Spectral Line and Continuum Emission Separation

Miriad task, uvlin separates line and continuum in a spectral visibility data set. An example is given here for taking out the continuum from a calibrated uv data with single source (e.g. SgrB2_M).

1. Find interested line transition

There could be many line transitions contained in the entire 2 GHz spectral window. First, one might check the spectrum using smauvspec:

smauvspec vis=SgrB2_M.H2CO select=ant(2)(3) \
        interval=1000  axis=freq,both  device=/xs nxy=1,1

Fig.5.3 shows the 24 spectral chunks conatining many line transitions.

Figure: The 2-GHz spectrum of SgrB2_M from the 24 chunks as function of frequency.

2. Find the line free channels

We now plot the spectrum on one of the baselines as function of channel numbers.

smauvspec vis=SgrB2_M.H2CO select=ant(2)(3) \
        interval=1000  axis=chan,both  device=/xs nxy=1,1

From Fig.5.4, line free channels might be defined. Careful users might want to go through all the baselines to make sure that the selected line-free channels are true for all the baselines.

Figure: The 2-GHz spectrum of SgrB2_M as function channel numbers.

3. Take out the continuum

The Miriad task uvlin since version 1.0 24-Jan-07 can handle uv data with multiple spectral windows with options=nowin.

It would be better to select line-free channels on both sides of the interested line feature so that uvlin can fit the continuum level. Also, one might choose several line-free windows in order to get a better fitting for the continuum level. Here is a usage of uvlin:

uvlin vis=SgrB2_M.H2CO chans=50,200,1650,1750,2300,2600 \
        out=SgrB2_M.H2CO.lin \
        order=1 mode=line options=nowin

Fig.5.5 shows the spectrum (amplitude and phase) after the continuum level is subtracted. From this point, one might go on to make spectral line images.

Figure: The continuum-free 2-GHz spectrum of SgrB2_M is plotted from SgrB2_M.H2CO.lin in which the continuum has been taken out.