Variations in EW(Hα) (top panel), in the APT and AAVSO V-band photometry (second panel), and in the soft (third panel) and hard X-ray fluxes (bottom panel) of γ Cas during the 2021 disc eruption. In the second panel, APT differential magnitudes from Smith & Henry (2021) were shifted by 6.49 mag and are shown as blue circles, and AAVSO photometry by observer Wolfgang Vollmann is shown as black crosses. The X-ray fluxes are in units of 10⁻¹¹ erg cm⁻² s⁻¹.