Ramesh BhatRamesh BhatSwinburne University of TechnologySwinburne University of Technology

Pulsars and the Interstellar Medium

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?

Pulsars make fabulous tools to study the ISM

• pulsed

• polarised

• spatially coherent

• dispersion / smearing

• Faraday rotation

• scattering / scintillation

ne

ne

BII

ISM

• And, they are distributed all over the Galaxy

• Most ISM effects become more pronounced at low radio frequencies; observables show steep dependence with frequency

ISM: Observable phenomena

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.

Steep dependence on the observing frequency and DM/distance

Scintillation basics

Pulsar Dynamic Spectrum

Pulse Broadening

TIME

FR

EQ

UE

NC

Y

“Strong” scattering: rms >> 1 radian

Electron density spectrum

Structure Function:

Power Spectrum:

Inner scale Outer scale

Most observables measure LOS integral of Cn2,

“Scattering Measure” (SM) = LOS integral of Cn2

Electron density fluctuations in the ISM can be characterized by their structure function or the power spectrum

Log (spatial wavenumber)

Log

(pow

er s

pect

rum

) Outer scale Inner scale

slope:

DISS

RISS

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

TIME

FR

EQ

UE

NC

Y

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?

Distribution of electron density

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

NE2001: Cordes & Lazio (2002)

Scattering in the Galaxy

SM DM for a uniform distribution of turbulence (Cn2)

However, much steeper increase at DMs larger than ~100 pc cm-3

Cordes & Lazio (2005)

Bhat et al. (2004)

DM

Pul

se B

road

enin

g (m

s)

The “composite” Electron density spectrum within ~1 kpc

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

Armstrong, Rickett & Spangler (1995)

100 km 1 AU 1 pc

20 yr DM variations of PSR B1937+21

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).

Ramachandran et al. (2006)

Estimation of inner scale

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

Bhat et al. (2004)

Estimation of outer scale

• 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

Haverkorn et al. (2008)

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

Interaction of Local Bubble

and Loop I

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)

Bhat & Gupta (2002)

Loop I

Local Bubble

Local Bubble

Loop I

Egger & Aschenbach (1998)

Refractive scintillation in ISM: dynamic spectra vary with time

Bhat, Rao & Gupta (1999)

Multiple imaging (fringing) event

PSR B1133+16 Time span = 2 months

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)

Daily scintillation monitoring of B0329+54

Wang, Yan, Manchester, Wang (2008)

Quasi-continuous observations, sampling the primary (dynamic) spectrum every 90 mins

Tests of quantitative predictions of theories (Romani, Narayan & Blandford 1986)

• Previous work - Stinebring et al. (1996); Bhat et al. (1999) [18 pulsars]

• Bottomline - results from observations are inconclusive!

The phenomenon of “scintillation arcs”

Primary (dynamic spectrum) Secondary spectrum2D 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

“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

Scintillation arcs opening up several new applications

Putney & Stinebring (2007)

Curvature of scintillation arc:

Effective velocity b/w pulsar, observer and the medium (screen):

Effects of DM changes - a result from Parkes timing array

Monitoring DM changes and applying corrections improve timing precision

You et al. (2007)

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!

Time variability of scattering delay

Changes in scattering delay are of the order of timing precision

Hemberger & Stinebring (2008)

Scattering delays from monitoring scintillation arcs over months

Important to assess effects on arrival times - a yet another source of low frequency noise!

New LF arrays will open up a new era in ISM studies

LWA LOFAR

MWA

• Large collecting area = raw sensitivity

• wide fields of view ~ 10 to 50 degree

• multiple tied array beams

• continuous frequency coverage

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

LOFAR/MWA surveys will discover lots of new pulsars!

MWA simulated pulsars (Bailes) ~600-800 pulsars (van Leeuwen) ~1000 pulsars with LOFAR (van Leeuwen)

Scattering measurements - current census

Bhat et al. (2004)

Only 370 measurements so far (out of ~1800 pulsars known)!

Scattering measurements - current census

Bhat et al. (2004)

Only 370 measurements so far (out of ~1800 pulsars known)!

Departures from current models (NE2001) - on a logarithmic scale

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|

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

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!)

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

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?