2. Observations and data analysis
We conducted VLBI monitoring from January to May in 1996 using J-Net, composed of three telescopes of National Astronomical Observatory, Japan, 10-m telescope at Mizusawa, 45-m telescope at Nobeyama and 6-m telescope at Kagoshima, as well as 34-m telescope belonging to Communications Research Laboratory at Kashima. Performance parameters of the telescopes and epochs of the observations are shown in Tables 1 and 2, respectively. The minimum fringe spacings of the baselines ranged from 2.1 to 14 mas at 22.2 GHz. We used the K-4 backend system (Kiuchi et al. 1991) which has 16 video channels with 2 MHz bandwidth each. The VLBI data were cross- and auto-correlated by using New Advanced One-unit COrrelator (NAOCO) (Shibata et al. 1994) at Mizusawa Astrogeodynamics Observatory/NAOJ. The 512 complex lags of NAOCO yielded high velocity resolution of 0.106 km s-1 in the cross-power spectrum and 0.053 km s-1 in the total-power spectrum at 22.2 GHz. We used the multiple fringe-rate mapping method (Walker 1981) to obtain wide-field map of the masing region.
Table 1. Performance parameters of the participating telescopes in J-Net
Table 2. Observing epochs in our J-Net observations and the participating telescopes
First, we selected a velocity channel containing a single maser spot as phase reference based on comparisons between temporal variations of cross-power flux density and gain curves of the baselines. The baseline gain curves were obtained from the auto-correlated data which were calibrated with respect to the frequency-band characteristics following the method presented in Diamond (1989). A velocity channel showing the temporal variation of the cross-power flux density very similar to that of the baseline gain curve was selected as containing a single spot of simple structure. Second, the VLBI data were integrated in the phase-referenced mode for 1200 seconds. The integration time provides appropriate number of () data sets with sufficiently high S/N ratios (approximately 10 Jy at 5 level) and spatial resolution (approximately 25 mas in right ascention). We thus obtained the fringe-rate maps with relative position accuracy of 5 to 15 mas in right ascenstion and 10 to 50 mas in declination in the single velocity channel.
We carefully identified spatially-distinguished water maser spots and measured their line-of-sight velocities. We first picked up those maser components in the successive velocity channels which are confined within the possible range of the position error (15 mas in right ascention and 100 mas in declination) and regarded them as originating from a single maser spot. The line-of-sight velocity of the maser spot was estimated at the flux peak of the maser spot. Accuracy in determining the velocity was estimated to be 0.1 to 0.2 km s-1 based on measured changes of flux peaks during a day due mainly to the random noise. Then we regarded the maser spots which existed in the same relative position through the successive epochs within the accuracy in position determination as the same maser spot. Such a procedure was safely performed for the stronger maser spots which are well determined in the relative position, especially, in right ascention.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998