Astron. Astrophys. 325, 282-294 (1997)

3. Observations and data reduction

3.1. Single dish Medicina observations

The Medicina 32-m VLBI antenna ¹ was used in single dish mode to observe the NH₃ (1,1), (2,2), and (3,3) inversion transitions. The observations were made during several runs in 1993 and 1994. At the frequency of the NH₃ (1,1) (23694.496 MHz), (2,2) (23722.634 MHz) and (3,3) (23870.130 MHz) transitions the half power beam width (HPBW) is 1:09. The zenith system temperature ranged from 120 K to 160 K depending on weather conditions. The antenna efficiency was 38% with a maximum gain of 0.11 K Jy^-1. The intensity scale of the spectra was calibrated on the continuum source DR21, resulting in an uncertainty of 20%. The spectra were corrected for telescope gain changes with elevation. The pointing accuracy was . The observations were made using position switching with 3-5 minutes integration on source. Several scans for each source were obtained for a total integration time between 35 and 45 minutes. This gave a r.m.s. noise of about 0.1 K in main beam brightness temperature. The spectra were obtained with a 1024-channel autocorrelation spectrometer. A 12.5 MHz bandwidth was used: the corresponding spectral resolution at 23.7 GHz is 0.155 km s^-1 and the total velocity coverage is 160 km s^-1. The conversion factor from main beam brightness temperature to flux is 5.6 Jy K^-1.

3.2. Interferometric VLA observations

We used the Very Large Array interferometer ² to measure the (2,2) and (3,3) inversion transitions of ammonia and the 1.3 cm continuum emission towards the sources listed in Table 1. The observations were carried out in December 1994 using the C configuration of the array. The (2,2) line and the continuum were observed simultaneously by centering the first IF at the rest frequency of NH₃ (2,2) and the second IF at 23737.810 MHz. For the (3,3) line only one IF was used, centred at the rest frequency of NH₃ (3,3). The bandwidth was chosen for both lines large enough to cover not only the main lines, but also the satellites. On-line hanning smoothing was applied in all cases. Further details of the observations are given in Table 2.

Table 2. Instrumental parameters for the VLA observations

The AIPS package developed at NRAO was used for calibrating the data and producing maps. The UV data of the continuum were averaged in such a way to produce a single channel from the original seven. All data, line or continuum, were Fourier transformed using natural weighting and then cleaned by standard methods (task MX of AIPS). Only towards G24.78 both line and continuum emission were detected; in this case continuum subtraction was performed by different methods and precisely: (i) line-free channels were averaged and then subtracted in the UV-plane by task UVLIN; (ii) a cube with continuum plus line was produced, a continuum map was made by averaging line-free channels, and finally subtracted from the cube; (iii) a continuum map obtained from the second IF was subtracted from the cube with continuum plus line. The results were all consistent: we thus chose method (iii) which gives a better signal-to-noise due to the larger bandwidth of the continuum observations. In the case of G24.78 the continuum emission was strong enough to allow self-calibration. We thus used task ASCAL to determine phase corrections to the continuum data and then transferred such corrections to the NH₃ (2,2) line data, observed simultaneously with the continuum. For G24.78 both natural and uniform weighting were used to produce the maps. For all sources the natural weighted maps were convolved with a gaussian to produce continuum or line channel maps with resolution 1:005. Note that the half power width (HPW) of the synthesised beam reported in Table 2 is an average value which refers to the maps obtained with natural weighting.

Fluxes per beam can be converted into beam brightness temperatures using the relation

where and indicate respectively the minor and major HPWs of the beam.

© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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