Chapter 4 Flashcards
What does a sudden drop in brightness of a source suggest?
That the source is eclipsed
What suggests a “spin up”?
A long term trend towards earlier pulse arrival times
What was recorded when 2 x-ray sources were detected and what does this suggest?
Pulsations and periodicity of pulsars and periodicity in the change of counts
It is consistent with binaries
What happens if a star is moving quickly?
The centrifugal force will destroy the star
What is the condition for finding the density of a star associated with an X-ray emission?
Gravitational force must exceed centrifugal force
What does a doppler shift of spectral lines show?
A measure of projected orbital velocity along our line of sight
How does the mass function change if the sun and a planet of significantly lower mass are compared?
Only the mass of the Sun is considered
How can we infer the size of the companion in binary system?
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
What is an X-ray binary taken to mean?
A neutron star with a non-degenerate companion
Why is there an upper limit on the mass of a neutron star?
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
In the binary of a radio pulsar, what showed consistency with emissions of gravitational waves?
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)
What causes the period of the neutron star to shorten?
When the accretion disk exerts a torque on the magnetosphere of the neutron star
What increases the rate of “spin up”?
When the mass accretion rate goes up (also means x-ray luminosity increases)
Why are accreting white dwarfs disregarded as an explanation for x-ray binaries?
White dwarfs have a radius between 5000 and 10,000 km
They would be 1000 times less luminous than a neutron star
What are High Mass Binaries (HMXBs)?
They contain a massive (> 10 solar mass) O/B star
Compact companion captures mass from star’s wind
What are Low Mass X-ray Binaries (LMXBs)?
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
How is mass transferred in HMXBs?
The young stars have strong stellar winds and the compact star can capture a fraction of that wind
how is mass transferred in LMXBs?
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
What does matter passing through the Lagrange point have?
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
What happens to overflowing material?
It can’t fall straight on to the compact star, it settles into an orbit (radius depends on l)
What does the disc which is formed around a binary do?
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
What happens when in falling material reaches the neutron star’s magnetospheric radius?
- 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
What happens if the magnetic axis is not equal to the spin axis?
X-ray pulsations are seen
What happens if a particle enters a B-field with a motion that is not perpendicular to the field?
It’s resultant motion is like a helix (Van Allen Belts)
Where are HMXBs (massive) found?
Short lived companion stars found near star forming regions (around galactic plane)
Where are LMXBs (old systems) found?
Away from galactic plane as they have had a chance to move away from birthplace
Also found in globular clusters
When can a binary system survive an SN explosion?
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
Why can we not see regular X-ray pulsations from a black hole?
As we cannot see inside the event horizon
What happens at the Schwarzschild radius?
Light is gravitational redshifted at v = 0 and the Schwarzschild radius can be considered the radius of a black hole
Where is the last stable orbit applied?
For any orbital motion about a non-rotating black hole
Any orbit smaller than this orbit will spiral into the BH singularity
What is the limit of the emission region?
It cannot be wider than r=ct
In binaries containing black holes, how big can they get compared to their companion?
The black hole may be more or less massive than its companion
What is a micro quasar?
Radio jets powered by accreting black holes
