Pulsar VLBI Kashima(Japan)-Kalyazin(Russia) Pulsar VLBI Experiment in 1995

M. Sekido, Y.Takahashi, Y.Koyama
Kashima Space Research Center
Communications Research Laboratory
893-1 Hirai, Kashima, Ibaraki 314, Japan

Y. Hanado, M.Imae
Communications Research Laboratory
4-2-1 Nukui-kita, Koganei, Tokyo 184, Japan

Yu. P. Ilyasov, A. E. Rodin, A. E. Avramenko, V. V. Oreshiko
Astro Space Center P.N. Lebedev Physical Institute, Russia

B. A. Poperchenko
Special Research Bureau of the Moscow Power Engineering Institute, Russia

1. Introduction

Pulsars are high-velocity objects (Lyne 1994) and their positions and proper motions are important for our understanding of the formation and evolution of pulsars. Accurate astrometric measurements by inter-continental VLBI observation will determine the positions and proper motions of pulsars in several years interval observations.

2. Observation and data acquisition

A VLBI experiment observing pulsars and reference sources was carried out by Kashima(Japan)-Kalyazin(Russia) baseline in March of 1995. This experiment was organized by collaboration between Communications Research Laboratory (CRL) and Lebedev Physical Institute of Russian Academy of Science (LPI). The antenna diameter and efficiency of Kashima and Kalyazin are 34m, 60%, and 64m, 60% respectively. Observation frequency band 1.4GHz was down converted to video band and divided into 16 channels with each 2MHz width. Data of each channel are sampled with 4Mbps sampling rate and recorded by K-4 VLBI recorder. This was the first VLBI experiment between Kashima and Kalyazin, therefore making sure of the direction of circular polarization, as well as of the coordinates of Kalyazin station was main purpose of this experiment.

3. Data processing

The K3 correlator, developed by CRL, was used for cross correlation processing. First fringes were successfully detected. K3 correlator has a gating function, which enhance the Signal to Noise Ratio (SNR) of pulsar data by accumulating only a part of the pulsar's signal. Initially we processed the pulsar data without the gating function. Correlation processing with gating function will be done soon. By using the gating function, weak pulsars, which have not yet been observed with interferometric methods, will become observable.

4. Preliminary result

A MkIII database, which is used for ordinary geodetic analysis data storage, was made for pulsar VLBI data. The analysis software Solve, which is widely used for geodetic analysis, was used to analyze the position of pulsars. In Fig.1, mark $\bullet$ shows the result of this experiment and open suare symbols shows the position from Taylor's catalog (J.H.Taylor 1993). The discrepancies between the results and catalog position may be due to several reasons. The main reason is ionospheric delay: ionospheric delay compensation cannot be done with single band (1.4GHz) data. Another reason may be the difference of coordinate systems. Timing positions are relative to a reference frame defined by an ephemeris of Solar system motions, whereas interferometric positions are relative to equatorial frame defined by the rotation of the Earth to background extragalactic sources.

5. Future

By applying the pulsar gating function of K3 correlator, SNR will be enhanced and accuracy of the position will be improved.

This experiment had been organized to analyze the difference of delay between pulsars and closest reference sources (Differential VLBI). By applying this method, calibration of ionospheric delay will be improved.

We are planning to equip K4 backend system at Kalyazin station for a few years. Then we will be able to organize pulsar VLBI experiments frequently.

Investigation for how we should compensate the ionospheric delay will be necessary. We are considering of using GPS observation for this purpose.

References



Figure 1. Open circles show the result of this experiment and open squares show the position from Taylor's catalog (J.H.Taylor 1993). The discrepancies between the results and catalog position may be caused from some reasons. Main reason may be caused from ionospheric delay because ionospheric delay compensation cannot be done with single band (1.4GHz) data. Other reason may be the difference of coordinate . Timing positions are relative to a reference frame defined by an ephemeris of Solar system motions, whereas interferometric positions are relative to equatorial frame defined by the rotation of the Earth to background extragaractic sources.

Updated on November 2, 1995.
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