Radio Diagnostics of Turbulence in the Interstellar & Intergalactic media

J. M. Cordes, Cornell Universitycordes@astro.cornell.edu

URSI 20 August 2002

• Probes of free electrons in the Galaxy and intergalactic medium (integrated measures)

• Why model ne & ne (mean and fluctuations) in the Galaxy?

• Conceptual infrastructure – multiphase components of the ISM

– Kolmogorov-like turbulence in ionized components the Galaxy?

• Modeling methods

• NE2001 = new release (July 2002)

• Applications & implications

• Preliminary results for the intergalactic medium

Why detailed modeling?• Distance scale for neutron stars

– Neutron star populations– Birth/death rates– Correlations with supernova remnant

• Turbulence in Galactic plasma• Galactic magnetic fields (deconstructing Faraday

rotation measures)• Interpreting scintillations of sources at

cosmological distances (AGNs, GRBs)• Baseline model for exploring the intergalactic

medium (dispersion & scattering in ISM, IGM)

Integrated Measures

• DM ds ne Dispersion Measure

• EM ds ne2 Emission Measure

• RM ds ne B Rotation Measure

• SM ds Cn2 Scattering Measure

Spectrum = Cn2 q-, q = wavenumber

(temporal spectrum not well constrained,

relevant velocities ~ 10 km/s)

= 11/3 (Kolmogorov value)

Scales ~ 1000 km to > pc

INTERSTELLAR DISPERSIONINTERSTELLAR DISPERSION

DM = 0D ds ne(s)

Known for ~1200 pulsars

DM ~ 2 to 1100 pc cm-3

Variable at ~10-3 pc cm-3

Variations with d,l,b show obvious Galactic structure

• Electron density irregularities exist on scales from ~ 100’s km to ~ pc as approximately a power-law spectrum (~ Kolmogorov)

• Pulsar velocities >> ISM, observer velocities500 km/s average (100 to 1700 km/s)

• Extragalactic sources: ISM, observer velocitiesdetermine time scales of scintillation~ 20 km/s

• Scattering is `strong’ for frequencies < 2 GHz

Interstellar Scattering Effects Used

• Angular broadening (seeing)

• Pulse broadening

• Diffractive interstellar scintillations (DISS)d = / ld , ld = diffraction scaled = scintillation bandwidth

=> Scattering Measure SM

Pulse broadeningPulse broadening Pulse broadening vs DMPulse broadening vs DM

Angular broadeningAngular broadeningDiffractive Scintillation Diffractive Scintillation

Dynamic spectrumVisibility functions:

Pulse broadening (recent Arecibo results, R. Bhat et al)

~ D2/2c -4

Low DM pulsar, no broadening High DM pulsar with broadening

SM = 0.92 ( / D)5/6 11/3 = scattering measure

Estimated Wavenumber Spectrum for ne

Similar to Armstrong, Rickett & Spangler (1995)

Slope ~ -11/3

Spectrum = Cn2 q-

SM = LOS integral of Cn

2

DM vs Galactic latitude for different longitude bins

SM vs latitude

Independent Pulsar Distances

• Parallaxes: Pulse timing Interferometry

• Associations: Supernova remnants Globular clusters

• HI Absorption: Galactic rotation

Very Long Baseline Array

PSR B0919+06S. Chatterjee et al. (2001) = 88.5 0.13 mas/yr = 0.83 0.13 mas

D = 1.2kpcV = 505 km/s

Brisken et al.

2001

NE2001 = New Model Cordes & Lazio 2002 astro-ph July

www.astro.cornell.edu/~cordes/NE2001

• Goal is to model ne(x) and Cn2(x) in the Galaxy

• Software to the community (cf web site)• Supercedes earlier model (Taylor & Cordes 1993, ApJ)• Investigate application spinoffs:

– Astronomical: • scattering degradation of pulsar surveys

• Imaging surveys at low frequencies (LOFAR, SKA)

• SETI

– Astrophysical:• Physics of interstellar turbulence

• Connection to magnetic fluctuations & CR propagation (scales probed match CR gyroradii over wide energy range)

NE2001 = New Model Cordes & Lazio 2002 astro-ph July

www.astro.cornell.edu/~cordes/NE2001

• Input data {DM, EM, SM, [DL, DU] = distance ranges}

• Prior input: – Galactic structure, HII regions, spiral-arm loci

– Multi- constraints on local ISM (H, NaI, X-ray)

• Figures of merit:– N> = number of objects with DM > DM (model) (minimize)

– Nhits = number of LOS where predicted = measured distance: d(model) [DL, DU] (maximize)

– L = likelihood function for distances & scattering (maximize)

• Basic procedure: get distances right first, then get scattering (turbulence) parameters

NE2001 = New Model Cordes & Lazio 2002 astro-ph July

• x2 more lines of sight (D,DM,SM) [114 with D/DM, 471 with SM/D or DM] (excludes Parkes MB obj.)

• Local ISM component (new) (new VLBI parallaxes)[12 parameters]

• Thin & thick disk components (as in TC93) [8 parameters]

• Spiral arms (revised from TC93) [21 parameters]

• Galactic center component (new)[3 parameters] (+auxiliary VLA/VLBA data ; Lazio & Cordes

1998)

• Individual clumps/voids of enhanced dDM/dSM (new)[3 parameters x 20 LOS]

• Improved fitting method (iterative likelihood analysis)penalty if distance or SM is not predicted to within the errors

NE2001 Spiral Arms

Electron density (log gray scale to enhance local ISM)

Local ISM components & results

Selected ApplicationsGalactic turbulence

anisotropy of fluctuations

relation to B and CR prop’n

expect correlations of

-ray emission & scattering

(GLAST needed)

IGM in local group

M33 giant pulses from

Crab-like pulsars DM,SM

IGM on cosmological scales

scattering/scint’n of AGNs

by intervening galaxies, Ly

clouds, turbulence in cluster

gas, HII regions at EOR

GRB & IDV scintillations

source sizes vs. t

ambient medium

IGM

Spatial fluctuations in ne

recall dSM = Cn2 ds F ne

2 ds F ne dDM

F = “fluctuation parameter” varies widely over Galaxy

F (ne / ne )2 / f (outer scale)2/3

(f = volume filling factor of ionized cloudlets)

F varies by >100 between outer/inner Galaxy

change in ISM porosity due to change in star formation rate (?)

outer scale ~ 0.01 pc in HII shells, GC > 1 pc in tenuous thin disk

estimate: ne / ne ~ 1

dSM F ne dDM

F (ne / ne )2 / f (outer scale)2/3

small F

large F

Evidence for variations in turbulence properties between inner & outer Galaxy

Constraints on IGM Scattering(work in progress with J. Lazio)

• Apparent scattering excess over Galactic scattering for some high-z objects

• Strong upper bounds on source size ‘seen’ by ISM for IDV sources that display RISS at ~5 GHz

• Ionized IGM contains most of baryons in the Universe: m b ~ 0.05. To satisfy observations, need scattering regions more numerous than L* galaxies.

Scattering of high z AGNs: Interstellar + Intergalactic ?

-2.2

Lazio et al. (unpublished)

Summary / Future

• 1500 lines of sight

• Reasonably detailed modeling of the Galaxy

• Galaxy contains significant, unsampled structures on large and small scales

• VLBI astrometry parallaxes on ~100 LOS in next few years

• Pulsar surveys will yield > 2000 pulsars (Arecibo MB) and ~ 104 pulsars (SKA)

definition of spiral arms complete sampling of significant HII regions

• Scattering may yield a unique probe of the ionized IGM

H Images of Pulsar Bow Shocks

Guitar Nebula (1600 km/s)MSP J0437-47 (100 km/s)

www.astro.cornell.edu/~shami/psrvlb

Modeling the Galactic ne & ne

• mean & fluctuations are modelled

• dSM = Cn2 ds F ne

2 ds F ne dDM F = “fluctuation

parameter” varies widely over Galaxy

• ne ~ Cn (outer scale)1/3

• outer scale ~ 0.01 to > 1 pc

• estimate: ne / ne ~ 1

Distance prediction on large scales