray (red/white) and optical (green/blue)
image of Black Widow Pulsar - shows elongated cloud,
or cocoon, of high-energy particles flowing behind the
rapidly rotating pulsar moving at a speed of almost a
million kilometers per hour. Bow shock wave due to this
motion optically visible - the greenish crescent shape.
Pressure behind the bow shock creates a second shock
wave that sweeps the cloud of high-energy particles
back from the pulsar to form the cocoon.
PHYS 3380 - Astronomy
The vela Pulsar
moving through
interstellar space
A recent change appears to
be connected to the
occurrence of a glitch
rotation speed, which
presumably released a burst
of energy that was carried
outward at near the speed of
light by the pulsar wind.
PHYS 3380 - Astronomy
Magnetars
Neutron stars with magnetic fields ~ 1000 times stronger than normal neutron
stars - 21 currently known. Much more massive than regular neutron stars.
Earthquake-like ruptures in the surface
crust of Magnetars cause bursts of soft
gamma-rays. Magnetars fizzle out in less
than 100,000 years, rendering them all but
undetectable - astronomers suspect that the
Milky Way might be littered with dead
magnetars.
On 27 December, 2004, a burst of
gamma rays arrived in our solar system
from SGR 1806-20 (artist's conception).
The burst was so powerful that it had
effects on Earth's atmosphere, at a
range of over 50,000 light years.
PHYS 3380 - Astronomy
Image of thin, glowing dust ring
around a magnetar. So thin it's almost
two-dimensional, and emits no
radiation other than a faint, infrared
glow. Probably formed after the
magnetic star emitted a giant flare,
spotted in 1998, which incinerated
surrounding dust in all directions,
leaving only the thin disk. The disk
glows from the heat emitted by nearby
massive stars, which the are probably
relatives of the magnetar's forebearer.
Researchers say they hope to nail the
original mass of SGR 1900+14 by
determining the masses of those
relatives, and the resulting data could
help them work out how heavy a star
needs to be to become a magnetar
rather than a neutron star.
Collision of two magnetars may
generate gravity waves - undeteced
so for
PHYS 3380 - Astronomy
Binary Pulsars
Some pulsars form binaries with other neutron stars (or black holes).
Binary Pulsars
Radial velocities resulting from the orbital motion
lengthen the pulsar period when the pulsar is moving
away from Earth … and shorten the pulsar period
when it is approaching Earth.
Similar to method of extrasolar planet discovery
First one discovered in 2004 - only 20km across and
have an orbital separation which is less than the size
of the Sun.
Already, four different effects have been measured consistent with Einstein's
general theory of relativity.
PHYS 3380 - Astronomy
Neutron Stars in Binary Systems: X-ray Binaries
Example: Her X-1
2 Msun (F-type) star
Neutron star
Orbital period =
1.7 days
Accretion disk material heats to several
million K => X-ray emission
Star eclipses neutron
star and accretion
disk periodically
PHYS 3380 - Astronomy
Pulsar Planets
Some pulsars have
planets orbiting
around them.
Just like in binary pulsars,
this can be discovered
through variations of the
pulsar period.
As the planets orbit
around the pulsar, they
cause it to wobble
around, resulting in
slight changes of the
observed pulsar period.
PHYS 3380 - Astronomy
Black Holes
Just like white dwarfs (Chandrasekhar limit: 1.4 M), there is a
mass limit for neutron stars (neutron degeneracy):
Neutron stars cannot exist with masses
> 3 M
We know of no mechanism to halt the collapse of a
compact object with > 3 M.
It will collapse into a single point – a singularity:
=> A Black Hole!
PHYS 3380 - Astronomy
Black Holes
Black holes are completely
collapsed objects - radius of the
“star” becomes so small that the
escape velocity approaches the
speed of light
Escape velocity for particle from
an object of mass M and radius R
2GM
v esc 
R
If photons cannot escape, then
vesc>c. Schwarzschild radius is

2GM
M
R  RS  2
 3 km
c
M Sol
Nothing (not even light) can escape from inside the Schwarzschild radius
- we have no way of finding out what’s happening inside the
Schwarzschild radius - the “event horizon”
PHYS 3380 - Astronomy
Size of black holes determined by mass. Example Schwarzschild
radius for various masses given by:
Object
M (M)
Rs
Star
10
30 km
Star
3
9 km
Sun
1
3 km
Earth
3x10-6
9 mm
The event horizon is located at
Rs - everything within the event
horizon is lost. The event
horizon hides the singularity
from the outside Universe.
If the entire mass of the Earth was confined to 9mm, it would be a
black hole - can’t collapse spontaneously into black hole because
mass < 3 M
PHYS 3380 - Astronomy
Black Holes in Supernova Remnants
Some supernova
remnants with no
pulsar / neutron star in
the center may contain
black holes.
PHYS 3380 - Astronomy
“Black Holes Have No Hair”
Matter forming a black hole is losing almost all
of its properties.
Black Holes are completely determined
by 3 quantities:
Mass
Angular Momentum
(Electric Charge)
PHYS 3380 - Astronomy
Types of Black Holes
Schwarzschild - Non-rotating black hole
-simplest black hole, in which the core does not rotate
- only has a singularity and an event horizon
Kerr - Rotating black hole
-probably the most common form in
nature,
- rotates because the star from which
it was formed was rotating. When the
rotating star collapses, the core
continues to rotate, and this carried
over to the black hole (conservation of
angular momentum).
-Has an Ergosphere - egg-shaped
region of distorted space around the
event horizon
-caused by the spinning of the
black hole, which "drags" the
space around it.)
-Static limit - The boundary between
the ergosphere and normal space
PHYS 3380 - Astronomy
Black Hole Gravity Well
At a distance, the
gravitational fields of a black
hole and a star of the same
mass are virtually identical.
At small distances, the
much deeper gravitational
potential will become
noticeable.
PHYS 3380 - Astronomy
General Relativity Effects Near Black Holes
An astronaut descending down
towards the event horizon of the
BH will be stretched vertically
(tidal effects) and squeezed
laterally - friction would heat the
astronaut to millions of degrees
emitting x-rays and gamma rays.
“Spaghettification”
PHYS 3380 - Astronomy
General Relativity Effects Near Black Holes
Time dilation
Clocks starting at 12:00
at each point.
After 3 hours (for an
observer far away from
the BH):
Clocks closer to the BH
run more slowly.
Time dilation becomes
infinite at the event horizon.
Event Horizon
PHYS 3380 - Astronomy
General Relativity Effects Near Black Holes
Gravitational Red Shift
All wavelengths of emissions from
near the event horizon are stretched
(red shifted).
 Frequencies are lowered.
Event Horizon
PHYS 3380 - Astronomy
Remember: General Theory of Relativity predicted gravity could bend space confirmed during a solar eclipse when a star's position was measured before,
during and after the eclipse.
An object with immense gravity (like a galaxy or black hole) between the Earth and
a distant object could bend the light from the distant object into a focus, much like a
lens can.
Einstein ring -the deformation of the light from a source into a ring through gravitational
lensing of the source's light by an object with an extremely large mass (such as
another galaxy, or a black hole).
PHYS 3380 - Astronomy
Lensing by a Black Hole
Animated simulation of gravitational lensing caused by a going past a
background galaxy.
- secondary image of the galaxy can be seen within the black hole
Einstein ring on the opposite direction of that of the galaxy.
- secondary image grows (remaining within the Einstein ring) as the
primary image approaches the black hole.
- surface brightness of the two images remain constant, but their
angular size varies
- produces an amplification of the galaxy luminosity as seen from a
distant observer. The maximum amplification occurs when the
background galaxy (or in the present case a bright part of it) is
exactly behind the black hole.
PHYS 3380 - Astronomy
Brightening of MACHO-96-BL5 happened when a gravitational lens passed between
it and the Earth. When Hubble looked at it, it saw two images of the object close
together - indicated a gravitational lens effect - intervening object was unseen.
- conclusion that a black hole had passed between Earth and the object.
PHYS 3380 - Astronomy
Stephen Hawking showed that black holes are not entirely black but emit small
amounts of thermal radiation.
- applied quantum field theory in a static black hole background.
- result - a black hole should emit particles in a perfect black body spectrum.
- Hawking radiation.
Virtual particle pairs constantly created near the horizon of the black hole, as they
are everywhere - quantum fluctuations. Normally, they are created as a particleantiparticle pair and they quickly annihilate each other. But near the horizon of a
black hole, it's possible for one to fall in before the annihilation can happen, in which
case the other one escapes as Hawking radiation.
- removes energy from black hole - evaporation
Temperature of the emitted black body spectrum is proportional to the surface
gravity of the black hole.
- large black holes are very cold and emit very little radiation
- black hole of 10 solar masses would have a Hawking temperature of
several nanokelvin, much less than the 2.7K produced by the Cosmic
Microwave Background.
- micro black holes on the other hand could be quite bright producing high
energy gamma rays.