Chapter 3

Question 1

Why was an alien civilisation ruled out as an answer for the “scruff” appearing in records for pulsars?

Answer 1

The pulse was exceptionally regular (no modulation as if carrying a message)

Jocelyn bell found 3 more in other directions, pulsating at different rates

Question 2

What was the ‘scruff’ dubbed?

Answer 2

Pulsars: pulsating radio sources

Timescale was too short to be associated with oscillations of a normal star

Question 3

Where do the majority of pulsars lie?

Answer 3

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

Question 4

What can pulsars be considered to be?

Answer 4

Highly magnetised rotating neutron stars

Question 5

What causes the magnetic field lines of the Earth to move?

Answer 5

Charged particles from the Sun which bombard the Earth

Question 6

What happens to an exhausted star?

Answer 6

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

Question 7

What happens close to a neutron star’s surface?

Answer 7

A huge electric field is induced

Question 8

What is the electric field cancelled out by?

Answer 8

Charges which are pulled off the surface, filling the surrounding magnetosphere

Question 9

Where does the closed magnetosphere extend to?

Answer 9

A radius of v =wr which tends to c, known as the light cylinder (upper limit to the size of the magnetosphere)

Question 10

What do the pulse periods show?

Answer 10

They are remarkably stable and have steady deceleration

Glitches have been observed, generally younger pulsars

Question 11

What happens when something rotates quickly?

Answer 11

There is an outward centrifugal force

Question 12

What happens if the centrifugal and gravitational forces are not balanced?

Answer 12

If centrifugal force changes and gravity stays the same, something must happen such as a breaking or shrinking of the star

Question 13

What is Starquake theory?

Answer 13

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.

Question 14

What causes glitches?

Answer 14

As a young pulsar slows down, the internal structure changes and there is a sudden change in rotation frequency which leads to a glitch

Question 15

What does a change in spin frequency correspond to?

Answer 15

A change in the moment of inertia

Question 16

What does plotting dP/dt vs P show?

Answer 16

Pulsars are associated with a visible SNR and therefore presumably quite young, have a relatively high spin down rate

Question 17

Why do we see vey few pulsars associated with SNR?

Answer 17

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.

Question 18

What happens beyond radius > c/w in a neutron star?

Answer 18

the magnetic field forms a spiral pattern

Charges accelerate away along open field lines and their velocity goes towards c

Question 19

What causes the neutron star to slow down?

Answer 19

Frictional losses/magnetic braking

Coupling of B field and charged plasma leads to drag

Question 20

Why is the characteristic age calculated for a pulsar sometimes greater than the actual age of it?

Answer 20

It may be that the spin down rate was higher in the past

Question 21

What is the braking index for dipole energy loss mechanism?

Answer 21

n=3

Question 22

On the P-Pdot diagram where are pulsars born and where do they move to?

Answer 22

They are born in the upper left and eventually disappear into the “graveyard” at the bottom right

Question 23

What is curvature radiation?

Answer 23

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)

Question 24

What is the problem with radio telescopes?

Answer 24

They only detect pulsars with their narrow radio beam pointing at us so that biased our point of view

Question 25

What do “gamma ray only” pulsars suggest?

Answer 25

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

Question 26

Where are electrons accelerated?

Answer 26

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.

Question 27

What happens close to the pulsar?

Answer 27

High energy curvature radiation photons pair produce with the strong magnetic field.

Question 28

Using which interactions, can electrons generate gamma rays?

Answer 28

Vis curvature radiation or inverse Compton scattering of low energy photons such as thermal X-rays.

Question 29

What are magnetars?

Answer 29

Pulsars with unusually long spin periods and very large spin down rates

Kinetic energy is transformed into magnetic field

Question 30

What is the size of the magnetic flux density in magnetars?

Answer 30

It has a large magnetic flux density much higher than normal. Internal magnetic field is generated in addition to compressed stellar field

Question 31

What do magnetar quakes cause?

Answer 31

Giant gamma ray flares

Question 32

What is a fireball?

Answer 32

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

Question 33

What indicates that millisecond pulsars are in binary systems?

Answer 33

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.