Interstellar Scintillation and IDV

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Interstellar Scintillation and IDV
Twinkle, twinkle quasi-star Biggest puzzle from
afar How unlike the other ones Brighter than a
billion suns. Twinkle, twinkle quasi-star How I
wonder what you are. George Gamow, "Quasar" 1964.
Giuseppe Cimò JIVE University of
Tasmania
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This Talk!

• IntraDay Variability and Interstellar
  Scintillation.
• Episodic scintillation 1326-5256
• ATCA observations of a sample of known
  scintillators
• Conclusions

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The IDV phenomenon (Radio Wavelengths)

• Variability of flat spectrum radio sources
• ?30 total flux density variations
• In some extreme cases variations up to 600 in
  less the one hour
• (i.e. J1819384, PKS 0405-385, PKS 1257-326)
• Factor 3 (or more) in the polarized flux density
• Common phenomenon present in 30 of compact
  radio galaxies
• Time scale ? 1 day Small sizes
  (?c?t)
• TB ? 1012K (Violation of the Inverse
• 
  Compton Limit)

Source-intrinsic explanation TBobs TBint
D3-a Relativistic effects can explain only
moderate values of D. IDV requires
D100-1000 Special geometry can be taken into
account (Qian et al. 1991) and the required
factors are similar to those involved from
superluminal motions.
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Example of IDV Ceduna data
The variability pattern of PMN J1326-5256 Modula
tion index 10 Time scalelt12 hours
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Interstellar Scintillation
The turbulence in the interstellar medium causes
a change in the phase of the incoming radio waves
the paths of the waves are distorted producing
spatial variations in the received flux density.
The phenomenon of scintillation then occurs since
the turbulent medium is in motion with respect to
the observer.
For extragalactic objects, one has to take into
account the source size, ? s.

• Scattering regimes
• strong diffractive ? s lt ?diff
• strong refractive ? s lt ?scatt
• weak ? s lt ?weak

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Annual Modulation
The observer moves through a (frozen) spatial
pattern caused by a screen at distance D on line
of sight. The relative velocity between the
orbital motion of the Earth and the scattering
screen is modulated by the composition of the
Earth's velocity vector and the velocity vector
of the screen it results in a seasonal change of
the variability time scale, so-called, annual
modulation
The time scale is defined as t
sspatial/vISM The spatial scale of the
scintillation pattern is sspatial D
?scat and vISM is the relative speed through the
spatial pattern
Rickett 2002
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PMN J1326-5256
COSMIC data
COntinous Single dish Monitoring of Intraday
variability at Ceduna

• PARKES-MIT-NRAO
• (Griffith et al. 1993, AJ 105, 1666)
• No optical identification/No redshift
• Inverted spectrum
• Monitored with the ATCA at 4.8 and 8.6GHz
• (Bignall et al. 2003, Bignall PhD Thesis).

(McCulloch et al. 2005.)
The variability pattern of PMN J1326-5256 m10,
?lt12hrs
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COSMIC data
COntinous Single dish Monitoring of Intraday
variability at Ceduna
The variability characteristics clearly change
after day 104 The modulation index dropped to 4
(Carter et al. in prep.)
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In the following months and up to now, the source
appears to be relatively stable around a mean
flux density of 1.5Jy apart of brief episodes of
rapid variability. During these sporadic events,
the flux density slightly increases and decreases
on a time scale of 1 day before coming back to
its quiescent phase.
mlt4
(Carter et al. in prep.)
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January 2004 An intriguing pattern
One can see a well defined variability even
though with a longer time scale (weeks) and
smaller amplitude that shown before.
Questions What are these sporadic variability
events ? Is PMN J1326-5256 trying to start up
again ? Does the source present multiple time
scales ?
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AT Compact Array Observations

• Target of Opportunity
• ATCA Observations 22, 23 and 25 January 2004
• Observing frequencies 20, 13, 6, 3 and 1 cm
• Calibrators 1934-638, 0823-500 (primary)
• 1421-490 (secondary)
• Data reduction with the software package Miriad

• Aim of the experiment
• Compare simultaneous observations at different
  frequencies.
• Analysis of the variability characteristics in
  different phases of the interstellar scattering
• Towards higher frequencies If a structural
  changes occurred, it is probable that the source
  is still varying in the short wavelengths regime
  where its size can be still smaller than the
  scattering size in the ISM.

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Some interpretation/ideas
A change in the mean properties of the ionized
ISM along the line of sight could be responsible
for the observed cessation of variability. One
should expect also a slow-down in both Q and U
In the case of PMN J1326-5256, we suggest that
the scintillation pattern has changed due to an
intrinsic long-term change in the source jet.
Q and U variability during Stokes I quiescent
phases has been observed.
compact structures in the jets
Only the component responsible for the total
intensity variations has become large enough to
quench the scattering. We observe the fast
scintillating polarized subcomponents in the jet.
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and now more sources!!!
ATCA Observations September 2005

• Two weeks of observing time with the split array
  (3 antennas)
• Observations at 1.6, 2.4, 4.8 and 8.6 GHz
• Full Stokes
• Aim
• Study of the Total intensity and Polarization
  characteristics of a sample of knows
  scintillators.
• Characteristic time scales and variability
  modulation indexes.

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In the case of 0405-385, we know that the screen
distance is about 30pc (or less). Therefore the
shorter time scale observed at 8.6 GHz gives a
scattering size of 230 µas Too
large!!! Assuming the value of 20 µas found by
Rickett et al 2004, we can evaluate the speed of
the scattering screen and an indication of the
anisotropy.
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Conclusions

• Long term changes in the IDV pattern can be due
  to source-intrinsic effect emission of new jet
  components?
• IDV measurements can provide information about
  the source size at microarcsecond scale (or the
  velocity of the scattering screen).
• Continuously monitoring flat spectrum radio
  sources at Ceduna (COSMIC) and Hobart to evaluate
  time scales and to look for annual modulation
  (McCulloch et al 2005, Carter et al. Cimò et
  al. in prep).
• ATCA data provided high precision measurements
  of time scales and modulation indexes, both in
  total intensity and polarization.

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Question Why IDV???

• Understanding the phenomenon
• source structure and ISM

• Micro-arcsecond imaging of AGNs
• via Earth Orbit Synthesis (Macquart and Jauncey
  2002)
• via polarization studies (Rickett
  Kedziora-Chudczer 2002)

• IDV sources are compact good candidates for
  geodesy experiments
• Geodetic VLBI RDV campaign VCS4 (CONT05?)