Setting the Scintillating Scene

1
Setting the Scintillating Scene

• Barney Rickett
• U. C. San Diego
• ASTRON/JIVE Workshop on
• Interstellar Scintillation of Extragalactic Radio
  Sources

2
Brightness Temperature

• Observe change DS in time Dt
• If Intrinsic Variation gt size cDt/(1z)
• Tbvar 5x1010 K DSJy lcm dMpc2
  Dtday(1z)-2
• Doppler Factor d µ (Tbvar)0.31 (1z)

0.1 Jy/day at 6cm, for a z1 quasar Tbvar
4x1016 K d 50 0.1 Jy/hr at 6cm, for a
z1 quasar Tbvar 2x1019 K d 360 But
from VLBI d 1 to 20
3
ISS Geometry
4
Screen simulation in weak scintillation (left)
point source (right) extended source 3
times qFr (kL)-0.5 Note reduced amplitude and
increased spatial scale
5
Interstellar Scintillation of extragalactic Radio
Sources
w
6
(No Transcript)
7
ISS and Source Diameter Plot Source diameter qso
vs frequency Dashed lines at constant
Tb/SJy Typical ISS at 45 deg Galactic
latitude Typical ISS timescale at right
Weak ISS if qso lt qweak Intra-Day Variation
IDV
Diffractive ISS if qso lt qdiss Pulsars only
Refractive ISS if qso lt qriss Low Freq
Variables LFV
Note GRB 970508
8
Brightness Temperature

• Observe change DS in time Dt
• If ISS(quenched)
• size qsrc VISSDt/LISS
• Tb DS/2k l LISS2 Dt VISS-2
• Doppler Factor d µ (Tb)0.83 (1z)

9
IDV RESULTS (published)
10
Source Diameter / Screen Distance trade-off

• mc mpt(1kLqso2)-0.5
• mpt(1qso2/ qFr2)-0.5
• where qFr (kL)-0.5
• contours of mc
• (with mpt fixed)
• gt qsoµ L-0.5

Log(qso)
tc constant
mc constant
tc L/k (Lqso)20.5/V (L/V)qFr2
qso20.5 contours of tc gt qsoµ L-1 qso gtgt
qFr tc µ L0.5 qso ltlt qFr
Log(L)
11
Source Dia / Screen Dist trade-off
ISS of PKS B0405-385 observed with
ATCA Rickett, Kedziora-Chudczer Jauncey (ApJ
2002)

• 8.6 GHz
• modulation index
• 0.08 lt mc lt 0.37

Tb constant
4.8 GHz
mc constant
and time scale 0.31hr lttclt 0.51hr
tc constant
12

• Polarization
• Anisotropic Scattering
• Annual Cycles due to Earth's Orbit

13
IDV in Stokes Parameters for Quasar 0405-38 at
3.5 cm
14
0405-385 correlations of I,Q
UII,QQ,UU,IQ,IU,QU
Solid lines are from a model fit (Rickett et al.,
ApJ, 2002)
15
Autocorrelation of intensity in weak
scintllation, axial ratio 41
16
41 anisotropic scattering
Point source Double source
17
Rapid IDV of Quasar J181938WSRT data from G de
Bruyn J Dennett-Thorpe

• Quasar J181938 exhibits ISS with a pronounced
  annual cycle in its characteristic timescale

Raypath from QSO
ISM
earth
18
Annual change in timescale of J181938 (J
Dennett-Thorpe and G de Bruyn 2003)

• Observed timescale for two years (6cm)
• Model is slice through an ellipse at angle of the
  effective Earth Velocity
• Effective velocity of scattering plasma relative
  to the Sun
• Best fit by adjusting ellipse parameters and the
  Va and Vd of the plasma relative to LSR.
• Ellipse represents anisotropy due to source or
  medium

19
IDV-ISS Surveys
20
Blazar 1514197 Green Bank Interferometer data
(Rickett, Lazio et al 2003)
ISS is the variation over day to week, stronger
at 2 GHz
Intrinsic variation is over month to year,
stronger at 8 GHz.
21
ISS statistics from GBI Monitoring

• m2 (rms/mean) at 2 GHz for each source
• Increases with a spectral index of source
• Increases toward Galactic Plane
• Increases with Emission Measure (via Ha from
  WHAM)
• Conclude
• Typical Tb 1011K
• Rickett, Lazio, Waltman Ghigo (2004)

22
MASIV Conclusion (preliminary)

• Observed Distribution of modulation index m with
  mean flux density

30 20 10 0
0.1Jy 1.0 Jy 10 Jy
Sources are best modelled by compact components
with brightness temperatures 1011 -1012 K
23
Key Topics in ISS Observations

• Basic quantities Srms, tISS gt mc
  Srms/ltScgt
• But what fraction of total flux to use for Sc
  ? Any skewness in the intensity statistics can
  be used to quantify mc near strong scintillation.
• Annual Changes in tISS due to VEarth (no change
  in Srms) 1819,1257,0917
• Time offset over v. long baseline (beware effects
  of anisotropy) 0405,1819,1257
• Time offset between two frequencies (source
  centroids offset vs frequency) 1257, 1819?
• Correlation ( offset?) in I,Q,U V gt
  polarization structure 0405. 1819
• Time asymmetry (with mc1) - rise-time vs
  fall-time gt angular structure 1819
• Annual changes in the above study transverse ISM
  vels
• Better to use power spectrum and cross power
  spectra or structure functions

24
Key Topics in Theory and Analysis

• Theory for extended source scattered by an
  extended medium
• Simulation methods between weak and strong scint
  gt scales intermediate between diffractive and
  refractive
• Anisotropic scattering
• Use more observables together in the
  interpretation
• ie scint index and full power spectrum at one or
  more frequencies, cross-spectra (or correlation),
  ISM mode multi-component source models

25

• The Scene is Set
• Let the Play Begin

26
(No Transcript)
27
MicroArcsec Scintillation InducedVariabilityLovel
l, Jauncey, Bignall, Kedziora-Chudczer, Macquart,
Rickett Tzioumis, AJ 2003

• 710 compact sources monitored at 6cm for 3days
  every 2hr using 5 subarrays of the VLA
• 85 showed variations with rms 2-20

28
Topics in Extra-Galactic ISS

• ISS screens and media
• Screen distance(s)
• Extended scattering medium (thickness)
• Anisotropy
• Velocity
• Sources
• Source models (multiple comps? elliptical?)
• Angular size and brightness
• Optical depth effects (ie diameter vs wavelength)
• Why so few fast scintillators?
• local screens are rare or
• Local ISS suppressed by distant scattering or?
• Methods for time-scale estimation gt Annual
  Cycles
• VLB time shifts
• Possible longer term monitoring gt GBI, Ceduna

29
Conclusions

• ISS is seen as rapid variation at cm wavelengths
  from compact sources at cm wavelengths.
• Source structure can be estimated via model
  fitting to the spectrum (or the correlation
  function) of the light curves gt angular
  resolution 10-100 µas
• Most IDV observations are explainable as ISS in
  the "normal" ionized ISM (Reynolds layer) with
  500pc thickness.
• MASIV survey found 85 new IDV sources with time
  scales of 6-48hrs gt 1012 K Brightness temps are
  typical.
• Green Bank Interferometer monitoring of Quasars
  and Blazars revealed 122 sources with ISS
  (5-50days) at 2.25 GHz gt 1011K brightness
  temps are typical.
• 3-4 very rapid IDV sources only explained by
  local scattering (10-30pc)
• New technique using scintillation "arcs" has
  potential for real imaging from single dish
  observations.

30
Spectrum of weak anisotropic ISS

• P?I(?x,?y) 8p2re2?2 (R?x2?y2/R)-(11/6) ?
• ? dz Cn2(z) sin2(?2?z/4p) V(kz/2p)2
• where Fresnel Filter Cohen-Salpeter
• ?x,?y,?z (3d) wavenumber
• V(u,v) Source Visibility
• Cn2(z)(R?x2?y2/R)-(11/6) electron density
  spectrum (Kolmogorov)
• Cn2(z) from the Taylor-Cordes 1993 model
• R axial ratio re classical electron radius
• ? radio wavelength
• ISS Correlation function
• R?I(t) ?? P?I(?x,?y) exp-i tVISM ?x d?xd?y
• Where VISM is the velocity of earth relative to
  ISM (defines x-axis)

31
Scint History
32
More scint history
33
Blazar 1514197

• Auto-correlations of ISS
• acf(t) corrected for noise spike
• ISS variance acf(0) Srms2
• Modulation index
• m Srms/ltSgt
• Timescale t
• acf(t) 0.5 acf(0)
• Cross-correlations normalized
• by product of Srms at 28 GHz

34
Scintillation scale/(Fresnel scale) versus axial
ratio for weak scint, kolmogorov, screen, point
source
Aappspar/sperp
0.7 Rf
35
Spectrum of weak anisotropic ISS

• P?I(?x,?y) 8p2re2?2 (R?x2?y2/R)-(11/6) ?
• ? dz Cn2(z) sin2(?2?z/4p) V(kz/2p)2
• where Fresnel Filter Cohen-Salpeter
• ?x,?y,?z (3d) wavenumber
• V(u,v) Source Visibility
• Cn2(z)(R?x2?y2/R)-(11/6) electron density
  spectrum (Kolmogorov)
• Cn2(z) from the Taylor-Cordes 1993 model
• R axial ratio re classical electron radius
• ? radio wavelength
• ISS Power Spectrum
• P?I(f) (V/2p) ? P?I(?x2pf/V,?y) d?y
• Where V is the velocity of earth relative to ISM
  (defines x-axis)

36
ISS spectra double source models
Isotropic scattering
Point source
Double source
37
ISS of Quasar B091762 (Quirrenbach et al, AA
1991)

• IDV is correlated between 11,6, 3.6 and 2cm, with
  a time-scale that increases (linearly) with
  wavelength - see overplotted smoothed curves from
  shorter wavelengths
• Source diameter smoothing of ISS explains this
  nicely if source diameter increases (linearly)
  with wavelength
• Conclusion from fitting an ISS model and source
  diameter is
• scattering distance is 200 pc
• brightness of core is 6.0x1012K with 44 of 6cm
  flux density (Rickett et al. AA 1995)