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Astron. Astrophys. 360, 49-56 (2000)

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2. Observation and analysis

Observations of NGC 5793 in both 18 cm continuum and OH spectral lines were made with the Very Long Baseline Array (VLBA) and the phased VLA on 1996 November 30. We observed the 1665 and 1667 MHz OH transitions of the [FORMULA], J=3/2 ground-state in absorption toward the background radio continuum source. A continuous full track observation of 7.3 hours was performed, of which about 4 hours were spent on NGC 5793. The data were recorded in left circular polarization in four 8 MHz bands, corresponding to 1440 km s-1 in velocity, with 2 bit sampling. Each of the four 8 MHz IFs was subdivided into 512 spectral points, yielding a frequency resolution of 15.6 kHz, resulting in a velocity resolution of 2.82 km s-1. In order to cover two OH absorption lines and the continuum we centered three IF bands on [FORMULA] = 3449, 3519, 3637 km s-1 and one on 3190 km s-1 (hereafter, all velocities are in the radio definition and with respect to LSR). These four Doppler velocity centers were referenced to the 1667 MHz main line. The first three IFs cover the absorption lines and were calibrated separately. However, only the first IF centered on 3449 km s-1 was used for absorption line imaging, the other two IFs were discarded. The fourth IF, an absorption-free band, was used for the continuum imaging.

Subsequently, observations of 6 cm continuum emission with the VLBA alone were carried out on 1997 October 15. At 6 cm we observed in a phase-referencing mode, using the calibration source 1507-168 about 2 degrees away from NGC 5793, although we later found that NGC 5793 could be detected without phase-referencing. The total time of one cycle was 6 minutes; 4 minutes for NGC 5793 and 2 minutes for 1507-168, resulting in a total integration time on NGC 5793 of about 4 hours. The data were recorded in dual circular polarization in four 8 MHz bands with 2 bit sampling. Individual IFs with 16 spectral channels were obtained after correlation.

The system temperature and antenna sensitivity for each VLBA antenna at the relevant observing frequencies are 30-100 K and 11 Jy K-1, respectively. The data were calibrated using the NRAO AIPS package. The delay and delay rate for each antenna were determined throughout the observation using 4C 39.25 and 1507-168. The absolute flux calibration of the VLA data was determined by observations of 3C 286. After the initial cleaned maps were obtained, self-calibration on the continuum sources at both wavelengths was employed, initially assuming point source models, to refine the phase solutions.

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000