Chapter 4

Question 1

What does a sudden drop in brightness of a source suggest?

Answer 1

That the source is eclipsed

Question 2

What suggests a “spin up”?

Answer 2

A long term trend towards earlier pulse arrival times

Question 3

What was recorded when 2 x-ray sources were detected and what does this suggest?

Answer 3

Pulsations and periodicity of pulsars and periodicity in the change of counts

It is consistent with binaries

Question 4

What happens if a star is moving quickly?

Answer 4

The centrifugal force will destroy the star

Question 5

What is the condition for finding the density of a star associated with an X-ray emission?

Answer 5

Gravitational force must exceed centrifugal force

Question 6

What does a doppler shift of spectral lines show?

Answer 6

A measure of projected orbital velocity along our line of sight

Question 7

How does the mass function change if the sun and a planet of significantly lower mass are compared?

Answer 7

Only the mass of the Sun is considered

Question 8

How can we infer the size of the companion in binary system?

Answer 8

From its spectral type

In HR diagram all objects with the same spectral type in the same location in HR diagram have the same mass

Question 9

What is an X-ray binary taken to mean?

Answer 9

A neutron star with a non-degenerate companion

Question 10

Why is there an upper limit on the mass of a neutron star?

Answer 10

The degenerate electrons are fighting gravity but there is a limit to how much they can counteract this gravity (3.2 solar masses) and above this gravity wins and the star collapses

Question 11

In the binary of a radio pulsar, what showed consistency with emissions of gravitational waves?

Answer 11

The orbit was shrinking by 1cm per day and the stars’ closest approach to each other was getting earlier

(binaries give off gravitational waves, if pulsar and neutron star)

Question 12

What causes the period of the neutron star to shorten?

Answer 12

When the accretion disk exerts a torque on the magnetosphere of the neutron star

Question 13

What increases the rate of “spin up”?

Answer 13

When the mass accretion rate goes up (also means x-ray luminosity increases)

Question 14

Why are accreting white dwarfs disregarded as an explanation for x-ray binaries?

Answer 14

White dwarfs have a radius between 5000 and 10,000 km

They would be 1000 times less luminous than a neutron star

Question 15

What are High Mass Binaries (HMXBs)?

Answer 15

They contain a massive (> 10 solar mass) O/B star

Compact companion captures mass from star’s wind

Question 16

What are Low Mass X-ray Binaries (LMXBs)?

Answer 16

They contain a low mass (1 solar mass) companion stars

Compact companion captures mass through Lagrange point when companion expands due to evolution and fills its Roche Lobe

Question 17

How is mass transferred in HMXBs?

Answer 17

The young stars have strong stellar winds and the compact star can capture a fraction of that wind

Question 18

how is mass transferred in LMXBs?

Answer 18

Mass transfer onto the compact star happens only as the companion star evolves and expands until its outer layers reach the gravitational saddle point between the two stars. Matter can then stream from it onto the compact star.

Mass loss via the wind of such a star is insignificant

Question 19

What does matter passing through the Lagrange point have?

Answer 19

It has angular momentum with respect to the accreting compact star (l = b^2 w)
b=distance between compact star and LP and w is orbital angular frequency

Question 20

What happens to overflowing material?

Answer 20

It can’t fall straight on to the compact star, it settles into an orbit (radius depends on l)

Question 21

What does the disc which is formed around a binary do?

Answer 21

It creates friction which:

allows material close to the compact star to lose angular momentum to slower material at larger orbits

heats the disc material which then radiates X-rays

Question 22

What happens when in falling material reaches the neutron star’s magnetospheric radius?

Answer 22

• is forced to follow magnetic field lines (B ~ 108 T)
• forms a narrow accretion column
• is channeled onto polar caps
• heats a small area of the neutron star’s surface

Question 23

What happens if the magnetic axis is not equal to the spin axis?

Answer 23

X-ray pulsations are seen

Question 24

What happens if a particle enters a B-field with a motion that is not perpendicular to the field?

Answer 24

It’s resultant motion is like a helix (Van Allen Belts)

Question 25

Where are HMXBs (massive) found?

Answer 25

Short lived companion stars found near star forming regions (around galactic plane)

Question 26

Where are LMXBs (old systems) found?

Answer 26

Away from galactic plane as they have had a chance to move away from birthplace

Also found in globular clusters

Question 27

When can a binary system survive an SN explosion?

Answer 27

If the system loses less than half of its mass (rare as most massive stars would evolve first)

And the neutron star should not get kicked not the companion stripped by ejecta of SN explosion

Question 28

Why can we not see regular X-ray pulsations from a black hole?

Answer 28

As we cannot see inside the event horizon

Question 29

What happens at the Schwarzschild radius?

Answer 29

Light is gravitational redshifted at v = 0 and the Schwarzschild radius can be considered the radius of a black hole

Question 30

Where is the last stable orbit applied?

Answer 30

For any orbital motion about a non-rotating black hole

Any orbit smaller than this orbit will spiral into the BH singularity

Question 31

What is the limit of the emission region?

Answer 31

It cannot be wider than r=ct

Question 32

In binaries containing black holes, how big can they get compared to their companion?

Answer 32

The black hole may be more or less massive than its companion

Question 33

What is a micro quasar?

Answer 33

Radio jets powered by accreting black holes