HST Polarization Observations of the Crab Nebula

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HST Polarization Observations of the Crab Nebula
Jeff Hester School of Earth Space
Exploration Arizona State University
Hester, 2008, Ann Rev Astron Astrophys, 46
127-155
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The Remnant of the Supernova of AD1054
I assume that the audience is generally familiar
with the Crab Nebula. As a brief review

• The Pulsar
• Spin-down luminosity 5 X 1038 erg s-1
• Carried away in a relativistic wind

• The Synchrotron Nebula
• Shocked wind
• Smaller at higher E
• Synchrotron loss times
• Extraordinarily Dynamic

• See structure on physically
• interesting spatial and temporal
• scales

X-rays (Chandra) Optical (HST) Radio (VLA)
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Time exposures provide an overall physical
context for discussing the Crab.
Axisymmetrical Geometry
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It is worth watching the movies for a few cycles
to show that the system is NOT simple.
X-rays Chandra X-ray Observatory
Optical Hubble Space Telescope
7 frames / 22 day spacing
Movies available at http//eagle.la.asu.edu/heste
r/crab_movies
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Temporal observations show that the torus is
confined by poloidal features that expand outward
into the body of the nebula rather than moving
radially outward from the pulsar. (Indicated by
A in the figure.)
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23 frames /11 day spacing
Visible Hubble Space Telescope
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Note rotated orientation.
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Note difference between unpolarized (clumpy and
mottled) and polarized (striated) morphologies.
This shows what features have coherent magnetic
structure and what features have tangled internal
field structures. There is a lot to point out
here, such as the unpolarized clumps around the
inner ring, and the fact that the inner knot is
strongly polarized while the outer knot and jet
features are not.
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For synchrotron radiation, Bsource ? Epol so
Bsource can be reconstructed
As with previous studies, white lines show
reconstructed B field running E-W across the
nebula, at an angle unrelated to much of anything
else associated with the nebula. BUT this
doesnt mean very much.
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Solution Isolate and background subtract
individual features in the three single-angle
polarization images then reconstruct the
polarization from those background subtracted
data. This gives VERY different results
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White lines show Magnetic Field direction in
individual physically discrete features.
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• The B field invariably runs along the length of
  striated features ? Striations are Flux Tubes
• Peak polarizations in wisps 70
• The torus is confined by a poloidal field
  component

This has significant implications for modeling of
the nebula
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Much has been made of qualitative comparisons of
2-D axisymmetrical MHD models and the Crab.
Del Zanna (2006) 2-D Relativistic MHD
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Much has been made of qualitatively comparisons
of 2-D axisymmetrical MHD models and the Crab.
BUT Axisymmetrical models with imposed
latitudinal variations in the wind properties
MUST produce axisymmetrical structure with
latitudinal variations. This is built into the
models. In fact, the models tend not to look
much like the real structure in the Crab at
all. This is true not only for overall
morphology, but for polarization of features such
as bright jet features.
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Much has been made of qualitatively comparisons
of 2-D axisymmetrical MHD models and the Crab.

• Physics that is left out
• Importance of non-azimuthal field structure.
  Polarization observations show that the torus is
  confined by a poloidal field component as the
  wind pushes out into the body of the nebula.
• Synchrotron cooling is left out of the dynamical
  calculations, even though locally synchrotron
  emission dominates the energetics (?synch 0.2
  years)

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• A few take-away points especially relevant to
  this meeting
• Polarized and unpolarized images of the Crab
  show what features have ordered fields and what
  features have tangled internal fields.
• Reconstruction of the magnetic field in the Crab
  based on overall polarization has little physical
  meaning because these observations integrate
  along the line of sight through the nebula.
• Polarization of physically discrete features
  shows that the striations (including bright
  wisps) seen throughout the Crab are invariably
  magnetic flux tubes with the magnetic field
  running along the striation.
• Polarizations close to the theoretical limit are
  observed in the bright wisps. (There are
  interesting things to say about pitch angle
  distributions as well.)
• The torus and other structure in the inner
  nebula is neither strictly axisymmetrical nor
  azimuthal. The expansion of the torus, for
  example, is clearly confined by a poloidal field
  component that arises as the wind pushes out into
  the disordered field in the body of the nebula.
• Current 2-D MHD models of the Crab leave out
  important first order physics, including
  synchrotron cooling. (Is gamma ray polarization
  an interesting constraint here?) While this work
  is very interesting, care is needed when applying
  these models to the real Crab Nebula or other PWN.