Blueward drift of the normalized Hβ profiles in velocity space (from Spectrum 1 (2015 September 24) to Spectrum 2 (2017 October 23), and Spectrum 3 (2018 July 20)) after subtraction of the narrow-line and the host-galaxy contribution. The Hβ profile from Spectrum 1 is the reconstructed Hβ profile (see Sect. 3.3). The blueward drift of the Hβ profile during the rising phase is clearly visible. The redshift of the Hβ line shifts from +(730 ± 50) km s⁻¹ to +(490 ± 50) km s⁻¹ and +(360 ± 50) km s⁻¹ from Spectrum 1 to Spectrum 2 and Spectrum 3, respectively.