SN 2023zaw: the low-energy explosion of an ultra-stripped star

Most stripped-envelope supernova progenitors are thought to be formed through binary interaction, losing hydrogen and/or helium from their outer layers. Ultra-stripped supernovae are an emerging class of transient which are expected to be produced through envelope-stripping by a NS companion. However, relatively few examples are known and the outcomes of such systems can be diverse and are poorly understood at present. Here, we present spectroscopic observations and high-cadence, multi-band photometry of SN 2023zaw, a rapidly evolving supernova with a low ejecta mass discovered in a nearby spiral galaxy at D = 39.7 Mpc. It has significant Milky Way extinction, $E(B-V{)}_{\mathrm{M}\mathrm{W}}=0.21$, and significant (but uncertain) host extinction. Bayesian evidence comparison reveals that nickel is not the only power source and an additional energy source is required to explain our observations. Our models suggest an ejecta mass of $M_{\mathrm{e}\mathrm{j}}\sim 0.07$ ${\mathrm{M}}_{\odot }$ and a synthesized nickel mass of $M_{\mathrm{N}\mathrm{i}}\sim 0.007$ ${\mathrm{M}}_{\odot }$ are required to explain the observations. We find that additional heating from a central engine, or interaction with circumstellar material can power the early light curve.