Extinction-corrected specific intensities, f_λ, observable at Earth from a RSG (in red) and BSG (in blue) located at z = 0.01, as expected from the model in Eq. (3). The fiducial parameters used in the computation for RSGs are R_* = 722 R_⊙, E = 10⁵¹ erg, M_ej = 2.8 M_⊙, and f_ρ = 1.455, and for BSGs R_* = 50 R_⊙, M_ej = 10 M_⊙, f_ρ = 0.0465, and E = 10⁵¹ erg. The solid lines show the expected emission with a median Galactic extinction E_B − V = 0.04, and the corresponding shaded region the minimum and maximum extinction values, i.e. E_B − V = 0.01 and E_B − V = 0.25, respectively (see Fig. 1 in the original manuscript). The vertical shaded bands represent the wavelength range covered by each instrument: ULTRASAT (in grey) and ZTF with the g filter (in cyan), r filter (in yellow), and i filter (in red). The horizontal dashed grey and black lines define the $f_{\lambda }^{\lim }$ of the two instruments, obtained following Eq. (7). Panels (a) to (f) show the different evolution times, from ∼5 minutes to ∼4 months after the SBO event, highlighting for each stellar progenitor the time range when the emission was visible to ULTRASAT and ZTF. The temporal evolution from 1 minute to 1 year of f_λ coming from RSGs and BSGs shown here is available as an online animation.