Download - Pulsar Scintillation Arcs and the ISM
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Pulsar Scintillation Arcsand the ISM
Dan StinebringOberlin College
Scattering and ScintillationIn Radioastronomy
Pushchino 19–23 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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0834+06 with ACF
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0834+06 with Secondary
DifferentialDoppler Shift
DifferentialDelay
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Some Examples
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Normal arc 1133+16
psr distance(kpc) V (km/s) sB0823+26 0.38 196 0.36B0834+06 0.72 174 0.33B0919+06 1.2 505 0.59B1133+16 0.27 475 0.49
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Normal arc 0823+26
psr distance(kpc) V (km/s) sB0823+26 0.38 196 0.36B0834+06 0.72 174 0.33B0919+06 1.2 505 0.59B1133+16 0.27 475 0.49
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B 2310+42
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B2021+25
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B2021+25B0450–18
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B1540–06
340 MHz
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B1508+55
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“Deflection of Pulsar Signal Reveals Compact Structures inthe 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 causesa random diffraction pattern
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Relative transverse velocities produce a dynamic spectrum
time
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Scattering in a thin screen plusa simple core/halo model canexplain the basics ofscintillation arcs
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Hierarchy of Power Levels
•Core-core
•Core-halo
•Halo-halo
Near origin of SS
Main scintillation arc features
Too weak to detect
Holographic Imaging
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Kolmogorov vs. Gaussian PSFHow to produce a “core/halo” psf?
A Gaussian psf will NOT work: No halo.
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Kolmogorov vs. Gaussian PSFKolmogorov 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
€
q=θ2x −θ1x =λ s
(1−s)Vpm
ft
€
p=θ22 −θ1
2 =2cs
D(1−s)fν
DsD
€
s ≡Ds
D
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Fringe frequencies
Veff
What if
€
θ1 =0
Then
€
p=θ2x2 +θ2y
2
€
q=θ2x
So that
€
p=q2 +θ2y2
(point source at the origin)
Parabolic arc with a positive definite offset
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Fringe frequencies
Veff
€
fν =η ft2
€
η=Dλ2
2cVpm2
s
1− s
⎛
⎝ ⎜
⎞
⎠ ⎟
Curvature of the Parabola
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Secondary spectrum basicsCurvature of the parabola
€
η=Dλ2
2cVpm2
s
1− s
⎛
⎝ ⎜
⎞
⎠ ⎟
Measure D, , V known Determinescreen location
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Needed: shallow (Kolmogorov) spectrum and “thin-screen”geometry
–25 25x (mas)
640 pc
450 pc
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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
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Sharpness of Arcs
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Effective Velocity
€
Veff⊥ =(1−s)Vp⊥+sVobs⊥−Vscr⊥
Cordes and Rickett 1998, ApJ, 507, 846
€
s ≡Dpsr−screen
Dtotal
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1929+10 velocity plot
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Scanning the Sky …
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The patchiness
MOVES !
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
Hill, A.S., Stinebring, D.R., et al.
2005, ApJ,619, L171 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?
Hill, A.S., Stinebring, D.R., et al.
2005, ApJ,619, L171
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Holographic Imaging
(very early stages)
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Walker, M.A. & Stinebring, D.R. 2005, MNRAS, 362, 1269
Mark Walker has made substantial progress on finding underlying “scattered wave components” in a secondary spectrum.
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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
• Stinebring et al. 2001, ApJ, 549, L97
• Hill et al. 2003, ApJ, 599, 457
• Hill et al. 2005, ApJ, 619, L17
•Walker et al. 2004, MNRAS, 354, 43
•Cordes et al. 2006, ApJ, 637, 346
•Walker & Stinebring 2005, MNRAS, 362, 1279
Observation
Theory