Pulsar Scintillation Arcs and the ISM Dan Stinebring Oberlin College Scattering and Scintillation In 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

with ACF

with Secondary Differential Doppler Shift Differential Delay

Some Examples

Normal arc

Normal arc

B

B

B0450–18

B1540– MHz

B

“Deflection of Pulsar Signal Reveals Compact Structures in the 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 causes a random diffraction pattern

Relative transverse velocities produce a dynamic spectrum time

Scattering in a thin screen plus a simple core/halo model can explain the basics of scintillation 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 PSF How to produce a “core/halo” psf? A Gaussian psf will NOT work: No halo.

Kolmogorov vs. Gaussian PSF Kolmogorov turbulence DOES work It produces a psf with broad wings

More Details …

Secondary spectrum basics

Fringe frequencies V eff

Fringe frequencies V eff DsDs D

Fringe frequencies V eff What if Then So that (point source at the origin) Parabolic arc with a positive definite offset

Fringe frequencies V eff Curvature of the Parabola

Secondary spectrum basics Curvature of the parabola Measure D,, V known Determine screen location

Needed: shallow (Kolmogorov) spectrum and “thin-screen” geometry –2525  x (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 Cordes and Rickett 1998, ApJ, 507, 846

velocity plot

Scanning the Sky …

The patchiness MOVES ! 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