A New Model for the Galactic Electron Density & its Fluctuations

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

BU Milky Way Workshop 17 June 2003

• New electron density model (ne & ne): NE2001 w/ J. Lazio

• How different from Taylor & Cordes ’93 and other models?

• Ingredients and performance

• VLBI astrometry = breakthrough

• Arecibo + GBT + VLA + Effelsburg + Jodrell = parallax machine

• Square Kilometer Array = Mother of all parallax machines

• Future modeling: radio+CO, radio+H, radio + -rays (GLAST)

• Future pulsar surveys (Arecibo/ALFA, SKA)

w/ S. Chatterjee, W. Brisken, M. Goss, S. Thorsett

NE2001 (uses data through 2001)

Paper I = the model (astro-ph/0207156)

Paper II = methodology & particular lines of sight (astro-ph/0301598)

Code + driver files + papers:www.astro.cornell.edu/~cordes/NE2001

Why detailed modeling?• Distance scale for neutron stars

– Neutron star populations (space density, luminosities)– Birth/death rates– Correlations with supernova remnants

• Designing Radio Pulsar Surveys• 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)

Deficiencies of TC93

• DM too small for distant, high latitude objects• Distances overestimated for many objects in the

Galactic plane (10% of now-known objects have DMs too large to be accounted for)

• Pulse broadening over/underestimated in some directions

• Spiral arms incompletely defined over Galaxy• No Galactic center component

Estimated Wavenumber Spectrum for ne

Similar to Armstrong, Rickett & Spangler (1995)

Slope ~ -11/3

Spectrum = Cn2 q-

ne2 = d3q Cn

2 q-

SM = ds Cn2 (s)

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

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

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

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; 2002

NE2001• Goal is to model ne(x) and Cn

2(x) Fne2(x) in the Galaxy

• 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 using distances & scattering (maximize)

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

NE2001• 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

Local ISM components & results

Model Components

Galactic Center Component

Thin disk

Thick disk (1 kpc)

Spiral arms

DM vs Galactic longitude for different latitude bins

DM vs Galactic longitude for different latitude bins

134 of 1143 TC93 distances are lower bounds

DM(psr)-DM(model, )

Asymptotic DM

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

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

New Parallax Programs

• 53 pulsars using VLBA antennas only at 1.4 GHz (systematics: ionospheric phase)

• Chatterjee, Brisken et al. (2002-2004)

• Currently can reach ~ 2 kpc

• 6 strong pulsars, VLBA-only at 5 GHz • Ionosphere less important

• Chatterjee, Cordes et al. (2001-ongoing)

• VLBA + Arecibo + GBT + … • Initial tests

• Expect to do ~100 pulsars in 5 years, some to 5 kpc

• Future: SKA superior phase calibration, sensitivity, can reach >10 kpc

Surveys with with Parkes, , Arecibo & GBT.

Simulated & actual pulsars shown

Yield ~ 1000 pulsars in ALFA survey

SKA pulsar survey

600 s per beam

~104 psr’s

Comments & Summary• NE2001 = large improvement over TC93

• Caveat: HII regions, etc are grossly undersampled by available LOS• Need ~ 104 DMs to adequately model the MW from pulsars alone

• Large-scale structures are imposed and parameterized • VLBI (esp. with Arecibo, GBT, Jodrell, Effelsberg, etc) will

yield many new parallaxes, obviating the need for DM distances for ~100 pulsars in a few yr

• New pulsar surveys will double sample in ~ 5 yr• Next version (NE200X) will

• Use scattering measurements of Parkes Multibeam sample• Define spiral arms more empirically using pulsar + HI, H, CO results

• Other distance approaches possible:• Radio = standard candles if beaming accounted for

• Expect tighter LX , L with better distance models.

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

• possible/probable ne / ne ~ 1

• not clear that ne on all scales due to same process

Electron density of TC93

Taylor & Cordes (1993 ApJ, 411, 674)

NE2001 • x2 more lines of sight (D,DM,SM) [114

with D/DM, 471 with SM/D or DM] (Parkes MB in next version)

• Local ISM component (new)[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’ of enhanced DM/SM (new)[5 parameters per clump] (Voids also)

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

Pulsar Velocities• Lyne & Lorimer 1994:

• Proper motions + TC93 <V> ~ 500 km/s

• Unimodal distribution

• Cordes & Chernoff 1997:• MSP analysis (TC93) <V> ~ 80 km/s

• Cordes & Chernoff 1998: • High-field pulsars (TC93), < 10 Myr, 3D velocities (z/t)

• No correction for selection effects bimodal V, 1~ 175 km/s, 2~ 700 km/s (14%)

• Arzoumanian, Chernoff & Cordes 2002:• Full analysis (beaming, selection effects, TC93) bimodal V, 1~ 90 km/s, 2~ 500 km/s (40%)

ACC ‘02

How might the results change using NE2001 instead of TC93?

Guitar Nebula & PSR B2224+65

Edot ~ 1033 erg/s P~0.6 sec

D(TC93) = 2 kpc V~1700 km/s

D(NE2001) = 1.7 kpc V~1450 km/s

H Palomar 5-m image

Is the DM distance Realistic?

Is the DM distance Realistic? Yes

Standoff radius and flux are consistent

Pulsar velocities using only objects with parallax measurements

Distribution shows high-velocity tail and is “not inconsistent” with ACC results on high-field pulsars and CC97 on MSPs

Parkes MB Feeds

Arecibo Multibeam Surveys

I. Arecibo Galactic-Plane Survey

• |b| < 5 deg, 32 deg < l < 80 deg

• 1.5 GHz total bandwidth = 300 MHz

• digital correlator backend (1024 channels)(1st quadrant available = WAPP)

• multibeam system (7 feeds)

• ~300 s integrations, 3000 hours total

• Can see 2.5 to 5 times further than Parkes(period dependent)

• Expect ~500 to 1000 new pulsars

II. High Galactic Latitude Survey

• Millisecond pulsars(z scale height ~ 0.5 kpc)

• High-velocity pulsars (50% escape) (scale height = )

• NS-NS binaries (typical z ~ 5 kpc)

• NS-BH binaries (typical z ~ few kpc ?)

Search for:

NE2001 Spiral Arms

Electron density (log gray scale to enhance local ISM)

Differential TOA from Multipath:

Quenching of pulsations for d > P.

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)

Deficiencies of TC93

• DM too small for distant, high latitude objects• Distances overestimated for many objects in the

Galactic plane (10% of now-known objects have DMs too large to be accounted for)

• Pulse broadening over/underestimated in some directions

• Spiral arms incompletely defined over Galaxy• No Galactic center component