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Astron. Astrophys. 322, 291-295 (1997)

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2. Observations

Single dish observations of the H2 O maser line at 22 GHz have been made since 1990 with the Effelsberg 100m and Medicina 32m telescopes at typical intervals of a few months. For details on the observational set-ups, we refer to Engels & Lewis (1996), Comoretto et al. (1990), and Brand et al. (1994). The spectra of OH 39.7+1.5 cover more than a full cycle including the minima in 1991 May and 1995 April, and the maximum in 1993 April. A log of the observations is given in Table 1.


Table 1. Observations at the Effelsberg, Medicina, and
VLA radiotelescopes. JD +2440000 is the Julian Date.

Interferometric maps were made with the Very Large Array (VLA) 1 in its A-configuration on four occasions in 1990 February and June, 1991 August (close to minimum), and 1992 December. All 27 antennas were used yielding an angular resolution of [FORMULA] at 22 GHz. We chose a receiver bandwidth of 3.125 MHz to obtain a total velocity range of 42 km s-1 and the bandwidth was split into 64 channels, yielding a velocity resolution of 1.32 km s-1 after on-line Hanning smoothing. Data from right and left circular polarization mode were averaged. Typical integration times were 30 min. on source and 12 min. on the phase calibrator 1923+210 with a sampling time of 30 sec. Flux calibration was obtained relative to 3C286, and 3C84 was used to calibrate the bandpass.

The data were reduced within AIPS using the spectral line data analysis routines. To improve the sensitivity as well as the dynamic range, self-calibration was applied. In 1991 August the 17.5 km s-1 line was used as phase reference and for the other epochs the most blueshifted maser component at 7.5 km s-1 was taken. The final sensitivity reached on line-free channels was 13-23 mJy (rms) depending on weather conditions and integration time.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998