Scintillation in Extragalactic Radio Sources

1
Scintillation in Extragalactic Radio Sources

• Marco Bondi
• Istituto di Radioastronomia CNR Bologna, Italy

2
References

• Conference Proceedings Review Papers
• AIP 74 Radio Wave Scattering in the
  Interstellar Medium 1988, Eds J.M. Cordes, B.J.
  Rickett D.C. Backer
• IAU Colloquium 182 Sources and Scintillations
  Refraction and Scattering in Radio Astronomy
  2001, Eds R. Strom
• Rickett 1990, Annu. Rev. Astron. Astrophys.
• Papers
• Bondi et al. 1994, AA 287, 390
• Blandford, Narayan Romani 1986, ApJL 301, 53
• Dennett-Thorpe De Bruyn 2000, ApJL 529, 65
• Ferrara Perna 2001, MNRAS 325, 1643
• Heeschen Rickett 1987, AJ 93, 589
• Padrielli et al 1987, AASS 67, 63
• Rickett et al. 1995, AA 293, 479
• Rickett et al. 2000, ApJL 550, 11
• Spangler et al. 1993, AA 267, 213
• Walker 1998, MNRAS 294, 307

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Outline

• Introduction
• Density and intensity fluctuations
• Scintillation Jargon
• Scintillation regimes weak, diffractive,
  refractive
• Low frequency variability
• Flickering and Intra-Day variability
• Intergalactic scintillation

4
Introduction

• Electromagnetic waves from an extragalactic radio
  source pass through several ionized media the
  intergalactic gas, the interstellar medium, the
  interplanetary medium and the ionosphere. In all
  these cases, the turbulent plasma produces a
  phase modulation of the wavefront and scattering.
• This produces a wide variety of observed
  phenomena such as intensity scintillation,
  angular broadening and pulse smearing.
• The study of these phenomena provides information
  on the angular size of the scattered sources and
  a unique method for the remote analysis of
  astrophysical plasmas.

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Density and Intensity Fluctuations

• Typically it is assumed a power-law spectrum for
  the spatial power spectrum of the density
  irregularities
• CN is a strength parameter and q is the wave
  number of density fluctuations in the plasma.
• This quantity is related to the power spectrum of
  intensity fluctuations through the source size
  (actually the source visibility in interferometer
  observations).
• In the case of refractive scintillation we
• have

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Scintillation Jargon

• Define the point source scintillation index (rms
  fractional intensity fluctuation)
• Define the scattering strength
• A relevant quantity in scintillation is the
  Fresnel scale (units are cm)
• The angular size of the Fresnel scale is given
  (in arcseconds) by

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Scintillation Regimes

• Scintillation is divided into weak and strong
  according to whether ? is much smaller or greater
  than unity. In the strong regime the wavefront is
  highly corrugated on scales smaller than the
  Fresnel scale, in the weak regime the phase
  changes over the Fresnel scale are small.
• Assuming a model for the distribution of the
  scattering material it is possible to map the
  transition frequency ?0 (the frequency at which
  ?1).

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Weak Scintillation

• The spatial scale for weak intensity variations
  is the Fresnel scale rf . For sources with
  angular extent greater than ?f the scintillation
  patterns from different parts of the source
  overlap and smear each other out, eliminating a
  detectable variation
• For a point source ( ) the following
  relations hold
• For a source with

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Strong Scintillation Diffractive

• It is an interference effect characterized by
  fast, narrow-band variations. The modulation
  index is unity for a point source and the
  interference fringes have a characteristic
  frequency scale
• It is necessary to observe with frequency
  resolution of ?? or better in order to be
  sensitive to diffractive scintillation.
• The angular size on which phase changes of order
  1 rad are introduced into the wavefront is
• The corresponding time-scale is
• For sources with ? ? ?d the modulation index is
  reduced to ?d/? and the time-scale for variations
  increased by a factor ?/?d .
• No recorded examples of diffractive scintillation
  of extragalactic sources.

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Strong Scintillation Refractive

• Can be understood in terms of ray-optics and
  correspond to lens-like phenomena.
• It is characterized by slow, broad-band
  variability.
• The refractive scale is given by the scattering
  disk, much larger than the Fresnel scale, and the
  time-scale is correspondingly longer.
• Again if ???r the modulation index is reduced by
  a factor while the time-scale
  increases with

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Low Frequency Variability - I

• Low frequency (lt 1 GHz) variability has been a
  puzzling phenomenon in the 70s and 80s.
• Variations of the order of 10 on time-scales of
  months to years.
• Variations could not be explained in terms of
  expansion of a synchrotron emitting cloud of
  plasma.
• Low frequency bursts would imply ? far higher
  than those derived from proper motion
  measurements

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Low Frequency Variability - II

• Refractive scintillation was proposed as the
  mechanism responsible for low frequency
  variability dependence of variability on
  galactic latitude.
• Results from analysis of a 15 years monitoring at
  408 MHz coupled with VLBI observations at 610 MHz
  to derive the source sizes
• Qualitatively and roughly quantitative agreement
  between the observed scintillation indices an
  time-scales and those derived from a standard
  model for interstellar plasma turbulence.

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Low Frequency Variability - III

• The time-scale of variability is determined by
  the distance of the effective screen and the
  pattern-observer velocity in the plane of the sky
  (pattern speed).
• Annual modulation in a sample of low frequency
  variable. This is interpreted as produced by the
  Earth orbital motion around the Sun on the
  pattern produced by refractive scintillation.
• Sources along the line of sight of the apex show
  longer time-scales.

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Low Frequency Variability - IV

• There is no measurable evidence for a finite
  propagation speed of the turbulent irregularities
  responsible for the refractive scintillation
• the scattering medium is extended along the line
  of sight. In this case the random velocities of
  the density irregularities will not produce any
  net motion
• the scattering medium is not uniformly
  distributed along the line of sight, but it is
  localized in a thin screen at a certain distance.
  In this case the velocity of the density
  irregularities should be low suggesting that they
  could be associated with the HII region
  envelopes, characterized by a Alfven speed

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Flickering Intra-Day Variability - I

• Low amplitude (1 --5 rms) short time scale (few
  hours to days) variability observed in the range
  2-20 cm in flat spectrum radio sources.
• In some cases the variations can have substantial
  amplitude (10-15 ) over few hours (e.g.
  0917624).
• If intrinsic these variations would imply Lorentz
  factors of the order of 100.
• Variations are observed also in polarized flux
  and position angle.

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Flickering Intra-Day Variability - II

• Refractive interstellar scintillation has been
  claimed to be the cause of this phenomena because
  of a significant trend of increasing flicker
  amplitude with decreasing galactic latitude.
• The combination of a steady and variable
  component with nearly orthogonal polarisation
  angles can produce the observed anticorrelation
  of total flux density and polarized flux

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Flickering Intra-Day Variability - III

• Assuming the source diameter is linearly
  dependent on wavelength it is possible to
  reproduce the amplitude and time scale trends
  with wavelength with a reasonable model of RISS.
• Annual modulation detected in IDV sources
  (0917624, J18193845).

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Scintillation as a Probe of the ICM - I

• Most of the baryons reside in a warm/hot
  component which is difficult to detect with
  standard absorption/emission line techniques.
• Refractive scintillation of a compact quasar
  behind a cluster can be used to probe the
  intracluster medium.
• The cluster will act as a foreground screen
  relevant parameters are
• radial profile of the cluster mass density
  (isothermal ? model)
• mass fraction of the gas (0.04 - 0.2)
• distance of the cluster (0.02) and of the quasar
  (1.0)
• velocity of the inhomogeneities (1000 km/s)
• the size of the quasar
• the impact parameter (depending on its value the
  propagation through the cluster can be in the
  weak or strong scattering regimes)

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Scintillation as a Probe of the ICM - II