interstellar scattering joseph lazio (naval research laboratory) j. cordes, a. fey, s. spangler, b....
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Interstellar ScatteringInterstellar Scattering
Joseph Lazio(Naval Research Laboratory)
J. Cordes, A. Fey, S. Spangler, B. Dennison, B. Rickett, M. Goss, E. Waltman, M. Claussen, D. Jauncey, L. Kedziora-Chudczer, R. Ojha
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Radio-wave Radio-wave ScatteringScattering
re ne ds
Electron density fluctuations
Refractive index fluctuations
Corrugated phase fronts Image distortions (cf.
atmospheric seeing) Characterized by a
scattering measureSM ne
2 dx
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Ionized Interstellar MediumIonized Interstellar Medium
H II regions EM > 104 pc cm-6
Powered by O or B star(s)
Warm ionized medium (WIM)
n ~ 0.1 cm-3
T ~ 8000 K f ~ 0.2 1/6 of O star
luminosityWHAM survey
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Scattering ObservablesScattering Observables Angular broadening
– Pulsars
– Extragalactic sources
– Masers and other Galactic sources
Intensity scintillations– Pulsars
– Extragalactic sources
Pulse broadening/scintillation bandwidth
Pulsars
(Spectral broadening)
Scattering characterized typically by scattering measure
SM ne2 dx
Not really scattering observables, but related observables include Rotation measure Optical emission from diffuse gas ( EM = ne
2 dx) Dispersion measure variations (DM = ne dx) Diffuse gamma-ray emission?
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Radio-wave Scattering AnalysesRadio-wave Scattering Analyses
ne ds
~ 2 SM
Scattering physics– Density spectrum
• Spectral index
• Inner scale
– Scattering genesis
Distribution– “regional”
– Galactic
slope
coefficient SM
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The Density Spectrum and Angular The Density Spectrum and Angular BroadeningBroadening
Point source at infinity
V(b) = e-D(b)/2
Phase structure function
D(b) = [(x) - (x+b)]2D(b) dq Pn(q)
[1–J0(bq)]
Pn(q) q- (or …)
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Density Fluctuation Power Density Fluctuation Power SpectrumSpectrum
Density spectrum in local interstellar medium
Power law, with spectral index near Kolmogorov value– Notable exceptions!– Large dynamic range!
Interstellar plasma has large Reynolds number.
Turbulent processes responsible for density fluctuations(?).
Density spectrum elsewhere in Galaxy similar, probably.
Armstrong, Rickett, & Spangler (1995)
1 pc 1 AU
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Extreme Scattering Extreme Scattering EventsEvents
Events simultaneous at 2.2 and 8.1 GHz
Duration of few weeks to months
intrinsic: Tb 1015 K
extrinsic: AU-sized refracting clouds in our Galaxy
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ESE of 1741-038:ESE of 1741-038:1992 June 20 (18 cm)1992 June 20 (18 cm)
Need a new monitoring program!
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Density Fluctuation Power Density Fluctuation Power SpectrumSpectrum
Armstrong, Rickett, & Spangler (1995)
1 pc 1 AUDensity spectrum in local
interstellar mediumPower law, with spectral
index near Kolmogorov value– Notable exceptions!– Large dynamic range!
Interstellar plasma has large Reynolds number.
Turbulent processes responsible for density fluctuations(?).
Density spectrum elsewhere in Galaxy similar, probably.
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Turbulence Inner ScaleTurbulence Inner Scale If density fluctuations result
from turbulence, inner scale would be a dissipation scale.
Scattering resolved if b ~ /d.
Inner scale important if l1 ~ b. Inner scale estimates are
roughly 200 km. Spangler & Gwinn attribute it
to the ion inertial length or ion Larmor radius.
Note gap in coverage from 30 km to 1000 km.
Spangler & Gwinn (1990)
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Sub-parsec magnetic fieldsSub-parsec magnetic fields
NGC 6334B and Cyg X-3 show rotation of image shape with frequency:– Different frequencies
sample different length scales in scattering medium.
Density fluctuations changing shape on these scales.
Magnetic fields aligning density fluctuations on this scale.
Yet Sgr A* and B1849+005 do not…
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Scattering GenesisScattering Genesis
Scattering traces star formation– NGC 6334B (Trotter et al.)– Cygnus region (many
studies) Direct link more difficult
to establish– Spangler et al. vs. Simonetti
& Cordes and Spangler & Cordes
Should be able to do much better today and in future
2013+370/G74.9+1.2CTA 1
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Where is the Scattering Medium?Where is the Scattering Medium?(“Regional”)(“Regional”)
Sources embedded in the medium are less scattered than background sources
Scattering must overcome the wavefront curvature.
Distance ambiguity for Galactic sources
No ambiguity for extragalactic sources
xgal = (DGC/GC) GC
Can solve for GC.
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Where is the Scattering Medium?Where is the Scattering Medium?
B1849+005
B1849+005
PSR B1849+00
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GC Scattered ImagesGC Scattered Images
Sgr A* displays enhanced angular broadening
OH/IR stars have maser spots with comparable diameters
GC scattering diameter: 1" @ 1 GHz
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GC Scattering—Where?GC Scattering—Where?
Likelihood Results: xgal sources: GC < 500 pc OH masers:
50 pc < GC < 300 pc GC 150 pc xgal 75” @ 1 GHz Angular extent 1 (Note 1°
150 pc.) Inhomogeneous on 10–20 pc X-ray emitting gas + molecular
gas
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Radial Extent of the WIMRadial Extent of the WIM(“Galactic”)(“Galactic”)
H I disks of nearby galaxies appear truncated
Due to extragalactic ionizing flux?
H II disk extends much farther?
Corbelli et al. 1989
H I
H (= H I + H II)
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Radial Extent and Warp of the WIMRadial Extent and Warp of the WIM
WIM radial extent equals or exceeds H I:
H I disks truncated at R ~ 25–50 kpc (Galaxy a prototypical z = 0 Ly α cloud?)
C IV absorption toward H1821+643, R ~ 25 kpc
HVC models often require pressure support at R ~ 25 kpc
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VLBA SurveyVLBA Survey
12 sources– 7 with |b| < 1°
– 5 with l ~ 180° and |b| < 10°
Cf. Dennison et al. 1984
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Best-fit Radial ModelBest-fit Radial Model
No Perseus spiral arm
Perseus spiral arm at 25% of TC93
truncated disk
sech2 disk
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Sources of ScatteringSources of Scattering
Truncated disk because of star formation?
Molecular clouds show radial truncation;
Star formation follows molecular clouds;
Scattering truncates where star formation does.
Similar to what is seen in other galaxies.
Molecular cloud distribution from CO survey by Wouterloot & Brand
26 kpc
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Ne2001Ne2001(Cordes & Lazio 2002, astro-ph/0207156)(Cordes & Lazio 2002, astro-ph/0207156)
Number of data have nearly doubled.
Modifications from TC93:– GC component added; Diffuse component
truncated at 20 kpc;– Diffuse component made
thicker; Spiral arms extrapolated; Spiral arms made thicker;– Orion-Cygnus arm added;– Local Bubble and similar
regions added; “Clumps” and “voids”
added.
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Anomalous Scattering EffectsAnomalous Scattering Effects
Multiple media can lead to anomalous scattering effects– Phase – Scattering angle 2
Effects occur because size of scattering region can become important in determining size of scattering disk.E.g., scattering of sources seen
through other galaxies. Important for LOFAR?
Infinitely extended scattering screen …
Or not.
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Cosmic Rays, Cosmic Rays, rays, and the WIMrays, and the WIM
CRs are charged particles
Smooth CR energy spectrum
Magnetic irregularities scatter CRs
Same magnetic irregularities cause scattering?
1 pc 1 AU
CR energy spectrum/gyroradii
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Summary: Interstellar ScatteringSummary: Interstellar Scattering
Exquisite probe of sub-parsec plasma physics– Density spectrum– Magnetic fields– Interstellar “clouds” (ESEs)– Cosmic rays?
Galactic distribution of scattering– Large-scale tracer of Warm
Ionized Medium (WIM)– Traces star formation
See also– Intraday variability– Pulsar parallax and proper
motions
VLBA itself has been an immense step forward.VLBA + other telescopes is good.
NMA will close gap around 100 km.
LOFAR will be wonderful instrument for scattering studies (2).Difficult to avoid scattering at
LOFAR frequencies! Space VLBI would be good,
if frequency is low enough. SKA will be even better.
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FINISFINIS
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GC Scattering—PulsarsGC Scattering—Pulsars
107–108 neutron stars: Massive star
formation High-energy sourcesSelection Effects:– beaming & LF– velocities– background– pulse broadening
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GC Scattering—Pulse GC Scattering—Pulse BroadeningBroadening
GC ~ 350 seconds GHz-4
Periodicity search: long-period, shallow spectra pulsars, > 8 GHz
Imaging search: steep-spectrum point sources, ~ 1” @ 1 GHz
10 seconds
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Characterizing ScatteringCharacterizing Scattering
Strong scattering at SIRA (and LOFAR) frequencies:
• Fresnel radius RF = 3 x 1012 cm (D/100 pc)1/2(/1 MHz)-1/2
• rms phase in Fresnel radius >> 1
• Two characteristic regimes within strong scattering:o Diffractiveo Refractive
Rickett 1990
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Refractive EffectsRefractive Effects
Unimportant time scales too long
• Refractive scintillation time scale -2
666
6660
tr (@ 1 MHz in yr)
Rickett et al. 1984
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Diffractive EffectsDiffractive Effects
Diffractive scintillation seen commonly in pulsar observations at meter and centimeter wavelengths.
• Characteristic bandwidth, d ~ 3 kHz (@ 1 MHz)
• Characteristic time, td ~ 60 s (@ 1 MHz for v ~ 100 km/s)
No objects will scintillate (twinkle). Frequency
Tim
e
Scintille
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Diffractive EffectsDiffractive Effects
• Pulse broadening smears out pulsar pulses.
• At SIRA frequencies, extreme pulse broadening can be obtained.
Most pulsars will not be seen as pulsed objects.
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Diffractive EffectsDiffractive Effects• Angular broadening
distorts view of sources.
• Magnitude is large. Current SIRA specs
more than sufficient!
Local Bubble
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Optical DepthOptical DepthElectrons responsible
for scattering also contribute to free-free optical depth.
0.24 MHz 0.4 MHz
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Cosmic Rays and Cosmic Rays and raysrays
Diffuse -ray emission:
– p + p – e + p – e +
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Cosmic Rays and Cosmic Rays and raysrays