Ramesh Bhat Swinburne University of Technology

1. Pulsars and the Interstellar Medium Ramesh BhatSwinburne University of Technology

2. Outline • Some basics - why pulsars? Why at LFs? • Observable phenomena and measurable quantities • Distribution of scattering in the Galaxy • Electron density power spectrum • New phenomena and techniques - e.g. scintillation arcs, holography • New instruments - what do they mean for ISM+pulsar studies?

3. Pulsars make fabulous tools to study the ISM • pulsed • polarised • spatially coherent ISM ne ne BII • dispersion / smearing • Faraday rotation • scattering / scintillation • And, they are distributed all over the Galaxy • Most ISM effects become more pronounced at low radio frequencies; observables show steep dependence with frequency

4. ISM: Observable phenomena Steep dependence on the observing frequency and DM/distance Dispersion and smearing of the pulse Faraday rotation Source gets broadened (scattering disk) Pulse gets lengthened (pulse broadening) Intensity variations with time and frequency Flux varies over weeks to months Image wandering, pulse arrival times DM variations, RM variations, etc.

5. Scintillation basics “Strong” scattering: rms  >> 1 radian Pulse Broadening Pulsar Dynamic Spectrum FREQUENCY TIME

6. Electron density spectrum Electron density fluctuations in the ISM can be characterized by their structure function or the power spectrum Structure Function: RISS slope:  Log (power spectrum) DISS Power Spectrum: Log (spatial wavenumber) Outer scale Inner scale Most observables measure LOS integral of Cn2, “Scattering Measure” (SM) = LOS integral of Cn2 Inner scale Outer scale

7. Refractive scintillation Density structures at scales much larger than the “Fresnel scale” give rise to refraction and partial focusing/defocusing effects ==> refractive scattering and scintillation FREQUENCY TIME

8. A lot can be learned about ISMfrom pulsar observations How are dispersive and scattering plasma distributed over the Galaxy? Distribution and orientation of the Galactic magnetic field Characterize turbulence in the ISM; try and understand the physics governing it. Structure of the Local ISM: characterize features such as bubbles, shells, HII regions, etc. Insights into radio wave propagation through plasma media Scintillation arcs - a rich phenomenon and there is a lot to learn! (e.g. what is causing it? And How?) How important are the ISM effects in precision timing?

9. Distribution of electron density NE2001: Cordes & Lazio (2002) NE2001: thin disk, thick disk, spiral arms, GC, local ISM, clumps, voids, etc. - a major improvement over TC93 However, lots of new measurements since 2001

10. Scattering in the Galaxy Cordes & Lazio (2005) Bhat et al. (2004) Pulse Broadening (ms) DM SM  DM for a uniform distribution of turbulence (Cn2) However, much steeper increase at DMs larger than ~100 pc cm-3

11. The “composite” Electron density spectrum within ~1 kpc Armstrong, Rickett & Spangler (1995) 1 pc 1 AU 100 km Kolmogorov spectrum is a good approximation (for at least within 1 kpc of Sun) However, data cannot distinguish between K^-4 and K^-11/3 Measurements for individual lines of sight - not all agree with alpha = 11/3

12. 20 yr DM variations of PSR B1937+21 Ramachandran et al. (2006) Phase structure function on scales from ~0.1 to ~100 AU Slope alpha = 1.66 \pm 0.04, remarkably agreement with Kolmogorov value 5/3 Most observations probe just a part of the spectrum That said many observations suggest/support discrete structures (drifts, arcs, etc).

13. Estimation of inner scale Bhat et al. (2004) Analysis of large sample yield ~300-800 km as a global value Pulse shape analysis of PSR J1644-45 suggests ~100-200 km (Rickett 2006) From early VLBI observations ~50-200 km (Spangler & Gwinn 1990) Large inner scale suggested from DM variations (e.g. You et al. 2007; Ramachandran et al. 2006) and flux variations of some pulsars

14. Estimation of outer scale Haverkorn et al. (2008) • Faraday rotation and depolarization of ~150 radio sources (SGPS data) • RM structure function analysis - lower amplitudes than expected! • outer scale ~ 1 pc in spiral arms ; could be much larger in inter arms

15. The ISM within ~1 kpc of the Solar neighborhood (the “local” ISM)

16. Interaction of Local Bubble and Loop I Loop I Local Bubble Egger & Aschenbach (1998) Loop I Local Bubble Bhat & Gupta (2002) Density estimates in good agreement with independent estimates from Sallmen et al. (2008) based on the FUSE data (ne ~ 0.1 cm-3, d~1-2 pc)

17. Refractive scintillation in ISM: dynamic spectra vary with time PSR B1133+16 Time span = 2 months Multiple imaging (fringing) event Bhat, Rao & Gupta (1999)

18. Refractive scintillation in the ISM: Flux variations of pulsars over 5 yrs Stinebring et al. (2000) • 21 pulsars at 610 MHz - daily measurements of flux density • depths of modulations - from 5% to 50% ; distant pulsars are stable • 16/21 consistent with Kolmogorov; the rest suggests a large inner scale (10^5 km)

19. Daily scintillation monitoring of B0329+54 Quasi-continuous observations, sampling the primary (dynamic) spectrum every 90 mins Tests of quantitative predictions of theories (Romani, Narayan & Blandford 1986) Wang, Yan, Manchester, Wang (2008) • Previous work - Stinebring et al. (1996); Bhat et al. (1999) [18 pulsars] • Bottomline - results from observations are inconclusive!

20. The phenomenon of “scintillation arcs” Primary (dynamic spectrum) Secondary spectrum 2D FT • Discovered by Stinebring, McLaughlin, Cordes, et al. (2001) - in Arecibo data! • Theoretical treatments - Cordes et al. (2006), Walker et al. (2004) • Lots of follow up work - Hill et al. (2005); Putney et al. (2005); Stinebring et al. 2005

21. “scintillation arcs” - basic picture Scattering in a “thin” screen and a simple “core + halo” model can explain the basics of scintillation arcs (see Stinebring et al. 2001) • Must be a point source - physical angular size << scattering disk size • Kolmogorov turbulence to produce a PSF with core + halo morphology

22. Scintillation arcs opening up several new applications Curvature of scintillation arc: Effective velocity b/w pulsar, observer and the medium (screen): Putney & Stinebring (2007)

23. Effects of DM changes - a result from Parkes timing array DM variations from data at 50 cm, 20 cm and 10 cm (10 and 50 cm simultaneously) Warning: DM changes can potentially mimic red noise! You et al. (2007) Monitoring DM changes and applying corrections improve timing precision

24. Time variability of scattering delay Scattering delays from monitoring scintillation arcs over months Important to assess effects on arrival times - a yet another source of low frequency noise! Hemberger & Stinebring (2008) Changes in scattering delay are of the order of timing precision

25. New LF arrays will open up a new era in ISM studies LWA LOFAR • Large collecting area = raw sensitivity • wide fields of view ~ 10 to 50 degree • multiple tied array beams • continuous frequency coverage MWA

26. Multibeaming is a big advantage for ISM studies • Most ISM phenomena involve time scales - weeks to months at LOFAR / MWA frequencies • Monitoring studies most efficient with multiple beams • 16 for MWA ; ~100 for LOFAR

27. LOFAR/MWA surveys will discover lots of new pulsars! MWA simulated pulsars (Bailes) ~600-800 pulsars (van Leeuwen) ~1000 pulsars with LOFAR (van Leeuwen)

28. Scattering measurements - current census Bhat et al. (2004) Only 370 measurements so far (out of ~1800 pulsars known)!

29. Scattering measurements - current census Bhat et al. (2004) Departures from current models (NE2001) - on a logarithmic scale Only 370 measurements so far (out of ~1800 pulsars known)!

30. Scattering at 200 MHz (at b=0) Contours at 1, 10, 100 and 1000 ms At ~100 MHz, scattering is 5 ms or larger even for low DMs, high |b|

31. Scintillation arcs at low frequencies Arecibo and GBT observations studied ~20 pulsars Arc curvature scales as ~ (frequency)2 Scintillation bandwidth ~ (frequency)4 Scintillation time scale ~ (frequncy)1.2 Tied array beams - less sky background Extensive study on a large sample of pulsars will become possible

32. RMs and Mapping out the (local) Galactic magnetic field Faraday rotation scales as lambda^2 Even very small RMs will be measurable at LOFAR / MWA - ionospheric contribution important for small RMs e.g. at 100 MHz, RM = 50, 180 degree over 0.35 MHz Currently only 1/3rd of known pulsars have RMs measured (Noustos et al. 2008) Lots of new (local) pulsars (+ transients!)

33. Pulsar+ISM science at LFs: summary and some questions Pulsars continue to be wonderful tools to study ISM; great prospects at LFs given steep dependence of obs phenomena Mapping out electron density, scattering, the B field in much greater detail - global models (pulsars + transients) Density spectrum - not all measurements consistent with the canonical form, however important for interpretations

34. Pulsar+ISM science at LFs: summary and some questions Models for refractive scattering, scattering due to discrete, dense structures? Scintillation arcs and related puzzles - the phenomenon itself, astrophysical structures causing them? How important are ISM effects in precision timing? How can the LF instruments help? Can we remove scattering? Coherent de-scattering? Reconstruction techniques?