Galactic Electron Density Models and Constraints on Scattering in the IGM Jim Cordes Cornell Univers

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Galactic Electron Density Models and Constraints
on Scattering in the IGMJim CordesCornell
University

• NE2001 Galactic Electron Density Model
• ne and ?ne (as Cn2)
• Relationship to TC93
• Input Data
• Structure of Model
• Implications
• Availability
• New Pulse Broadening Times
• New Parallaxes
• Constraints on the IGM
• New Pulsars

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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,
  velocities, 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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Integrated Measures

• DM ?ds ne Dispersion Measure
• EM ?ds ne2 Emission Measure
• RM ?ds neB 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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Dispersion Measures
Pulse broadening times
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Dynamic Spectrum Diffractive Interstellar
Scintillations
Dnd
2D Autocorrelation Function ? Characteristic DISS
frequency and time scales
Dtd
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Angular broadening
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Pulsar Distances
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New Parallaxes from VLBI
Brisken et al. (2002) Chatterjee et al. (2004)
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PSR B192910
Chatterjee et al. 2003
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Brisken et al. 2001 2002
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ne in local ISM (Brisken PhD thesis)
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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) (relies on 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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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

• 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
• F 105 in the Galactic center scattering
  region
• Small dDM can ? large dSM
• (bears on issue of IDV screens)
• ? 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
large F
Evidence for variations in turbulent properties
between inner outer Galaxy
small F
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Implications and Comments

• Large scale features of NE2001 are required by
  available data
• Clumps of excess SM do not correlate well with
  known HII regions or other features
• The available lines of sight grossly undersample
  fine-scale structure in the Galaxy, thus
  limiting the predictive value of the model (the
  SKA will alter this) for specific lines of sight
• ISS limits pulsar surveys even at 1.4 GHz (need
  gt8 GHz for GC)
• Next model NE200X
• Parkes MB pulsar scattering measurements
• Redefined spiral arms and Sun-GC distance
• Expect VLBI parallaxes to double in number

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Screen shot of NE2001 usage (command-line mode)
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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, Vlemmings, 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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New Pulse Broadening Measurements(Bhat et al.
2004)

• 96 pulsars
• Parkes MB Survey pulsars in Arecibo dec range
• uninteresting Hulse-Taylor pulsars
• 0.43 2.4 GHz (3 to 4 frequencies)
• Deconvolution of pulse broadening functions
• PBF1 one sided exponential (thin screen)
• PBF2 finite rise, slower decay (thick medium)
• Some LOS prefer one or the other PBFs
• Scaling with frequency depends on PBF form
• Identification of inner scale in global fit to
  all pulse broadening times

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Example of pulsar that gives better fit using
PBF2 thick medium case
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0.43 1.18 1.48 2.4 GHz
Some pulsars are too scattered to detect at 0.43
GHz, others too weak at 2.4 GHz
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Identifying an inner scale in the scaling law for
pulse broadening ?d ? ?x with x different for
different levels of scattering
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c.f. similar constraints on inner scale from
Moran et al. 1990 Spangler and Gwinn 1990
Molnar et al. 1996(?) based on angular broadening
measurements of AGNs and Cyg X-3
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Scattering in the IGM
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SM vs Galactic latitude and z
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Cosmological Integrals

• IGM Fresnel scale 103 times larger than for ISM
• Need to transform relevant integrals, e.g.
• ?(x?,D) -re ?0D ds ? ne(x?,s)
• ?(x?,zs) -re?0cH0 ?0zs dz ne(x?,s)
  (1z)-2/E(z)
• E(z) ?m(1z)3 1 - ?m)1/2
• Takes into account that ne larger but ? smaller
  at higher z, etc
• Phase structure function samples autocorrelation
  of ne at spatial lag ?b ? 0 as z?zs faster than
  in flat space.
• ? higher weighting of low-z material (extent
  depends on the particular observable)

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Baseline vs. z
zs
b(z)
z 0
b0
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Aspects of Intergalactic Scattering

• Intervening galaxies
• c.f. 1 mas scattering ? Galactic plane
• Expect 1 L galaxy across LOS at z2
• IGM clouds
• Most of IGM is ionized
• Filling factor, turbulent level not known
• Could have large-enough F parameter to produce
  measureable SM

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IGS Summary

• Contribution from IGM is still very uncertain
• Empirically accuracy of foreground Galactic
  model is crucial and perhaps not sufficient
• General IGM no knowledge of what the level of
  turbulence really is, though the ingredients
  (ionized gas, shocks, winds) are there
• Intervening galaxies will contribute to some
  LOS ? selection effects
• Progress
• Angular broadening vs l,b for large number of
  AGNS
• Search for AGNs and measure scattering through
  known galaxies
• LOFAR, SKA can measure large numbers of sources
  though wide-field studies

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New Pulsars

• Ongoing Parkes surveys
• Arecibo ALFA surveys ? 1000 new pulsars over the
  next 5 yr
• SKA 104 pulsars
• Full sampling of Galactic plane
• Sample every significant HII region, solve for
  magnetic field, etc.

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