2025MNRAS.544.1488M

We present synthetic light curves and spectra from three-dimensional (3D) Monte Carlo radiative transfer simulations based on a 3D core-collapse supernova explosion model of an ultra-stripped $3.5\, \mathrm{M}_{\odot }$ progenitor. Our calculations predict a fast and faint transient with $\Delta m_{15} \sim 1\!-\!2\, \mathrm{mag}$ and peak bolometric luminosity between $-15.3$ and $-16.4\, \mathrm{mag}$. Due to a large-scale unipolar asymmetry in the distribution of $^{56}\mathrm{Ni}$, there is a pronounced viewing-angle dependence with about $1\, \mathrm{mag}$ difference between the directions of highest and lowest luminosity. The predicted spectra for this rare class of explosions do not yet match any observed counterpart. They are dominated by prominent Mg II lines, but features from O, C, Si, and Ca are also found. In particular, the O I line at ${7}\mathord {,}{774}\, {\mathring{\rm A}}$ appears as a blended feature together with Mg II emission. Our model is not only faster and fainter than the observed Ib/c supernova population, but also shows a correlation between higher peak luminosity and larger $\Delta m_{15}$ that is not present in observational samples. A possible explanation is that the unusually small ejecta mass of our model accentuates the viewing-angle dependence of the photometry. We suggest that the viewing-angle dependence of the photometry may be used to constrain asymmetries in explosion models of more typical stripped-envelope supernova progenitors in future.