Transcript Pulsar_mag_Russbach_16x

Magnetism in Neutron stars,
pulsars and magnetars.
N.J.Stone,
Oxford and Tennessee
Russbach, March 2016
Outline
Magnetism is the cinderella interaction – it’s time she came to
the party
- very general survey talk at an introductory level
- to take any topic further consult references and move on
Magnetism in cosmic bodies
Planets and the sun
Field configurations in rotating, conductive bodies
Neutron stars (NS)
Internal and external structure
How large are NS magnetic fields?
Possible mechanisms for generation of sufficient fields
Is stellar magnetism significant?
strong; coulomb: gravity … ?
Start with the earth
Surface field
weak
non-axial
wandering axis with infrequent reversals
understood(?)
Environmental field: magnetosphere
ionosphere
rotating charges
deflection of solar wind
importance
Other planets
Mercury
Earth-like field 1% of H(E)
Venus
No magnetic field
Mars
No inner core field
Jupiter
Field ~ 2000 H(E), 10o off axis. Moon Ganymede has its own field
Saturn
Field ~ 600 H(E) on axis
Uranus
Field ~ 50 H(E), 60o off axis
Neptune
Field ~ 25 H(E), 47o off axis
General reference
Space Science Review vol 152 (2010)
The Sun
(C. A. Jones et al., Space Sci Rev 152 591 (2010))
Global field mainly quasi -axial, dipolar poliodal.
Internal fields more complex.
Regions
Solid body core rotation with outer convection zone where there is
differential rotation, faster at equator and slower at poles.
Medium of convection zone is plasma – conductive fluid
Convection leads - through Coriolis effect – to spiral motion about polar field
lines– induction - and toroidal fields which penetrate surface as Sunspots.
Resulting field configuration has coupled poloidal and toroidal components – the
aW dynamo – complex theory!
The direction of the poloidal field reverses every ~ 11 years
Poloidal field deflects galactic cosmic rays which produce isotopes such as 14C and
10B in the Earth’s atmosphere. These in trees and polar ice caps reveal the
persistence of the sun spot cycle over hundreds of years.
How different are neutron stars compared to the Earth?
Earth
Neutron star
Mass
6 x 10^24 Kg
~ 1.5 x solar … 3 x 10^30 Kg
Radius
6400 km
10 km
Surface gravity g
~ 10 m/s^2
1.6 x 10^11 m/s^2
Volume
1.1 x 10^21 m^3
4.2 x 10^12 m^3
Mean density
5.5 x 10^3 Kg/m^3
7.1 x 10^17 Kg/m^3
Expect application of the laws of physics in a very different regime
No practical possibility of terrestrial experimentation
Dependent upon observation (all aspects: spectrum. timing, deduced location
….)
and theory, pushed to extreme limits
The Challenge: magnetism in neutron stars
Neutron stars
Products of cataclysmic explosions of stars at the end of their burning cycle
when gravity causes collapse and disruption with loss of > 70% of material
Properties:
Position
Motion
Mass ~ 1.5 M(s), Radius ~ 10 km, Energy ~ 10^51 erg
Temp initially ~ 10^10, cooling to ~ 10^8 K
In the vicinity of supernova remnants
Can be travelling at > 10,000 km/s wrt surroundings
Pulsars
Subgroup (~10%) of neutron stars which
exhibit ‘lighthouse’ like emission of
EM radiation over a wide wavelength range.
Directed emission thought to be result of
magnetic dipole field rotating at an angle
to the axis of rotation of the star.
Fields in the region of 10^11 – 10^12 G
[10^7 – 10^8 T].
Properties:
Slow down of period. Glitches (abrupt small changes)
Magnetars
Further subgroup (~3%) with even higher fields ~ 10^14- 10^16 G
Neutron star structure
Atmosphere - thin
Outer crust
Inner crust
Core
beta equilibrium requires presence of protons, electrons and
(great majority) neutrons.
Inner core
Likely to contain hyperons and heavier mesons
Conductivity
By highly relativistic electrons in all regions and
(superconductive) protons until mesons appear.
Beyond the stellar surface.
Magnetosphere
standard model considers this divided into
regions of open and closed field lines.
Magnetosphere rotates with the NS out to
where this would imply velocities > vel. of light..
Closed field region filled by charged particles
such that their electric field, in the rotating
reference frame, cancels that from rotation of the star.
Open field region has strong electric field which accelerates charged particles to
close to speed of light. Particle motion in magnetic field causes emission of cyclotron
radiation.
Pulsars also emit gamma, x-ray and thermal radiation.
Role of magnetosphere in radiation propagation: detailed analysis of radiative
propagation in plasma is complex . Observable radiation varies with B field and
temperature, which are not uniform across the star surface. Composition of thin
atmosphere ions (H, He) affects spectrum.
Such questions of great importance in attempts to extract neutron star radii.
A. Y. Potehkin - Physics Uspekhi 57 735 (2014) and arXiv:1403.007v5 5 Jan 2016.
Evidence for magnitude of magnetic field in pulsars
Some spectra (in pulsars accreting material from a binary
partner) show lines which have been associated with electron
cyclotron resonance involving radiation from electrons orbiting
the poloidal field lines.
Frequencies correspond to fields of up to ~ 1.4 x 10^12 G.
For isolated pulsars use is made of the relation between the
period of rotation and its slowing, assuming magnetic dipole
radiation and star parameters: Radius R, MoI I, angular velocity
W and angle between field and rotation axes a.
dW/dt = - 2 R6B2W3sin2a/3I
The results again show B ~ 10^12 G.
Magnetic fields beyond and within the star.
General remarks.
Simple dipolar magnetic fields are unstable in systems which can support
currents such as NS interiors. In these there is a more stable configuration
involving a dipolar component ( a poloidal field) and a toroidal field. The latter
may not be ‘visible’ at the star surface – same idea as sun spots.
Conduction is by highly relativistic electrons and superconductive protons.
Superconductive currents are considered to flow at the core – inner crust
interface
Fujisawa and Kisaka MNRAS 445 2777 (2014)
More elaborate field profiles
combining poloidal and toroidal fields
Fujisawa and Kisaka MNRAS 445 2777 (2014)
Origin of magnetic fields in N S:
circulating currents or
intrinsic magnetization of constituents
Spruit H. C. AIP Conf Proc 983 391 (2008)
1. Fossil fields
Hansson and Ponga ISRN Astr Astro
2011 article 378493
Inherited from precursor star by flux conservation through
reduced surface area.
How is problematic.
2. Dynamo processes
circulating currents
(see references to
a-W dynamo)
Rossby Number: Ratio of inertial to convective forces. Governs Coriolis as
opposed to convective contribution to dynamo magnetic field amplification in
stars.
Reynolds Number: ratio of inertial to viscous forces. Low value – streamline
flow: High value – turbulent flow with eddies. Dynamo action requires R
above a specified value to be sustained. In neutron star matter R >> 1
(Thompson and Duncan 1993)
3. Constituent magnetisation
Polarisation of electrons, protons,
neutrons. All have intrinsic magnetic moments and VERY HIGH DENSITY
Fossil fields
For the first years after the discovery of pulsars the thinking was that these
remnants of precursor main sequence stars could ‘inherit’ their huge
magnetic fields, either direct from the progenitor or even from the primordial
pre-stellar medium.
Some stars have fields in the 10^4 G range and radii of (few x10^6) km. If
magnetic structure were somehow maintained during the supernova process
then, with radius reduced to ~ 10 km, fields (flux/area) could reach 10^4 x [(4
10^6/10)]^2 ~ 10^15 G.
Later thinking realised that such flux conservation is unlikely (Spruit ). Mass
loss during the explosion and the smaller initial area of the remnant as part
of the precursor core brings the maximum below that observed in magnetars,
even for an initial 10^4 G precursor. Furthermore only very few precursors
have fields ~ 10^4 G.
These and other technical arguments have led to the dropping of this idea as
the source of neutron star fields.
Dynamo effect and field amplification
(no detail - ref NOVA What drives the earth’s Magnetic Field ?)
When a conducting medium moves through a magnetic field
it constitutes a current, which, in turn, produces a second
field. This field adds to the first and produces a field
which is larger.
This ‘amplification’ requires energy input to sustain the fields
and currents, and there are losses through viscous and other
resistive forces.
In the earth, energy is derived from slow ‘freezing’ of the
inner surface of the molten outer core onto the inner solid
core, the heat so generated driving convective action which
keeps the molten, conductive, outer core moving through the field.
Coriolis effects also contribute by imparting a spiraling motion
to the moving liquid (just as in the bath or so they say!) and such spirals
generate additional magnetic field sources, which are roughly
aligned and add to the total field. This may contribute to the SN
explosion (Obergaulinger, Janka, Aloy MNRAS 445 3169 (2014)
Convective motion of a conducting medium is a typical feature of many types of star
structure: seen as the principal means of generation of magnetic fields.
The convection may be driven by any type of entropy gradient: T ….. Composition, ….
Requirements for dynamo existence and persistence.
N.B. Field is not static or ‘permanent’ but must be continuously generated.
Existence
1. An electrically conductive, fluid medium – exists in NS central regions.
2. Kinetic energy – provided by planetary rotation.
3. Internal energy source to drive convective motions in the fluid – this is
often provided by Coriolis forces associated with rotation. In the earth
thermal convection in the liquid iron outer core drives the motion.
Persistence
There will be losses associated with the field generation, largely from
viscous forces. If these are too large the field will die away. The ratio of the
generating power to the resistive losses must exceed unity for the field to
grow and this limits the maximum field through Lorentz forces if nothing
else acts first.
Collective magnetism in (neutron) stars
History: Brownell and Callaway 1969, Rice (1969) Silverstein (1969)
see Heansel and Bonazzola (Astron Astro 314 1017 (1996))
First ideas : Neutron pair interaction favours triplet state (S = 1) at high density which
avoids short range strongly repulsive n-n interaction in singlet (S = 0) state by Pauli
Principle. This would lead to ‘ferromagnetic’ (F) transition above some critical
density and below a critical temperature T(c). Estimated T(c) ~ 10^10 K.
Later:
Detailed calculations showed dense pure n matter would not undergo F
transition. However the addition of a small proton density was shown to
predict a stable polarised system (Kutschera and Wojcik P Lett B 223 11 (1989).
Full polarisation produces far too HIGH a magnetisation density (10^16 G) for the observed
normal pulsar fields, but high enough to produce magnetars (~ 10^15 G).
Details
existence of Domains as in Ferromag materials reduce effective magnetisation
Screening of core field by inner crust proton superconductivity
(Type I would give total screening, Type II partial screening)
Non-coaxiality: result of complex domain structure?
Magnetisation in NS produced during initial cooling phase since ordering temp ~ 10^10 K.
- some memory of precursor star field direction (?).
a
Angle a between rotation and
magnetic field axes
Inspiration…
Toy model of two dipoles
α
1950’s
2000 ?
Assumptions: 1. Fixed moment of inertia
2. Fixed magnitude M1, M2, θ1 and r
3. M1 and M2 co-planar
M1 – dynamo
M2 - polarized matter
Coexistence of dynamo and collective magnetism
Possible evidence
General: Wide range of observed magnetisations in N stars and white dwarfs
Specific: Angle a between resultant magnetic dipole field and rotation axis is
slowly changing.
(Lyne et al., Science 342 598 (2013) Crab pulsar pulse analysis)
Data:
Pulsar pulses have structure. Separation in time of different
components is found to change over the 22 years of observation of
this pulsar in a way consistent with change in angle a at rate of 0.62
degree per century towards orthogonality of the two axes.
Mechanism for a change.
Two non-coaxial dipoles exert couple tending either to
perpendicular or parallel configuration dependent upon
initial configuration.
Conclusions
Two possible mechanisms (dynamo, magnetization) for origin of NS
magnetism.
Both claim to produce fields as observed in both regular pulsars and in
magnetars.
There seems no clear reason why they cannot co-exist.
Both are consistent/compatible with other aspects of NS structure including
Superconductivity and Superfluidity.
Pulsars also show rapid small step changes in period. Origin not fully
determined.
Glitches:
Possibly related to adjustment as superfluid component
shares angular momentum with the normal component
after this has lost energy and slowed.
Stellar Magnetism.
A wonderfully open field for
new ideas which demand
new observations and
sophisticated analysis.
Thank you
Source: presentation by C. Auer ‘ABC of
Magnetars’ 2008
Alternative to dynamo polarised Neutromagnets
[Hansson and Ponga]
Polarised neutrons (major constituent – others too few to be important)
NS born at temp ~ 10^10 K. Estimated bound neutron spin triplet energy in
presence of strong gravity ~ 0.5 MeV – predicts ordering transition also with
T(c) ~ 10^10 K . Thus as neutron star is made and cools it falls below neutron
ordering temperature – Curie Temperature – and forms an ordered system
comparable to a ferromagnet
If all neutrons align in NS of mass ~ 1.4 solar masses, combined neutron
moments would generate field ~ 10^12 T that is 10^16 G – comparable to
highest estimates for magnetar fields.
Positive feature of this mechanism – existence of domain structure allows
partial magnetisation having axis independent of the pulsar rotation axis – as
observed.
Other - dynamo - mechanism associated with circulation of superfluid,
superconducting protons have difficulty providing fields with non-axial
symmetry.