ner_tags: O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Instrument O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O B-Wavelength I-Wavelength B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Wavelength I-Wavelength O O O O O O O O O O O
tokens: (black circles), and He i λ 3889 (red diamonds). Bottom panel: EW of the same emission lines (with the same color and marker combination as the top panel). The dotted blue lines indicate the value of the He ii line measured from the STIS spectra of 2000 February 09 (MJD 51584). The He i lines were not detected in that spectrum. On the X -axis we plot the Modified Julian Date of the observations. The FWHM and EW of the He ii line show a gradual decrease over the duration of our follow-up campaign: the FWHM decreases from ≈2700 km s −1 to ≈1350 km s −1 and the EW from ≈ 14 Å to ≈7 Å . Neither the FWHM nor the EW have returned to the values measured during the STIS spectrum. The evolution in the EW and FWHM of the He i lines is less clear, but especially that of the λ5876 line seems to follow a similar trend as that of He ii . In contrast, the other lines of similar width, that is, the broad bases of the other emission lines, do not show significant evolution with time in their FWHM, while the EW of some of them (mainly H β and [O iii ] λ 4959, λ 5007) show an increase between MJD 57280 and 58345 to subsequently decay back to the initial value (see Table 2 ). Table 2 Results of the Line Fitting of the Most Prominent Broad Lines Note. With ellipsis dots we indicate an epoch in which the line in question could not be fit. For the He i λ 5876, the values marked with an asterisk are the combination of the values measured for the narrow and broad component. We measure the offset of the lines with respect to their rest-frame wavelength. It is important to note that in good observing conditions (i.e., when the value of the seeing in arcseconds is smaller than the slit width, which is the case for many of our observations, see Table 1 ), the wavelength calibration of the source spectrum could be slightly shifted with respect to that derived by the arc lines (as the latter do fill the whole slit), due to possible imperfections in centering of the source in the slit. This in turn can affect the velocity offset measurements by a few times 100 km s −1 . However, the narrow lines do not show a significant offset with respect to their rest-frame wavelength (its absolute value is always below 200 km s −1 ), implying that the shifts in the wavelength scale caused by the potential imperfect centering of the source in the slit are smaller than a few hundred km s −1 . This is in line with the fact that the extended nature of the source acts as a mitigating factor for any small imperfect centering of the source in the slit. The broad bases of the emission lines are all blueshifted. We measure the shift of the broad bases with respect to the central wavelength of the narrow lines, assuming that these are at their rest-frame wavelength. Each broad line has a different blueshift, but none of them show a clear evolution with time. The velocity offset values are all ≲10 3 km s −1 . For the He i λ 5876 lines, we only separate the narrow and broader component of the