1. Jim Cordes SINS Workshop
The Fluctuating Ionized ISM: Galactic Distribution and Related Issues Jim Cordes (Cornell), Joe Lazio (NRL), Ramesh Bhat (Swinburne)
Modeling the warm ionized ISM (primarily)
Measureables:
what scales do we have evidence for? (kpc  few  100 km)
inversion into physical parameters (wavenumber spectrum, etc.)
# sightlines grossly undersamples Galactic structure
Relationship between measures (DM, EM, SM)
Galactic models
Prior vs fitted structures
NE2001 (Cordes & Lazio 2003); supercedes TC93
NE200X: new data and new priors (esp. parallaxes out to 4 kpc)
Constraints on the ionized IGM
Role of foreground plasma (IPM, ISM)
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Jim Cordes SINS Workshop

2. Jim Cordes SINS Workshop
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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Jim Cordes SINS Workshop

3. Jim Cordes SINS Workshop
Observables
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Jim Cordes SINS Workshop

4. Jim Cordes SINS Workshop
Observables
ℓ1= inner scale
ℓ0= outer scale
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5. Jim Cordes SINS Workshop
Observables
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Jim Cordes SINS Workshop

6. Galactic Structures Necessary to Explain DM data
Thin disk
Thick disk
Spiral arms
Local ISM components
Clumps of enhanced ne
Voids of low ne
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7. NE2001: Galactic Distribution of Free Electrons + Fluctuations
Paper I = the model (astro-ph/ )
Paper II = methodology & particular lines of sight (astro-ph/ )
Code + driver files + papers:
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Jim Cordes SINS Workshop

8. Jim Cordes SINS Workshop
Asymptotic DM
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Jim Cordes SINS Workshop

9. Empirical Distances vs. Model Distances
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Jim Cordes SINS Workshop

10. The highest measured velocity using direct distance measurement
Possibly born in Cyg OB 6
,b = 91.3o, 52.3o
D = 2.450.25 kpc
V = km s-1
P = 0.74 s
B = 2x1012 G
s = P/2Pdot = 2.36 Myr
The highest measured velocity using direct distance measurement
2.5x further than electron density model based distance estimate (NE2001)
The orbit of B with respect to the Cygnus Superbubble and the
Cygnus OB associations using an age of 2.34 Myr and a radial velocity
of 200 km/s. The solid dot denotes the pulsars current position and
the thick solid line its orbit traced back in time. The dashed circle
represents the Cygnus superbubble while the solid ellipses are the
Cygnus OB associations with positions and extents as tabulated by
Uyaniker et al. (2001). From left to right and top to bottom these are
OB 7, OB 6, OB 4, OB 2, OB 8, OB 9, OB 1, OB 3 and OB 5. The starred
symbols are the Supernova remnants identified in this region. The
solid horizontal line is the galactic plane with the horizontal dashed
lines representing the pulsar scaleheight determined by ACC at the
distance of the Cygnus superbubble.
l,b = 91.3, 52.3 deg + D = 2.45 kpc => (x,y,z) = (1.50, 8.53, 1.94) kpc
X || l=90 deg, y || l=180deg, z perpendicular to the Galactic plane

11. Estimated Wavenumber Spectrum for ne
Constraints on spectrum:
Small scales: (“micro tiny” ?)
Scaling of DISS parameters with 
angle, frequency, time
Scintillation arcs
DM(t) on months to years
DM() in globular clusters
RISS parameters
Large scales:
RM vs 
EM, DM, SM on same LOS
Bow shock contours (Guitar Neb)
All scales:
Cosmic ray scattering on B and linkage ne/ne ~ B/B
 ~ 11/3
see also Armstrong, Rickett, Spangler 1995
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Jim Cordes SINS Workshop

12. Methods for Probing ne on Small Scales
Technique
Quantity
Good for:
Results
Scaling of visibility function with baseline
(b)
 = spectrum index
ℓi = inner scale
ℓi  few x 100 km
Image anisotropy
(vector b)
anisotropy of ne
Up to 3:1 for high scattering
Scaling of DISS parameters with 
d()
d()
, ℓi
ℓi  300 – 800 km
Scintillation Arcs
arc parameters
, ℓi, anisotropy of ne
Existence  <4, inner scale “small”
DM vs time
DM vs angle on sky
DM(t)
DM()
, ℓ0 = outer scale
, outer scale
  11/3
ℓ2 > 10 AU
RISS and DISS modulation indices 
DM, EM, SM on same LOS & distance
EM, DM,SM
ℓ0 =outer scale
filling factor
RM vs angle on sky
RM()
, ℓ0
Constraints on ℓ0
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Jim Cordes SINS Workshop

13. Summary of Constraints on Length Scales in WIM
Spectral index of wavenumber spectrum
  11/3  0.3 commonly inferred
Exceptions most likely due to special geometries or inner scale effects (e.g. “anomalous” pulse broadening)
Excess power inferred on ~ AU scales from refractive ISS probably due to deterministic structures
Outer scale:
At least ~ 100 AU from DM(t) measurments
~ 0.01 pc in high scattering regions with EM obs
> 1 pc inferred from EM, DM, SM of thick disk component
Inner scale:
100 to 103 km from visibility, pulse broadening measurements
Scintillation arcs (TBD)
N.b. proton gyro-radius
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Jim Cordes SINS Workshop

14. Jim Cordes SINS Workshop
Pulse Broadening
Bhat, Cordes et al. 2004, unpub.
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Jim Cordes SINS Workshop

15. Scaling of Pulse Broadening with DM and 
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16. Jim Cordes SINS Workshop
Scaling of pulse broadening with DM and 
c.f. similar constraints on inner scale from Moran et al. 1990; Spangler and Gwinn 1990; Molnar et al based on angular broadening measurements of AGNs and Cyg X-3
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Jim Cordes SINS Workshop

17. Jim Cordes SINS Workshop
Constraints from Arcs
c.f. Dan Stinebring’s talk
Large inner scales and media with single scales or steep wavenumber spectra (4) suppress arcs
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Jim Cordes SINS Workshop

18. Jim Cordes SINS Workshop
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
F varies by 100 between inner and outer Galaxy
 Change in porosity from different SFR?
small F
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Jim Cordes SINS Workshop

19. Jim Cordes SINS Workshop
NE200X
3 more lines of sight with pulsar parallaxes
Brisken, Chatterjee et al. published, unpublished
~ 1000 more pulsar DM values
DM() on globular-cluster scales (dozens)
~150 new pulse-broadening measurements
Use scintillation arcs for local ISM screens
More linkage between optical and radio tracers
Alternative Sun-GC distance
New spiral arm definitions
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Jim Cordes SINS Workshop

20. The SKA as a Pulsar/Gravity/ISM Machine
Relativistic binaries (NS-NS, NS-BH) for probing strong-field gravity
Orbit evolution of pulsars around Sgr A*
Millisecond pulsars < 1.5 ms (EOS)
MSPs suitable for gravitational wave detection
100s of NS masses (vs. evolutionary path, EOS, etc)
Galactic tomography of electron density and magnetic field; definition of Milky Way’s spiral structure
Target classes for multiwavelength and non-EM studies (future gamma-ray missions, gravitational wave detectors)
Millisecond Pulsars
Relativistic Binaries
Today
Future
Today
Future
Pulsars as gravitational laboratories and gravitational wave detectors are one of the five key areas.
The primary source classes to be bound are
(a) relativistic binaries (NS-NS, NS-BH)
(b) Millissecond pulsars
The Galactic center is a primary target:
(a) Identifying radio pulsars there and timing them requires observations at > 9 GHz to combat radio-wave scattering;
(b) current instrumentation appears to have insufficient sensitivity to detect pulsars at these frequencies in the GC
(c.f. attempts with Parkes (64m), Effelsberg (100m) and the GBT (100m)
The yield from a Galactic survey of binary pulsars and MSPs is terrific!
Not shown on the slide are the prospects for detecting single, giant pulses from local group galaxies
and probably as far as the Virgo cluster. Such pulses can be used to characterize the local intergalactic medium.
SKA
SKA
Blue points: SKA simulation
Black points: known pulsars
only 6!
~104 pulsar detections
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Jim Cordes SINS Workshop

21. Scattering in the IGM? (Lazio, Cordes, Fey in preparation)
SM vs Galactic latitude
AGNs
pulsars
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Jim Cordes SINS Workshop

22. Jim Cordes SINS Workshop
IGS Summary
Contribution from IGM is still very uncertain
IPS contribution
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  selected lines of sight
Empirically: accuracy of foreground Galactic model is crucial and perhaps not sufficient
Progress:
Angular broadening vs l,b for large number of AGNS
Search for AGNs and measure scattering through known galaxies and clusters
Secondary spectrum analyses of IDVs and GRB afterglows to get best estimates of angular size “seen” by the ISM
LOFAR, SKA: can measure large numbers of sources though wide-field studies
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Jim Cordes SINS Workshop

23. Jim Cordes SINS Workshop
Summary
Strong evidence exists for an inner scale on LOS toward heavily scattered pulsars (few  100 km)…not inconsistent with the proton gyro-radius of WIM (or ion-inertial scale) in typical ISM B fields
The fluctuation parameter F = (ne/ne)2 / f ℓ02/3 varies by a factor of 100 between the solar circle and inner Galaxy; this may reflect the larger SFR in the inner Galaxy
Available LOS indicate the broad structures needed for a Galactic model for ne and ne (thin, thick disks, spiral arms, strong GC component, clumps and voids) (NE2001)
A new model NE200X (X=6, maybe 7) is in the works that will make use of new parallaxes, pulse broadening, new methods for modeling the local ISM (arcs)
VLBI on large samples (esp. IDV sample) can constrain or detect IGM scattering
The SKA will provide adequate LOS to critically sample most HII structures in the Galaxy, define spiral arms from pulsar birth sites
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Jim Cordes SINS Workshop

24. Jim Cordes SINS Workshop
Extra Slides
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Jim Cordes SINS Workshop

25. Jim Cordes SINS Workshop
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:
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
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26. Jim Cordes SINS Workshop
Local ISM components & results
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27. Jim Cordes SINS Workshop
Model Components
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28. Galactic Center Component
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29. Jim Cordes SINS Workshop
Thin disk
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30. Jim Cordes SINS Workshop
Thick disk (1 kpc)
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31. Jim Cordes SINS Workshop
Spiral arms
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32. Jim Cordes SINS Workshop
DM vs Galactic longitude for different latitude bins
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33. Jim Cordes SINS Workshop
DM vs Galactic longitude for different latitude bins
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34. 134 of 1143 TC93 distances are lower bounds
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35. Jim Cordes SINS Workshop
DM(psr)-DM(model, )
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36. Jim Cordes SINS Workshop
Asymptotic DM
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Jim Cordes SINS Workshop

37. Estimated Wavenumber Spectrum for ne
Constraints on spectrum:
Small scales:
Scaling of DISS parameters with 
angle, frequency, time
Scintillation arcs
DM(t) on month to years
DM() in globular clusters
RISS parameters
Large scales:
RM vs 
EM, DM, SM on same LOS
Bow shock contours (Guitar Neb)
All scales:
Cosmic ray scattering on B and linkage ne/ne ~ B/B
Slope ~ -11/3
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Jim Cordes SINS Workshop

38. Jim Cordes SINS Workshop
Pulse Broadening
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Jim Cordes SINS Workshop

39. Constraints on the Inner Scale From DISS Scaling
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40. Jim Cordes SINS Workshop
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41. Jim Cordes SINS Workshop
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Jim Cordes SINS Workshop

42. Is the DM distance Realistic? Yes
Standoff radius and flux are consistent
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43. Jim Cordes SINS Workshop
Galactic Center Region
Sgr A* = 3106 black hole with a surrounding star cluster with ~ 108 stars. Many of these are neutron stars.
Detecting pulsars in Sgr A* is difficult because of the intense scattering screen in front of Sgr A*.
Multipath differential arrival times
d ~ 2000 ν-4 sec
Solution: high sensitivity at high frequency
327 MHz VLA image
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Jim Cordes SINS Workshop

44. Jim Cordes SINS Workshop
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Jim Cordes SINS Workshop

45. Jim Cordes SINS Workshop
Observables
l1= inner scale
l0= outer scale
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Jim Cordes SINS Workshop

46. Jim Cordes SINS Workshop
Scaling of pulse broadening with DM and 
c.f. similar constraints on inner scale from Moran et al. 1990; Spangler and Gwinn 1990; Molnar et al based on angular broadening measurements of AGNs and Cyg X-3
9/21/2018
Jim Cordes SINS Workshop