Chapter 3 Flashcards
Why was an alien civilisation ruled out as an answer for the “scruff” appearing in records for pulsars?
The pulse was exceptionally regular (no modulation as if carrying a message)
Jocelyn bell found 3 more in other directions, pulsating at different rates
What was the ‘scruff’ dubbed?
Pulsars: pulsating radio sources
Timescale was too short to be associated with oscillations of a normal star
Where do the majority of pulsars lie?
Close to our galactic disc which is consistent with their being neutron stars formed in core-collapse supernovae in star forming regions
Some are at high latitudes due to a kick velocity
What can pulsars be considered to be?
Highly magnetised rotating neutron stars
What causes the magnetic field lines of the Earth to move?
Charged particles from the Sun which bombard the Earth
What happens to an exhausted star?
It is still hot and has highly conducting plasma and it becomes frozen in field lines
So magnetic flux is conserved as the stelar core shrinks to form a neutron star
What happens close to a neutron star’s surface?
A huge electric field is induced
What is the electric field cancelled out by?
Charges which are pulled off the surface, filling the surrounding magnetosphere
Where does the closed magnetosphere extend to?
A radius of v =wr which tends to c, known as the light cylinder (upper limit to the size of the magnetosphere)
What do the pulse periods show?
They are remarkably stable and have steady deceleration
Glitches have been observed, generally younger pulsars
What happens when something rotates quickly?
There is an outward centrifugal force
What happens if the centrifugal and gravitational forces are not balanced?
If centrifugal force changes and gravity stays the same, something must happen such as a breaking or shrinking of the star
What is Starquake theory?
In equilibrium, the gravitational, centrifugal and solid state forces in the crust are in balance. As the star slows down, centrifugal forces weaken and the surface shifts and cracks to find a new equilibrium.
What causes glitches?
As a young pulsar slows down, the internal structure changes and there is a sudden change in rotation frequency which leads to a glitch
What does a change in spin frequency correspond to?
A change in the moment of inertia
What does plotting dP/dt vs P show?
Pulsars are associated with a visible SNR and therefore presumably quite young, have a relatively high spin down rate
Why do we see vey few pulsars associated with SNR?
Pulsars could have moved away from their SNR as they are positioned away from galactic plane at high speeds. SNR moves in opposite direction of supernova due to angular momentum.
What happens beyond radius > c/w in a neutron star?
the magnetic field forms a spiral pattern
Charges accelerate away along open field lines and their velocity goes towards c
What causes the neutron star to slow down?
Frictional losses/magnetic braking
Coupling of B field and charged plasma leads to drag
Why is the characteristic age calculated for a pulsar sometimes greater than the actual age of it?
It may be that the spin down rate was higher in the past
What is the braking index for dipole energy loss mechanism?
n=3
On the P-Pdot diagram where are pulsars born and where do they move to?
They are born in the upper left and eventually disappear into the “graveyard” at the bottom right
What is curvature radiation?
When electrons are dragged from the polar cap and accelerated (by E = vxB) and they rapidly loose any velocity perpendicular to the field lines via synchrotron radiation. They then stream out along the curved magnetic field lines emitting curvature radiation.
(similar mechanism to synchrotron as electrons follow curved path, accelerating and therefore radiating)
What is the problem with radio telescopes?
They only detect pulsars with their narrow radio beam pointing at us so that biased our point of view
What do “gamma ray only” pulsars suggest?
That gamma rays are not produced above the neutron star’s polar cap where the radio beams form, but must come from a broader region
The pulsar’s radio beams never cross our line of sight but pulsed gamma rays do
Where are electrons accelerated?
Above the polar caps or near the outer gaps, where complex balance of magnetic and induced electric fields results in separation of positive and negative charges in the magnetosphere, leaving gaps with a huge potential difference across them.
What happens close to the pulsar?
High energy curvature radiation photons pair produce with the strong magnetic field.
Using which interactions, can electrons generate gamma rays?
Vis curvature radiation or inverse Compton scattering of low energy photons such as thermal X-rays.
What are magnetars?
Pulsars with unusually long spin periods and very large spin down rates
Kinetic energy is transformed into magnetic field
What is the size of the magnetic flux density in magnetars?
It has a large magnetic flux density much higher than normal. Internal magnetic field is generated in addition to compressed stellar field
What do magnetar quakes cause?
Giant gamma ray flares
What is a fireball?
Neutron star curst rearranges itself it drives electric currents which energise particles trapped inside the magnetic field lines above it. The magnetic field rearranges itself and releases a dense cloud of electrons positrons and photons (fireball)
The photons slowly leak out
What indicates that millisecond pulsars are in binary systems?
When a spectrum is taken from around planets, it shows absorption lines in a star that are doppler shifted (blue towards you and red away from you)
If it is moving towards you, pulsars would be moving with high frequency and away from you at frequency
Change in pulse period is doppler shifted and this shows that they are in a binary system.
