X-ray Binaries

Question 1

When do X-ray binary orbits occur?

Answer 1

If binary orbit is close enough, a few radii, accreted matrial flows from normal star onto compact object.

Question 2

Why is radiation produced by the accretion disk and how does it form?

Answer 2

mass flows to surface and some fraction of rest mass is ‘transferred’ into radiation. but conservation of momentum stops direct infall. so in the accretion disk, viscous forces arise as different layers move a different speeds (friction).

Question 3

At what point is a body radiating at the Eddington Luminosity?

Answer 3

When radiation pressure is equal to the gravitational pressure.

Question 4

Describe LMXRB.

Answer 4

donor is slowly evolving, low-mass star. <1.4

Question 5

Describe HMXRB.

Answer 5

donor is young massive star >10 with strong winds. formation requires two massive stars after first SN.

Question 6

Describe an X-ray Pulsar.

Answer 6

compact object. strong B-field affects accretion, disk material funnelled to hot spots on poles.

Question 7

Dipole Magnetic Field

Answer 7

B_s (r*/r)^3

Question 8

How can the Alfven radius be determined?

Answer 8

Ram pressure (rho v^2, rate of change of momentum per unit area) is equal to the radiation pressure (B-field energy density)

Question 9

What happens within the Alfven radius?

Answer 9

B-field funnels material onto poles. X-ray emitting hotspots.

Question 10

Describe a millisecond pulsar.

Answer 10

old pulsar that was spun down can be spun up to very short periods, by the accretion disk.

Question 11

Derive the equation for minimum period.

Answer 11

d/dt(transfer of angular momentum in disk) = d/dt(increase of angular momentum of star)

Question 12

Differences between x-ray emission from BH and x-ray pulsars.

Answer 12

pulsar: bright and pulsed. BH: not pulsed (some flickering, causality limit)