2 Observations

SBS 1543+593 was observed on 31 December 2000 using the GMRT. The observing setup gave a total of 128 spectral channels over the total bandwidth of 2 MHz, corresponding to a channel separation of $\sim$3.1 km s^-1. The standard calibrators 3C 48 and 3C 286 were observed at the start and end of the observing run and used to calibrate the visibility amplitudes and the bandpass shape. Phase calibration was done using observations of the nearby continuum sources 1438+621 and 1549+506, one of which was observed once every 35 min.

The data were converted to FITS from the raw telescope format and analysed in classic AIPS. The brightest continuum source in the field of view is the steep spectrum source 7C 1543+5920. The flux as measured at GMRT is 42 mJy, in excellent agreement with that obtained in the NVSS (Condon et al. 1998). The source is at a distance of 2.2$\arcmin$ (corresponding to an impact parameter of $\sim$24.1 kpc) from SBS 1543+593. No statistically significant 21 cm absorption is seen against this source; the 3 $\sigma$ limit to the optical depth after smoothing to a velocity resolution of 10 km s^-1 is $\sim$0.1. This yields column density limits of $\sim$ $1.8\times 10^{20}$ cm^-2 and $\sim$ $1.8\times 10^{21}$ cm^-2 for HI with spin temperatures of 100 K and 1000 K respectively, along this line of sight. As an aside, we note that 7C 1543+5920 has a spectral index of $\sim$-0.5 and is, at best, marginally resolved on our longest baselines - the upper limit on its angular size is $\sim$$1\arcsec$. Given its relatively high galactic latitude ($\sim$ $46^{\rm o}$), it is likely that 7C 1543+5920 is a compact steep spectrum (CSS) source.

The GMRT has a hybrid configuration; 14 of its 30 antennas are located in a central compact array with size $\sim$1 km ($\sim$5 k$\lambda$ at 21 cm) while the remaining antennas are distributed in the three arms of a Y configuration, giving a maximum baseline of $\sim$25 km ($\sim$120 k$\lambda$ at 21cm). The baselines obtained from antennas in the central square are similar in length to those of the VLA in its D configuration, while the arm antennas provide baselines similar in length to the B array of the VLA. A single observation with the GMRT hence yields information on both large and small angular scales. The search for absorption against the compact source 7C 1543+5920 was made using all available baselines. However, the HI emission from SBS 1543+593 is heavily resolved on the longest baselines. To study this emission, cubes were hence made at a variety of UV ranges, viz. 0-5 k$\lambda$, 0-10 k$\lambda$ and 0-20 k$\lambda$, corresponding to spatial resolutions of 39 $\arcsec \times 39\arcsec$, $29\arcsec \times 25\arcsec$ and $13\arcsec \times 11\arcsec$. The two low resolution cubes were CLEANed, using the AIPS task IMAGR. At the highest resolution, the signal to noise ratio is too low and the emission too diffuse for CLEAN to work reliably. However, at the signal to noise ratio of the map at this resolution, convolution by the dirty beam does not greatly degrade the dynamic range (or image fidelity). The morphology should hence be accurately traced, apart from an uncertainty in the scaling factor (which is discussed in more detail below).