1. Impact of Interstellar Scintillation on Astrometric VLBI Dave Jauncey 1, Jim Lovell 1, Roopesh Ojha 1, Alan Fey 2, Yasuhiro Koyama 3, Hayley Bignall 4, Lucyna Kedziora-Chudczer 5, J-P Macquart 6, Barney Rickett 7, Oleg Titov 8 & Tasso Tzioumis 1 1) Australia Telescope National facility, CSIRO, Australia 2) U.S. Naval Observatory, Washington DC, USA 3) Kashima Space Research Centre, CRL, Japan 4) Joint Institute for VLBI in Europe, Dwingeloo, Netherlands 5) School of Physics, Sydney University, Australia 6) Kapteyn Astronomical Institute, Groningen, Netherlands 7) Department of Electrical & Computer Engineering, UCSD 8) Geoscience Australia

2. The talk is in two parts: The first is to demonstrate that ISS is the principal cause of the intra-day variability commonly seen at cm-wavelengths in many flat-spectrum radio sources. The second to show what effect this may have on astrometric S/X Band VLBI.

3. What evidence supports an ISS origin? Firstly, there is the presence of a time delay in the variability pattern arrival times at widely spaced telescopes. Here it is for PKS 1257-326 between the ATCA and the VLA.

4. X = VLA, + = ATCA Pattern Time Delay Observed between the ATCA and the VLA.

5. The presence of a time delay of minutes establishes ISS unequivocally. Such time delays can only be measured for the most rapidly variable sources PKS 0405-385, PKS 1257-326 and J1819+3845.

6. The second method to establish ISS has been through the discovery of an annual cycle in the characteristics of the variability over the course of a year.

7. When the Earth is moving in the same direction as the ISM, the relative speed is small and the variations slow. Six months later the Earth is moving in the opposite direction, the relative speed is high and the variations will be much more rapid.

8. This has been found for a number of sources, and it looks like the behavior is widespread. Here it is beautifully illustrated again by PKS 1257-326

9. Red = 8.6 GHz Black = 4.8 GHz

10. In summary; there is conclusive evidence for Interstellar Scintillation. This Vincent Van Gogh understood very well:

12. Interstellar scintillation implies source sizes ~ the angular size of the first Fresnel zone. That is micro-arcseconds. So scintillating sources are amongst the most compact AGN known; a good starting point for them to be position reference sources. But how to find more scintillators?

13. We have undertaken the MASIV (Micro-Arc-second Scintillation- Induced Variability) Survey to find a sample of 200 such scintillators; we surveyed 700 sources with 4 epochs of at 5 GHz on the VLA in 5 sub- arrays, for a total of 13 days. The MASIV Survey

14. VLBA images of MASIV scintillators Compact little so-and-sos aren’t they?

15. What’s a perfect ICRF source and how close are the scintillators? should be a strong radio source at cm-wavelengths should be a “point” source, with no structure at sub-VLBI resolution Should not eject jets the position should be stable on the sky.

16. Should be strong at cm-wavelengths: Relatively easily satisfied. With MkV recording bandwidths sources as weak as ~ 100 mJy should be easily reached; there are plenty of MASIV scintillators above 100 mJy.

17. Should be a “point” source: Most difficult to fulfill: ISS shows that sources which scintillate possess  as components; thus scintillators are the most compact known AGN, but just how compact?

18. We define the core fraction, CF, as the ratio of the flux density on our longest VLBA baseline, to that on the shortest. 49% of the scintillators have CF > 0.8 and 19% with CF 0.8 and 58% with CF < 0.6. There is a very clear VLBI difference between scintillators and non-scintillators: the scintillators are much more compact and core-dominant.

19. Should not eject jets Very difficult to fulfill. However, high core fraction sources have weaker jets. But even such weak jets should not move. In 0059+581, for example, the weak extended structure shows NO apparent change over 7 years.

21. So while 0059+581 does have weak extended mas structure, it does not show significant structural variability. In addition, as you might expect, it also provides excellent positional stability:

23. Compare the scintillators, S, with defining, D, sources for Feissel-Vernier positional stability: D:55% are class 1 or 2 S:67% are class 1 or 2 The defining sources have been pre- selected over a decade. It is remarkable that the scintillators, with no other selection criterion, possess the better positional stability!

24. What problems might be expected with the scintillators? Scintillation flux density variability may produce position shifts due to refraction in the ISM. But this should be no more than the few  as scattered source size. Position shifts from an ISS Christmas-tree effect. But this is offset by their high compactness. It is, however, worth trying to measure. Position shifts due to intrinsic variability. But this is no different for the defining sources.

25. In conclusion, we suggest that the IVS discuss selecting the next generation ICRF from the population of scintillating sources. Their high level of compactness and remarkable positional stability make them a potentially powerful population of sources from which to extend and improve the ICRF.

26. The last several years have seen a conclusive demonstration that interstellar scintillation (ISS) is the principal cause of the rapid intra-day variability (IDV) seen at cm wavelengths in many flat-spectrum extragalactic sources. The presence of ISS implies that the source possesses an ultra-compact component with microarcsecond angular size. We have recently undertaken the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey of the northern sky at 5 GHz with the VLA (Lovell et al., 2003, AJ., 126, 1699), and find that about 20% of all compact, flat-spectrum sources exhibit IDV. Our VLBA images of a sample of the MASIV IDV sources show them, as a class, to be amongst the most compact, core-dominated sources known. Such sources are ideal as VLBI astrometric reference sources, and this is confirmed by the positional quality of the MASIV scintillators in the list of stable astrometric sources (Feissel-Vernier 2003, A&A., 403, 105). We therefore suggest that with the advent of the new MkV broad-band VLBI recording system and associated increase in sensitivity, the next generation of the VLBI astrometric and geodetic celestial reference frame, be made up of a selection of the most compact IDV sources from the MASIV Survey.

29. To find a sample of 150 – 200 such scintillators we have undertaken the MASIV (Micro- Arc Scintillation-Induced Variability) Survey; 700 sources with 4 epochs of 3 days at 4.8 GHz on the VLA in 5 sub-arrays.