# 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 29 km/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 2 mm 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.