2. Observations and data reduction
The 1.3 cm continuum and H2O maser (616523; = 22235.080 MHz) observations were made with the VLA of the National Radio Astronomy Observatory (NRAO) 1 in the A configuration on 1998 May 14 and 21. We observed simultaneously two different bandwidths of 25 MHz (7 channels of 3.125 MHz each) and 3.125 MHz (63 channels of 48.8 kHz each), respectively. Both the right and left circular polarizations were sampled in the two different bandwidths. The broad bandwidth was centered at the frequency of 22285.080 MHz for continuum measurements, while the narrow bandwidth was centered at the frequency of the H2O 616523 maser line (22235.080 MHz) with = -69.0 km s-1. The absolute amplitude calibrator was 1331+305 and the phase calibrator was 1733-130, both names in the J2000 coordinate system. The bootstrapped flux densities of 1733-130 were found to be 3.710.06 and 3.790.08 Jy, for 1998 May 14 and 21, respectively. Once the strongest H2O maser component was identified in a particular spectral channel of the narrow bandwidth (the channel at = -61.1 km s-1), we self-calibrated its signal in phase and amplitude. The self-calibration procedure on the first day was initiated with a point source model, whose position was that of the strongest maser component. To measure it, we used the AIPS task MAXFIT on the corresponding channel map externally calibrated with 1331+305 and 1733-130. The phase and amplitude corrections finally obtained were then applied (as cross-calibration) to both the narrow and broad bandwidth data. To better align the positions of both days of observation, we used another cross-calibration procedure for the second day, i.e, taking the self-calibration phase solution of the 1998 May 14 as the first self-calibration model for 1998 May 21. For more details of the system parameters and calibration procedures see Torrelles et al. (1996).
In order to study the continuum emission with maximum sensitivity, we produced a map with the task IMAGR of AIPS and the "robustness" parameter set to 5 (Briggs 1995), equivalent to natural weighting. The resulting angular resolution was 0 1 and the map is shown in Fig. 1.
With regard to the H2O maser emission, individual spectral channel maps (63 channels of 0.66 km s-1 each) were produced with the same weighting of the (u,v) data as for the continuum. The spectrum observed in 1998 May 14 is shown in Fig. 2. The uncertainty in the relative positions between the masers spots is dominated by the noise statistics and can be estimated (e.g., Meehan et al. 1998) to be = (beam size)/(2signal-to-noise ratio). These relative uncertainties turn out to be in the range of a few mas. In Fig. 3 we show the relative positions of the water masers spots for 1998 May 14.
On the other hand the accuracy of the absolute positions of both continuum and masers depends on the accuracy of the position of the phase calibrator, and is estimated to be 0 05.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999