A New Model for the Galactic Electron Density

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A New Model for the Galactic Electron Density
its FluctuationsJ. M. Cordes, Cornell
Universitycordes_at_astro.cornell.eduBU 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 radioCO, radioH?, radio
  ?-rays (GLAST)
• Future pulsar surveys (Arecibo/ALFA, SKA)

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

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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

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Estimated Wavenumber Spectrum for ?ne
Similar to Armstrong, Rickett Spangler (1995)
Slope -11/3
Spectrum Cn2 q-? ?ne2? ?d3q Cn2 q-? SM ?ds
Cn2 (s)
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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 gt pc

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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 gt pc

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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 gt pc

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Independent Pulsar Distances

• Parallaxes Pulse timing
  Interferometry
• Associations Supernova remnants Globular
  clusters
• HI Absorption Galactic rotation

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Very Long Baseline Array
PSR B091906 S. Chatterjee et al. (2001) ? 88.5
? 0.13 mas/yr ? 0.83 ? 0.13 mas
D 1.2kpc V 505 km/s
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Brisken et al. 2001 2002
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NE2001

• 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
• Ngt number of objects with DM gt 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

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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

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Local ISM components results
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Model Components
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Galactic Center Component
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Thin disk
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Thick disk (1 kpc)
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Spiral arms
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DM vs Galactic longitude for different latitude
bins
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DM vs Galactic longitude for different latitude
bins
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134 of 1143 TC93 distances are lower bounds
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DM(psr)-DM(model, ?)
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Asymptotic DM
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Spatial fluctuations in ne

• 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 gt100 between outer/inner Galaxy
• ? change in ISM porosity due to change in
• star formation rate (?)
• outer scale 0.01 pc in HII shells, GC
  gt 1 pc in tenuous thin disk
• estimate dne / ne 1

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dSM ? 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
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Selected Applications
Galactic turbulence anisotropy of fluctuations relation to ?B and CR propn 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/scintn 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
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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 gt10 kpc

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Surveys with Parkes, Arecibo GBT. Simulated
actual pulsars shown Yield 1000 pulsars in
ALFA survey
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SKA pulsar survey 600 s per beam 104 psrs
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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.

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Modeling the Galactic ne 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

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Electron density of TC93
Taylor Cordes (1993 ApJ, 411, 674)
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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

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Pulsar Velocities

• Lyne Lorimer 1994
• Proper motions TC93 ? ltVgt 500 km/s
• Unimodal distribution
• Cordes Chernoff 1997
• MSP analysis (TC93) ? ltVgt 80 km/s
• Cordes Chernoff 1998
• High-field pulsars (TC93), lt 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)

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ACC 02
How might the results change using NE2001 instead
of TC93?
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Guitar Nebula PSR B222465
Edot 1033 erg/s P0.6 sec D(TC93) 2 kpc
? V?1700 km/s D(NE2001) 1.7 kpc ?
V?1450 km/s
H? Palomar 5-m image
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Is the DM distance Realistic?
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Is the DM distance Realistic? Yes
Standoff radius and flux are consistent
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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
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Arecibo Multibeam Surveys
Parkes MB Feeds
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I. Arecibo Galactic-Plane Survey

• b lt 5 deg, 32 deg lt l lt 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

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II. High Galactic Latitude Survey
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 ?)

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Electron density (log gray scale to enhance local
ISM)
NE2001 Spiral Arms
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Differential TOA from Multipath Quenching of
pulsations for ?d gt P.
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NE2001 New Model Cordes 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)

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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