A New Model for the Galactic Electron Density & its Fluctuations
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A New Model for the Galactic Electron Density & its Fluctuations J. M. Cordes, Cornell University [email protected] BU Milky Way Workshop 17 June 2003. New electron density model (n e & n e ): NE2001 w/ J. Lazio How different from Taylor & Cordes ’93 and other models?

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A New Model for the Galactic Electron Density & its FluctuationsJ. M. Cordes, Cornell [email protected] 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
NE2001 (uses data through 2001) Fluctuations

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
Why detailed modeling? Fluctuations

  • 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
Deficiencies of TC93 Fluctuations

  • 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 Fluctuationsne

Similar to Armstrong, Rickett & Spangler (1995)

Slope ~ -11/3

Spectrum = Cn2 q-

ne2 = d3q Cn2 q-

SM = ds Cn2 (s)


Integrated measures
Integrated Measures Fluctuations

  • 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 measures1
Integrated Measures Fluctuations

  • 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 measures2
Integrated Measures Fluctuations

  • 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
Independent Pulsar Distances Fluctuations

  • Parallaxes: Pulse timing Interferometry

  • Associations: Supernova remnants Globular clusters

  • HI Absorption:Galactic rotation


Very long baseline array

PSR B0919+06 Fluctuations

S. Chatterjee et al. (2001)

 = 88.5  0.13 mas/yr

 = 0.83  0.13 mas

Very Long Baseline Array

D = 1.2kpc

V = 505 km/s


Brisken et al. Fluctuations

2001; 2002


Ne2001
NE2001 Fluctuations

  • Goal is to model ne(x) and Cn2(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


Ne20011
NE2001 Fluctuations

  • 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



Model components
Model Components Fluctuations



Thin disk
Thin disk Fluctuations


Thick disk 1 kpc
Thick disk (1 kpc) Fluctuations


Spiral arms
Spiral arms Fluctuations





DM(psr)-DM(model, Fluctuations)


Asymptotic DM Fluctuations


Spatial fluctuations in n e
Spatial fluctuations in n Fluctuationse

recall dSM = Cn2 ds  F ne2 ds  F ne dDM

F = “fluctuation parameter” varies widely over Galaxy

F  (dne / 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: dne / ne ~ 1


dSM Fluctuations F ne dDM

F  (dne / ne )2 / f (outer scale)2/3

Evidence for variations in turbulence properties between inner & outer Galaxy

large F

small F


Selected applications
Selected Applications Fluctuations


New parallax programs
New Parallax Programs Fluctuations

  • 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 Fluctuations with Parkes, Arecibo & GBT.

    Simulated & actual pulsars shown

    Yield ~ 1000 pulsars in ALFA survey


    SKA Fluctuationspulsar survey

    600 s per beam

    ~104 psr’s


    Comments summary
    Comments & Summary Fluctuations

    • 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 n e d n e
    Modeling the Galactic n Fluctuationse& dne

    • mean & fluctuations are modelled

    • dSM = Cn2 ds  F ne2 ds  F ne dDM F = “fluctuation parameter” varies widely over Galaxy

    • ne ~ Cn (outer scale)1/3

    • possible/probable dne / ne ~ 1

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


    Electron density of TC93 Fluctuations

    Taylor & Cordes (1993 ApJ, 411, 674)


    Ne20012
    NE2001 Fluctuations

    • 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
    Pulsar Velocities Fluctuations

    • 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 Fluctuations

    How might the results change using NE2001 instead of TC93?


    Guitar nebula psr b2224 65
    Guitar Nebula & PSR B2224+65 Fluctuations

    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 yes
    Is the DM distance Realistic? Yes Fluctuations

    Standoff radius and flux are consistent


    Pulsar velocities using only objects with parallax measurements
    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


    Arecibo Multibeam Surveys measurements

    Parkes MB Feeds


    I arecibo galactic plane survey
    I. Arecibo Galactic-Plane Survey measurements

    • |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
    II. High Galactic Latitude Survey measurements

    Search for:

    • 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 ?)



    Differential TOA from Multipath: measurements

    Quenching of pulsations for d > P.


    Ne2001 new model cordes lazio 2002 astro ph july www astro cornell edu cordes ne2001
    NE2001 = New Model measurementsCordes & Lazio 2002 astro-ph Julywww.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 tc931
    Deficiencies of TC93 measurements

    • 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


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