IDV Sources as ICRF Sources: Viability and Benefits

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IDV Sources as ICRF Sources Viability and
Benefits

• Roopesh Ojha, Alan Fey, David Jauncey, Kenneth
  Johnston, Jim Lovell and Lucyna Kedziora-Chudczer

Email rojha_at_atnf.csiro.au, afey_at_usno.navy.mil
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Abstract
Radio sources that exhibit rapid variability in
their light curves, as a result of radio wave
propagation through turbulent electron density
fluctuations in the interstellar medium, appear
to be the most compact sources in the sky. In
particular, the most variable weak sources, might
be the most point-like and, thus, some of the
best candidates for densification of the
International Celestial Reference Frame (ICRF)
and consequent improvement in its
accuracy. Further, the advent of the Mk IV/V VLBI
system will make use of weaker sources easier. We
will discuss the viability of this idea and state
the benefits that might flow from this approach.
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The International Celestial Reference Frame
Very Long Baseline Interferometry (VLBI)
observations of selected strong compact
extragalactic radio sources have been used to
define and maintain a radio reference frame with
sub-mas precision. This ICRF was adopted as the
fundamental celestial reference frame at the
XXIII General Assembly of the International
Astronomical Union (IAU) held on 20th August 1997
in Kyoto, Japan (Ma et al. 1998, AJ 116, 516
). The ICRF is currently defined by the radio
positions of 212 extragalactic objects obtained
using the technique of VLBI at radio frequencies
of 2.3 and 8.4 GHz over the past 20 years.
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ICRF Current Limitations

• The ICRF is currently limited by
• Deficit of defining sources, particularly in the
  Southern Hemisphere
• Sources have variable core-jet structure which
  causes position variations (can in principle be
  corrected).
• The ICRF is composed mostly of the brighter (gt
  0.2 Jy at 8.4 GHz) sources many of which suffer
  the most from structure problems

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ICRF Future Improvement

• Thus, future improvement will involve
• Increasing the number of defining sources
• Incorporating sources that have little or no
  structure (presumably leading to increased
  position stability)

IDV/ISS sources may be the answer!
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What are IDV/ISS Sources ?

• IDV Intra Day Variability
• Flat-spectrum extragalactic sources
• Show amplitude variations at centimeter
  wavelengths
• Variation timescales are less than a day
• Interstellar Scintillation (ISS) is the principal
  cause of this IDV
• This behaviour is illustrated by the light curves
  shown in Fig 1

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Fig 1 Light curves of IDV/ISS sources with
timescales ranging from a few hours to
approximately one day (Lovell et al 2003)
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VLBA Observations of IDV Sources
We used the VLBA to make 8.4 GHz snapshot images
of a sample of Low Flux Density Scintillating
(LFDS) sources ( 0.1 Jy) from the MASIV 5 GHz
VLA survey for scintillating extragalactic
sources (Lovell et al. 2003, to appear in AJ,
astro-ph/0306484) 8.4 GHz VLBA images of 18 High
Flux Density Scintillating (HFDS) sources ( 1
Jy) and 40 High Flux Density Non-scintillating
(HFDN) sources were obtained from the USNO Radio
Reference Frame Image Database (RRFID). First
results from imaging 38 sources are presented
here and a representative selection of images is
shown in Fig 2.
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Milliarcsecond Scale Structure of Scintillating
Sources
The Lovell et al. 2003 VLA survey results showed
an increase in both the fraction of scintillators
and in their amplitude of variability. This
raised the possibility that the Low Flux Density
Scintillating (LFDS) sources may form a distinct
population with differing morphology Our VLBI
observations address the following questions

• Are there any morphological differences, at mas
  scales, between Scintillating and
  Non-scintillating sources ?
• Are there any morphological differences, at mas
  scales, between high flux density, 1 Jy, and
  low flux density, 0.1 Jy, scintillating sources ?

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Fig 2 Examples of LFDS sources
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Results of Analysis I
We calculated a Core Fraction which is the
ratio of flux on longest (VLBA 250 Ml) baseline
to flux on shortest baseline. Using Core
Fractions, i.e. a quantitative classification of
the sources, we find that the Low and High Flux
Density Scintillating Sources do not differ
significantly in their core fraction.
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Results of Analysis II

• Calculation of the mean and standard deviations
  of the Core Fractions for the three samples
  yields
• No significant difference between Low and High
  Flux Density Scintillating Sources (difference
  1.5?).
• Significant difference between High Flux Density
  Non-scintillating sources and both Low and High
  Flux Density Scintillating sources (difference
  6.4 ? and 3.3 ? respectively).
• A limitation at the moment is the small number
  (18) of known High Flux Density Scintillating
  (HFDS) sources.
• These results are summarized in Fig 3

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Fig 3 Histogram illustrating distribution of
core fraction for the three samples
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Summary of VLBA results

• Scintillating Sources have different mas scale
  morphologies from Non-Scintillating Sources. They
  have a higher fraction of their total flux in an
  unresolved (lt 1 mas) core.
• Amongst the high flux density scintillating
  (HFDS) sources, it would appear that only the
  core components scintillate.
• Low and High Flux Density Scintillating sources
  do not, on average, have different morphologies
  (Note The HFDS sample is small, 18 sources at
  present).

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Implications for Improvement of the ICRF

• Scintillating sources have proved to be some of
  the most compact sources. This may make the LFDS
  sample ideal reference sources for the next
  generation (Mk IV/V) astrometry and geodesy
  reference frames.
• The increased sensitivity of Mk IV/V VLBI will
  probably be required to observe the generally
  weaker IDV sources
• While the compact morphology of IDV sources
  suggests their use as ICRF sources, their
  position stability will need to be established.
• The 18 HFDS sources are ICRF sources and may
  provide a good test of the positional stability
  of IDV sources in general (this is the subject of
  future work)
• As VLBI assumes that sources do not change over
  the period of observation, the amplitude
  variability of IDV sources may introduce spurious
  structure into the images