class 12 : x-ray binaries and the discovery of black …chris/teaching/astr398b_fall..."...
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ASTR398B Black Holes (Fall 2015) Prof. Chris Reynolds
Class 12 : X-ray binaries and the discovery of black holes
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RECAP ! Development of radio astronomy (1960s) lead
to the discovery of pulsars " Very regular and rapid pulses of emission " Soon identified as the rotation of a magnetized
neutron star
! Neutron stars " Exotic objects! " Average density comparable to that of atomic nucleus " Much of the interior has been “neutronized” (protons
have been converted to neutrons) " Center probably contains exotic forms of quark matter
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This class
! Discovery of black holes! ! Story of X-ray astronomy…
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I : Atmospheric Windows
! The atmosphere blocks many wavelengths of e/m radiation. Essentially, we just see " Optical " Microwave (if we go to dry place!) " Radio
! So how do we do X-ray astronomy?
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II : X-ray astronomy
! Sounding rocket flights… " 1949 : Launch of X-ray detector on a V2-
rocket; first detection of X-rays from Sun " 1962 : First detection of an X-ray source
from outside of the solar system (Sco-X1) " 1964 : Detection of Cygnus X-1 " 1970 : Uhuru satellite launched
! Properties of Cygnus X-1 " Associated with a star 6000 ly away " Powerful X-ray source… 105x power of Sun! " Rapidly variable X-ray source
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III : Figuring out Cygnus X-1
! Once companion star identified, intense study began
! Blue supergiant with mass of 40Msun
! Star found to wobble… " Period 5.6 days " Velocity ±75km/s " Clearly a binary star…
something is orbiting the star, causing it to swing around
" Analysis of orbit says that the companion must be 7-13Msun
! Nature of companion " Must be very small (X-ray
variability) " Too massive for neutron star " … hence a black hole!
Brocksopp et al. (1999)
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! So… why the X-rays? " Gas from star flows towards black hole " Forms an accretion disk around black hole " Inflowing/spirally gas gives up gravitational
potential energy and heats up to 107K " Hence X-rays! " Physics of accretion disks actually quite
interesting…
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IV : Neutron star X-ray binaries
! Many X-ray binaries consist of a neutron star (rather than a black hole) orbiting a normal star
! Important new aspect… " Neutron star can have a magnetic field! " Appearance of source depends on magnetic
field
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! Strongly magnetized neutron star X-ray binaries… " Magnetic field disrupts accretion disk " Matter flows down field lines and strikes neutron star
surface at magnetic poles… get “hot spots” " In hotspots, temperatures get high enough to fuse
the incoming hydrogen to helium (continuously) " As poles spin around (due to NS rotation), the hot-
spots flash in and out of view… get an accretion-powered X-ray pulsar.
" Unlike case of the normal pulsars these are spinning-up due to the action of the accretion disk
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From http://astronomy.swin.edu.au/cosmos/X/X-ray+Pulsar
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! Weakly magnetized neutron star X-ray binaries " Accretion disk extends all of the way to the
neutron star surface… hydrogen gas is dumped onto surface and spreads around.
" So… there’s a growing layer of hot hydrogen gas on surface
" Once layer grows thick enough, runaway fusion starts at base of layer… layer explodes!
" Explosion lasts about 10s and blows most of the layer into space… “X-ray burst”
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