Pulsar Scintillation Arcsand the ISM

Dan StinebringOberlin College

Scattering and ScintillationIn Radioastronomy

Pushchino 19–23 June 2006

Collaborators

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

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

0834+06 with ACF

0834+06 with Secondary

DifferentialDoppler Shift

DifferentialDelay

Some Examples

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

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

B 2310+42

B2021+25

B2021+25B0450–18

B1540–06

340 MHz

B1508+55

“Deflection of Pulsar Signal Reveals Compact Structures inthe Galaxy, ” A. S. Hill et al. 2005, 619, L17

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

Schematic Explanation

Coherent radiation scatters off electron inhomogeneities

Multi-path interference causesa random diffraction pattern

Relative transverse velocities produce a dynamic spectrum

time

Scattering in a thin screen plusa simple core/halo model canexplain the basics ofscintillation arcs

Hierarchy of Power Levels

•Core-core

•Core-halo

•Halo-halo

Near origin of SS

Main scintillation arc features

Too weak to detect

Holographic Imaging

Kolmogorov vs. Gaussian PSFHow to produce a “core/halo” psf?

A Gaussian psf will NOT work: No halo.

Kolmogorov vs. Gaussian PSFKolmogorov turbulence DOES work

It produces a psf with broad wings

More Details …

Secondary spectrum basics

Fringe frequencies

Veff

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

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

Fringe frequencies

Veff

€

fν =η ft2

€

η=Dλ2

2cVpm2

s

1− s

⎛

⎝ ⎜

⎞

⎠ ⎟

Curvature of the Parabola

Secondary spectrum basicsCurvature of the parabola

€

η=Dλ2

2cVpm2

s

1− s

⎛

⎝ ⎜

⎞

⎠ ⎟

Measure D, , V known Determinescreen location

Needed: shallow (Kolmogorov) spectrum and “thin-screen”geometry

–25 25x (mas)

640 pc

450 pc

Multiple Screens

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

Sharpness of Arcs

Effective Velocity

€

Veff⊥ =(1−s)Vp⊥+sVobs⊥−Vscr⊥

Cordes and Rickett 1998, ApJ, 507, 846

€

s ≡Dpsr−screen

Dtotal

1929+10 velocity plot

Scanning the Sky …

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!

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

Holographic Imaging

(very early stages)

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.

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 …)

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!

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