Interstellar Holography at Low Frequencies

1. Interstellar Holographyat Low Frequencies Dan Stinebring, Willem van Straten, Leon Koopmans • Mark Walker • (Manly Astrophysics)

2. Overview • Background • Technique • Results and current directions • Issues at low frequency • Applications • Interstellar medium • Pulsar timing

3. Background • Patterns in dynamic spectra • DRS: Lots of high quality data • emphasis on Fourier Plane • Discovery of Parabolic Arcs • Spectrum is FT{U} A FT{U*} • Sparse power in Fourier Domain • Model FT{U} by “CLEANing” • Holographic Image of ISM

4. Iterative modelling Delay Doppler Shift *

5. Noise-limited performance

6. Current Directions • Mitigate propagation-induced timing delays • B1937+21 test case (430 MHz baseband data) - UCB • Virtual phase conjugation • PD: dynamic cyclic spectra supercede dynamic spectra • Migrating to physical representations • Parametric descriptions of DM(x) and B(x) • Predict fields at any frequency / epoch / location • Determine all geometric / kinematic parameters ? • Continuum frequencies - slow transforms • Stationary Phase Points { DMk , DMk’ , DMk’’ }

7. Low frequency issues • Small scintillation bandwidths & timescales • Sampling limits are delay: Tobs / 2, Doppler: 2 / P • Low signal / noise per scintle • Use a compact representation of the wave field • Degeneracies in complicated environments • Extended media ? Complex velocity fields ? • Systematics in phased arrays ?

8. Imaging the ionised ISM • Lots of potential in regular monitoring of many L.O.S. • B0834+06 holographic imaging tells us: • Linear image (aspect ratio > 50) tens of AU long • Strong magnetic field most likely cause • Propagation anomalies show us high-stress regions - this and size scale common to ESEs, IDVs and Arcs - probable physical association • Such regions are common

9. Physical context of anomalies Self-Gravitating “Bullets” of Neutral Gas Shock Heating of ISM to T ~ 1 MK Dark Matter Filamentary Magnetotail Field Stretched to B ~ 10-4 G

10. Timing • Understanding propagation delays will be critical • Characterise ISM at “low” frequencies, because • Get most detailed picture of ISM structure • Get widest field image of ISM • Minimise the chance of missing features • Stitch together images of ISM with large overlap • Wide-angle multi-beaming - regular monitoring • Best spatial resolution at edge of FOV • Time at high or low frequencies ??

12. Imaging through the ISM • Know Phi(x) - determine source visibility vs. baseline • Just like terrestrial interferometric imaging • Highest resolution at lowest frequency • Resolution limit ~ FMHz km or better • Pulse-phase resolved images in any polarisation state • Image emission regions - new insights • Differential astrometry at ~ 100 pico-arcsecond level • Image reflex motions of PSRs with companions • Refraction & broadening in winds • Refraction in gravitational potential