Plots - $uv$ Distance

We present plots of flux density (in Jy) as a function of $uv$-distance (in kilo wavelength) for the 235 MHz GMRT data.

The input files used for getting these plots are the final $uv$-data files got from the 'amplitude and phase self calibration'. The AIPS task UVPLT was used for getting the $uv$-distance plots using the following parameters : DOCALIB=1; BCHAN=1; ECHAN=0; BPARM=0 0 1 0 20 0 3 10 1 0. DOCALIB=1 is to apply the calibration, BPARM(3)=1 is to put fixed scale for X & Y axis, BPARM(4) & BPARM(5) are used for the fixed scale of X-axis (0 kL to 20 kL). BPARM(7)=3 is to change the Y axis maximum value to desired one. This value has been changed for each plot as per the flux density of each source, BPARM(8)=10 is number of bins to be plot, BPARM(9)=1 is for the values and standard deviations in each bin will be put in the message file (prio=5); has no effect if the plot is not binned.

Unlike the case of 610 MHz where most of the sources have flat $uv$-distance plots, at 235 MHz, the sources have mostly non-flat $uv$-distance plots, because of several reasons e.g source is resolved on the observed $uv$ scale, presence of strong secondary source(s) near the source etc. This behavior was not unexpected since the source were selected from the 1420 MHz calibrators in VLA calibration list which are usually compact flat spectrum objects for which there is no information at low frequencies. Many of them were very found to be weak or non detectable at 235 MHz and hence could not be considered as calibrators.

In figure 4.5, we are presenting $uv$-distance plots for the 21 sources for which flux density (peak as well as integrated flux density) is greater than 2.5 Jy. For the observing setup at 235 MHz, with 5 minutes of integration, and 1 MHz bandwidth, the $rms$ noise on a given baseline is of the order of a hundred mJy and we have chosen the cutoff of 2.5 Jy as the minimum acceptable for a calibrator.

Figure 4.5: $uv$-distance plots of the sources at 235 MHz.

Figure 4.5: $uv$-distance plots of the sources at 235 MHz.

Figure 4.5: $uv$-distance plots of the sources at 235 MHz.

Figure 4.5: $uv$-distance plots of the sources at 235 MHz.