3 Mass Transfer in Binary Stars In a binary system, each star controls a finite region of space, bounded by the Roche Lobes (or Roche surfaces).Lagrange points = points of stability, where matter can remain without being pulled towards one of the stars.Matter can flow over from one star to another through the Inner Lagrange Point L1.
5 Two mechanisms of mass transfer in a binary system How the matter from a star can be brought to L1 point?Two mechanisms of mass transfer in a binary systemAccretion through Roche lobe outflowAccretion from stellar wind
8 Formation of an Accretion Disk The 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.
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 frictionFriction leads to heating of the disk and intense radiation!!
10 Accreting binary systems White dwarf binariesNeutron star binariesBlack hole binaries
11 Nova Explosions: a mechanism Hydrogen accreted through the accretion disk accumulates on the surface of the WDVery hot, dense layer of non-fusing hydrogen on the WD surfaceNova Cygni 1975Explosive onset of H fusionNova explosion
12 Accreting neutron stars and black holes Black holes and neutron stars can be part of a binary system.Matter gets pulled off from the companion star, forming an accretion disk.=> Strong X-ray source!Infalling matter heats up to billions K. Accretion is a very efficient process of energy release.
13 The Universe in X-ray and gamma-ray eyes Giacconi: Nobel prize 2002
14 Accretion onto a neutron star Figure 11.8: Sometimes the X-ray pulses from Hercules X-1 are on, and sometimes they are off. A graph of X-ray intensity versus time looks like the light curve of an eclipsing binary. (Insets: J. Trümper, Max-Planck Institute) (b) In Hercules X-1 matter flows from a star into an accretion disk around a neutron star producing X rays, which heat the near side of the star to 20,000 K compared with only 7000 K on the far side. X rays turn off when the neutron star is eclipsed behind the star.
15 X-ray pulsar: an accreting neutron star Compare with a radio pulsar
16 Pulsars are slowing down with time. Millisecond pulsars: how can an old neutron star rotate at a rate 1000/sec?
17 Accretion onto black holes There is no hard surface. Will there be any radiation from the infalling matter??
19 Measurement of binary system parameters gave M ~ 7 Msun
20 High-Mass X-ray binary: accretion from a wind Cygnus X1
21 Low-Mass X-ray binary: accretion through Roche-lobe overflow
22 Binary systems a – in AU P – in years M1+M2 – in solar masses If we can calculate the total mass and measure the mass of a normal star independently, we can find the mass of an unseen companiona – in AUP – in yearsM1+M2 – in solar masses
24 Low-mass X-ray binaries are best candidates because the mass of a red dwarf is much less than a black-hole mass
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.
27 Black Hole X-Ray Binaries Accretion disks around black holesStrong X-ray sourcesRapidly, erratically variable (with flickering on time scales of less than a second)Sometimes: Quasi-periodic oscillations (QPOs)Sometimes: Radio-emitting jets
28 Radio Jet SignaturesThe radio jets of the Galactic black-hole candidate GRSV ~ 0.9 c
29 Gamma-ray bursts Discovered in 1968 by Vela spy satellites Occur ~ 3 times a day at random positions in the sky
35 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 burstAn X-ray image of the gamma-ray burst GRB , obtained by the team of Italian and Dutch scientists at 5:00 AM on Friday 28th February, 1997, using the BeppoSAX satellite.
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 universeThey must be the most powerful explosions in theuniverse: ~ 1 solar mass is converted into gamma-raysin a second!
38 Known types of supernovae Type II: hydrogen lines; collapse of a massive starType I: no hydrogen linesFigure 10.18: Type I supernovae decline rapidly at first and then more slowly, but type II supernovae pause for about 100 days before beginning a steep decline. Supernova 1987A was odd in that it did not rise directly to maximum brightness. These light curves have been adjusted to the same maximum brightness. Generally, type II supernovae are about 2 magnitudes fainter than type I.Fig , p. 202
39 Hard to imagine a supernova without ejection of a star shell