Keywords: mrtcal-check

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# Quantitative comparison of line intensities

Figure 7: Relative difference of integrated line intensities in MRTCAL and MIRA-calibrated spectra versus line center frequency for the four frequency setups. One data point is shown per line and scan. Vertical bars indicate the 3 uncertainties estimated from the baseline rms and number of channels in the line. Horizontal blue solid and dotted lines denote the mean value and 1 standard deviation of all points of the respective setup.

We selected all the lines present in the FTS200 spectra with a signal-to-noise-ratio larger than four. We disgarded blended lines, lines from the image side band and ``ghost'' lines in the E230 setup arising from mixing with higher order harmonics of the local oscillator frequency. For each selected line, the intensity was integrated over a fixed interval of 29km/s, equal to the typical linewidth of the source, after subtraction of a baseline of first order in a region of three times the linewidth. The relative difference of integrated line intensities between MIRA and MRTCAL-calibrated spectra (i.e., the difference of integrated intensities divided by the mean integrated intensity) as a function of frequency is shown in Fig. 7.

The mean of the relative difference over all lines and scans is consistent with zero difference for all frequency setups (0.70.7% for setup E150/POINT2MM, 118% for setup E150/L170250, 0.02.2% for setup E230, and 1.21.8% for setup E330). This indicates an on-average good agreement of line intensities between MIRA- and MRTCAL-calibrated spectra. For the E230 setup the maximum relative difference for individual lines is 12%, for the E330 and E150/POINT2MM setup it does not exceed 5%, compatible with the typical expected calibration accuracy1. Significant deviations are noted at the high frequency end of the E150/L170250 setup, close to the edge of the 2mm band atmospheric window, with integrated line intensities in MRTCAL-calibrated spectra up to a factor of two higher than those from MIRA-calibrated spectra. This particular case will be considered in detail in the following section.

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