Relative variation of the fitted Gaussian area for the ¹³CO (1–0) line toward the Horsehead core in percentage. The left panel shows the variations as a function of the time of the measurements, while the right panel shows the histogram of the variations. The black and red colors are used for the H and V polarizations of the EMIR receiver. The vertical error bars around each point show the uncertainties on the fitted area due to thermal noise. The horizontal dashed lines show the mean variation for each polarization. The horizontal dotted lines show the ± 1σ level for all the measurements.