Slide 1 Calculate the net force acting on a particle Mass transfer in a binary system.

Slide 1

Calculate the net force acting on a particle

Mass transfer in a binary system

Slide 2

Gravitational potential in the corotating frame

Slide 3

Mass Transfer in Binary StarsIn a binary system, each star controls a finite region of space,

bounded by the Roche Lobes (or Roche surfaces).

Matter can flow over from one star to another through the Inner Lagrange Point L1.

Lagrange points = points of stability, where matter can

remain without being pulled towards one of the stars.

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Accretion from stellar windAccretion through Roche lobe outflow

Two mechanisms of mass transfer in a binary system

How the matter from a star can be brought to L1 point?

Slide 6

Accretion from a stellar wind

Slide 7

Star overflows its Roche lobe

Slide 8

Formation of an Accretion DiskThe rotation of the binary systems implies that gas flowing through the L1 point will have relatively high specific angular momentum - too much to directly accrete onto a compact companion star.

Slide 9

Initial ring of gas spreads into the disk due to diffusion.

To be able to accrete on the star, matter should lose angular momentum as a result of viscous friction

Friction leads to heating of the disk and intense radiation!!

Slide 10

Accreting binary systems

• White dwarf binaries

• Neutron star binaries

• Black hole binaries

Slide 11

Nova Explosions: a mechanism

Nova Cygni 1975

Hydrogen accreted through the accretion

disk accumulates on the surface of the WD

Very hot, dense layer of non-fusing hydrogen

on the WD surface

Explosive onset of H fusion

Nova explosion

Slide 12

Accreting neutron stars and black holes

Black holes and neutron stars can be part of a binary system.

=> Strong X-ray source!

Matter gets pulled off from the companion star, forming an accretion

disk.

Infalling matter heats up to billions K. Accretion is a very efficient process of

energy release.

Slide 13

The Universe in X-ray and gamma-ray eyes

Giacconi: Nobel prize 2002

Slide 14

Accretion onto a neutron star

Slide 15

X-ray pulsar: an accreting neutron star

Compare with a radio pulsar

Slide 16

Pulsars are slowing down with time.

Millisecond pulsars: how can an old neutron star rotate at a rate 1000/sec?

Slide 17

Accretion onto black holes

There is no hard surface. Will there be any radiation from the infalling matter??

Slide 18

Cygnus X1 – first black hole

Slide 19

Measurement of binary system parameters gave M ~ 7 Msun

Slide 20

High-Mass X-ray binary: accretion from a windCygnus X1

Slide 21

Low-Mass X-ray binary: accretion through Roche-lobe overflow

Slide 22

;2

3

21 P

aMM

a – in AUP – in yearsM1+M2 – in solar masses

Binary systems

If we can calculate the total mass and measure the mass of a normal star independently, we can find the mass of an unseen companion

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Low-mass X-ray binaries are best candidates because the mass of a red dwarf is much less than a black-hole mass

212

3

21 ; MMP

aMM

Slide 25

Black-Hole vs. Neutron-Star Binaries

Black Holes: Accreted matter disappears beyond the event horizon without a trace.

Neutron Stars: Accreted matter produces an X-ray flash as it impacts on the

neutron star surface.

Slide 26

Soft X-ray transients (X-ray Novae)

Slide 27

Black Hole X-Ray Binaries

Strong X-ray sources

Rapidly, erratically variable (with flickering on time scales of less than a second)

Sometimes: Quasi-periodic oscillations (QPOs)

Sometimes: Radio-emitting jets

Accretion disks around black holes

Slide 28

Radio Jet Signatures

The radio jets of the Galactic black-hole candidate GRS 1915+105

V ~ 0.9 c

Slide 29

Gamma-ray bursts

Discovered in 1968 by Vela spy satellitesOccur ~ 3 times a day at random positions in the sky

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Variability on a less than 1 ms timescale – must be a very small object R < ct ~ 100 km

Slide 32

Compton gamma-ray observatory discovered two puzzles:

• GRBs are distributed isotropically on the sky

• There is a deficiency of weak bursts – are we looking over the edge of their distribution?

Slide 33

Gamma-ray sky

GRB distribution

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Breakthrough: in 1997 when BeppoSAX satellite was able to detect the burst position at 1 arcmin resolution and coordinate with optical telescopes within 1 hour after the burst

An X-ray image of the gamma-ray burst GRB 970228, obtained by the team of Italian and Dutch scientists at 5:00 AM on Friday 28th February, 1997, using the BeppoSAX satellite.

Slide 36

Discovery of the optical and radio counterparts of GRBs

Spectral lines with redshift from 0.8 to almost 4!

• GRBs are at the edge of the observable universe• They must be the most powerful explosions in the universe: ~ 1 solar mass is converted into gamma-rays in a second!

Slide 37

Gamma-ray burst models

Hypernova??

Slide 38 Fig. 10-18, p. 202

Known types of supernovae

Type II: hydrogen lines; collapse of a massive starType I: no hydrogen lines

Slide 39

Hard to imagine a supernova without ejection of a star shell

Slide 40

Colliding neutron stars

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Slide 42

Continuing cycle of stellar evolution

Slide 43

Our Earth and our bodies are made of atoms that were synthesized in previous generations of stars