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New electron density model (n e & n e ): NE2001 w/ J. Lazio

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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

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

- 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)

- DM too small for distant, high latitude objects
- Distances overestimated for many objects in theGalactic 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 Cn2 q-

SM = ds Cn2 (s)

- DMds neDispersion Measure
- EMds ne2Emission Measure
- RMds neB||Rotation Measure
- SMds 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

- = 11/3 (Kolmogorov value)

- DMds neDispersion Measure
- EMds ne2Emission Measure
- RMds neB||Rotation Measure
- SMds 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

- = 11/3 (Kolmogorov value)

- DMds neDispersion Measure
- EMds ne2Emission Measure
- RMds neB||Rotation Measure
- SMds 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

- = 11/3 (Kolmogorov value)

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

PSR B0919+06

S. Chatterjee et al. (2001)

= 88.5 0.13 mas/yr

= 0.83 0.13 mas

D = 1.2kpc

V = 505 km/s

Brisken et al.

2001; 2002

- 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

- 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

DM vs Galactic longitude for different latitude bins

DM vs Galactic longitude for different latitude bins

DM(psr)-DM(model, )

Asymptotic DM

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 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

- 53 pulsars using VLBA antennas only at 1.4 GHz(systematics: ionospheric phase)
- Chatterjee, Brisken et al. (2002-2004)
- Currently can reach ~ 2 kpc

- Ionosphere less important
- Chatterjee, Cordes et al. (2001-ongoing)

- Initial tests
- Expect to do ~100 pulsars in 5 years, some to 5 kpc

Surveys with Parkes, Arecibo & GBT.

Simulated & actual pulsars shown

Yield ~ 1000 pulsars in ALFA survey

SKApulsar survey

600 s per beam

~104 psr’s

- 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

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

- Radio = standard candles if beaming accounted for

- 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

Taylor & Cordes (1993 ApJ, 411, 674)

- 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

- Lyne & Lorimer 1994:
- Proper motions + TC93 <V> ~ 500 km/s
- Unimodal distribution

- MSP analysis (TC93) <V> ~ 80 km/s

- 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%)

- 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?

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

Standoff radius and flux are consistent

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

Arecibo Multibeam Surveys

Parkes MB Feeds

- |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

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 ?)

Electron density (log gray scale to enhance local ISM)

NE2001 Spiral Arms

Differential TOA from Multipath:

Quenching of pulsations for d > P.

- 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)

- Astronomical:

- DM too small for distant, high latitude objects
- Distances overestimated for many objects in theGalactic 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