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Scintillation in Extragalactic Radio Sources

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Title: 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

3
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.

5
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

6
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

7
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).

8
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

9
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.

10
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

11
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

12
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.

13
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.

14
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

15
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.

16
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

17
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).

18
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)

19
Scintillation as a Probe of the ICM - II
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