Transcript theory.physics.helsinki.fi

Plasma universe
Fluctuations in the primordial
plasma are observed in the
cosmic microwave background
ESA Planck satellite
to be launched in 2007
Data from WMAP of NASA
Accretion disk around a black hole:
MHD in general relativity regime
Shock wave from a dying star
Neutron stars
- Radius ~10 km
- Mass 1.4 Msun
- Born from core collapse
supernova
(or possibly from
white dwarf accreting
mass from companion;
Type Ia supernova)
- Spindown and cooldown
in ~107 years, after which
difficult to observe (faint)
- Highly magnetised
neutron stars (B ~1011 T)
are called magnetars
Neutron star formation
- Massive star’s core burns into iron
- Iron core collapses. Angular momentum conservation causes rotation to
increase, and rotation is also differential. BA=const causes existing magnetic
field to multiply.
- When neutron star density reached, gravitational collapse energy has
heated matter to ~0.1 fraction of its rest mass ( ~ 100 MeV, 1012 K, per
nucleon)
- URCA-process cooling, T8
- Indirect URCA cooling, T6
- Convection due to temperature and lepton number gradients (density so
high that neutrinos trapped inside core) ==> dynamo action, even larger Bfield
- Radiative cooling, T4
- Dynamo action takes ~30 seconds
Neutron star life
- Initially (most probably) rapidly rotating, ~1 ms
- Spindown due to magnetic breaking (dipole radiation)
- Spindown rate depends on strength of magnetic field (this is the main
reason we know the values of the fields)
- Some modest decraese of the magnetic field may also occur (this is not
well known)
- Neutron star magnetosphere contains electron-positron plasma, if the
rotation rate is high enough
- Somehow, this plasma produces coherent radio emission ==> pulsar
- When rotation rate decreases below critical limit, radio emission stops, after
which detection is only possible by thermal X-rays (difficult)
- Irregularities: Glitches (abrupt spinrate changes), Starquakes, Decoupled
rotation rates of superfluid neutrons and iron lattice in the crust
Magnetars (magneettitähdet)
- Very highly magnetised neutron stars
- Strong magnetic breaking, rapid spindown (~10000 years), easily
observable (=”live”) only short time, therefore probably much more common
than low number of known examples (~ 10) would indicate
- Starquakes and glitches produce gamma ray bursts. The most energetic
ones (gamma flares) are so strong that they increase conductivity of Earth’s
ionosphere from galactic centre distance (10 kpc)
- “Soft gamma-ray repeaters” (SGRs) and “anomalous X-ray pulsars” (AXPs)
- Biosphere-killing potential of the same order of magnitude as that or
supernovae and gamma ray bursts (?)
- Short gamma ray bursts (GRBs) may be due to magnetar gamma flares
Neutron star magnetospheres
Fast rotating neutron stars
can be observed as pulsars
(fastest ones about 1 ms)
→ speed of light limits the
size of the pulsar
Very high energies → quantum effects
e.g., e– - e+ pair production and annihilation:
e– + e+ → 2  (511 keV gamma rays).
Plasma is necessary for radio emission.
Pulsar model
Neutron star observational issues
- Gravitational redshift
- Dependence of electron energy levels and ionisation potentials on magnetic
field (generally, they increase in high field) ==> difficulty of doing spectral
analysis
- Recent indications for “solar-type”, non-dipolar and complex, locally strong
magnetic fields. Magnetar-class fields of 1010-1011 T may occur locally even
on normal neutron stars (?)
- Magnetic dipole radiation (note: NOT the same as pulsar radiation, which
has higher frequency) lower than any plasma frequency around ==> it must
heat the surrounding plasma (??)
Pulsar statistics
- Spindown: motion to the right
- The higher the magnetic field,
the faster the spindown
→ magnetars observable
only for ~104 years here
- normal pulsars observable
for ~107 years
- Critical field: electron Larmor
radius equal to its deBroglie
wavelength → photon splitting,
possible disappearance of
e+e- plasma from high-field
region
- the Galaxy may contain
millions of dead magnetars
Equation of state is unknown!
Accretion to a compact object
Millisecond pulsars
- Very high rotation rate (~1 ms)
- Very slow decline of rotation rate ==> “weak” magnetic field
- Always (?) in binary star systems
Scenario:
- Double star, heavier partner undergoes supernova and becomes neutron
star. Probably it has time to slowdown and “die” (107 years) while companion
still in main sequence
- Lighter partner becomes red giant, fills his Roche limit ==> mass flow,
accretion disk
- The SMALLER the magnetic field, the SMALLER the corotating inner
magnetosphere, the HIGHER the Keplerian angular velocity at the corotation
boundary and the HIGHER the spinup effect of mass accretion
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The End