Pulsar Scintillation Arcs and the ISM

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Pulsar Scintillation Arcsand the ISM

• Dan Stinebring
• Oberlin College

Scattering and Scintillation In
Radioastronomy Pushchino 1923 June 2006
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Collaborators

• Bill Coles
• Jim Cordes
• Barney Rickett
• Volodya Shishov
• Tania Smirnova
• and many Oberlin College students

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Motivations

• Interstellar inhomogeneity spectrum
• Single-dish imaging of the ISM on AU size
  scales on a continuing basis
• Imaging the pulsar magnetosphere?
• Improving high-precision pulsar timing
• Reducing the effects of scattering

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083406 with ACF
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083406 with Secondary
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Some Examples
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Normal arc 113316
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Normal arc 082326
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B 231042
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B202125
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B154006
340 MHz
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Deflection of Pulsar Signal Reveals Compact
Structures in the Galaxy, A. S. Hill et al.
2005, 619, L17
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Key Points

• 1) scintillation arcs are detectable toward most
  bright pulsars
• 2) they provide single-dish snapshots of the 2d
  distribution of scattering material (fov 40
  mas ?? 4 mas)
• 3) they scan the sky at the large proper motion
  rate of most pulsars

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Schematic Explanation
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Coherent radiation scatters off electron
inhomogeneities
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Multi-path interference causes a random
diffraction pattern
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Relative transverse velocities produce a dynamic
spectrum
time
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Scattering in a thin screen plus a simple
core/halo model can explain the basics
of scintillation arcs
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Hierarchy of Power Levels

• Core-core
• Core-halo
• Halo-halo

Near origin of SS
Holographic Imaging
Main scintillation arc features
Too weak to detect
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Kolmogorov vs. Gaussian PSF
How to produce a core/halo psf?
A Gaussian psf will NOT work No halo.
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Kolmogorov vs. Gaussian PSF
Kolmogorov turbulence DOES work
It produces a psf with broad wings
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More Details
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Secondary spectrum basics
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Fringe frequencies
Veff
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Fringe frequencies
Veff
Ds
D
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Fringe frequencies
What if
(point source at the origin)
Then
So that
Veff
Parabolic arc with a positive definite offset
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Fringe frequencies
Curvature of the Parabola
Veff
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Secondary spectrum basics
Curvature of the parabola
Determine screen location
D, l, V known
Measure
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Needed shallow (Kolmogorov) spectrum and
thin-screen geometry
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?x (mas)
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Multiple Screens
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• Multiple Scintillation Arcs
• Each is telling us about a scattering screen
  along the los
• The curvature of the arc (plus distance and
  proper motion info) locates the screen along the
  los
• Sharp arc boundaries imply thin screens
• Screen locations are constant over decades of
  time

see Putney et al. poster for details
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Sharpness of Arcs
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Effective Velocity
Cordes and Rickett 1998, ApJ, 507, 846
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192910 velocity plot
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Scanning the Sky
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The patchiness MOVES !
This is the angular velocity of the pulsar across
the sky!
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There is considerable bending power in the
entities that give rise to the arclet features (a
- d). Our estimates Size 1 AU Density
200 cm-3 Are these the same objects that give
rise to ESEs?
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Holographic Imaging

• (very early stages)

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Mark Walker has made substantial progress on
finding underlying scattered wave components in
a secondary spectrum.
Walker, M.A. Stinebring, D.R. 2005, MNRAS, 362,
1269
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It may be possible to form an image of the
scattering material in the ISM with
milliarcsecond resolution. The searchlight beam
that illuminates the medium is swept along by the
pulsar proper motion.
(Work in progress with Mark Walker and others )
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Summary Comments

• There are many opportunities for focused
  observational projects
• Early stage of interpretation of results many
  fundamental puzzles remain!
• Larger more sensitive telescopes will provide
  breakthroughs!

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Some references
Observation

• Stinebring et al. 2001, ApJ, 549, L97
• Hill et al. 2003, ApJ, 599, 457
• Hill et al. 2005, ApJ, 619, L17

Theory

• Walker et al. 2004, MNRAS, 354, 43
• Cordes et al. 2006, ApJ, 637, 346
• Walker Stinebring 2005, MNRAS, 362, 1279