A pre-launch low frequency VLBI survey for RadioAstron
Introduction
One of the observing bands of the forthcoming RadioAstron Space VLBI (SVLBI) mission is the low-frequency band at 327 MHz. The important property of this band is the presence of strong propagation effects in the interstellar and interplanetary medium. It is a well-known fact (Ananthakrishnan et al., 1989; Kardashev, 1997) that the scattering affects encountered at low frequencies seriously affect the coherence properties of VLBI data and this is expected to be a particular problem for the RadioAstron mission, which employs a relatively small 10-m antenna and forms baselines that are much larger than the diameter of the Earth. Hence, improved knowledge of the turbulence distribution in both the interstellar and interplanetary medium, as well as its negative influence on interferometer sensitivity and angular resolution, are important for any low frequency SVLBI mission.
It is also important to establish a suitable list of compact radio sources that may be reasonable targets for the RadioAstron mission. We began this process by selecting sources that were dominated by compact components that were unresolved on the longest ground baselines. These investigations were started in 1994 (Molotov et al., 1996) as part of an extensive pre-launch survey of various kinds of sources with different galactic latitude and longitude, and different elongations and position angles from the Sun. These experiments enable us to get a better understanding of the origin and physics of the turbulence spectrum in the interstellar and interplanetary medium.
Section snippets
Experiment description
We describe two VLBI experiments at 327 MHz. Both were correlated with the JPL/Caltech Block-II correlator in 1998. The first experiment, (EA009), was carried out on the European VLBI Network (EVN) on 25–26 October 1995, using the MkIII recording format, and with the participation of Jodrell Bank, Noto, Simeiz and Westerbork. Moreover, two Russian telescopes at Puschino and Ussuriysk supported these observations recording with the MkII system. In principle, the Block-II correlator has the
Data processing
Observations for the EVN experiment, EA009, began at 16:30 UT on October 25, and ended at 12:00 UT on October 26, 1995. The longest baseline in the array was Jodrell Bank-Simeiz with an extent of ∼2800 km. The beam size was 46×32 milliarcseconds (mas). Correlation was made for only a single 2 MHz band (319.99–321.99 MHz). All observed sources were detected on at least one baseline.
Observations for (INTAS1) began at 00:00 UT on November 30, and ended at 23:55 UT on December 2, 1997. The maximum
Conclusions and future plans
We have found that all the compact sources we selected were detected on the longest baselines. This result is rather surprising. Of the 37 sources observed, all of them were resolved on the longest baseline (Noto – GMRT). Of the 29 sources observed on the baseline Jodrell Bank – Simeiz, only 5 sources were not resolved. For both experiments, it was found that 20 sources have complex structures. We plan to re-observe 25 radio sources in our sample next year, aiming to obtain much better
Acknowledgements
We thank Dr. Arnold Benz (Institute of Astronomy, Zurich, Switzerland) for coordinating the INTAS 96-0183 Project. We express our special thanks to Dr. Valery Altunin (Jet Propulsion Laboratory, Pasadena, USA) for correlation of the data, for help in the preparation of the observations and for useful discussions. We are also grateful to JIVE for providing the financial support that permitted two of the co-authors of this paper (Igor Molotov and Andrey Chuprikov) to attend the Fourth EVN/JIVE
References (6)
- Altunin, V., Beasley, A., Ludke, E., Alekseev, V., Lipatov, B., Dagkesamansky, R., Kardashev, N., Kovalenko, A.,...
- Ananthakrishnan, S., Kulkarni, V.K., Posnoby, J.E.B., Spencer, R.E., Graham, D.E., Porcas, R.E., Van Ardanne, A.,...
- Kardashev, N.S., 1997, Exp. Astron., 7,...
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C.R.S. wishes to acknowledge that this research was supported by INTAS 96-0183.