SN 2019tua: A Type IIb Supernova with Multiple Bumps in the Light Curves

We present photometric and spectroscopic observations and analysis of the Type IIb supernova (SN) SN 2019tua, which exhibits multiple bumps in its declining light curves between 40 and 65 days after discovery. SN 2019tua shows a time to peak of about 25 days, similar to other Type IIb SNe. Our observations indicate a decrease in its brightness of about 1 mag in the 60 days after the peak. At about days 50 and 60, its multiband light curves exhibit bumpy behavior. The complex luminosity evolution of SN 2019tua could not be well modeled with a single currently popular energy source model, e.g., radioactive decay of ^56 Ni, magnetar, interaction between the ejecta and a circumstellar shell. Even though the magnetar model has a smaller χ ^2 /dof value, the complex changes in SN 2019tua’s brightness suggest that more than one physical process might be involved. We propose a hybrid circumstellar medium (CSM) interaction plus ^56 Ni model to explain the bolometric light curve of SN 2019tua. The fitting results show that the ejecta mass M _ej ≈ 2.4 M _⊙ , the total CSM mass M _CSM ≈ 1.0 M _⊙ , and the ^56 Ni mass M _Ni ≈ 0.4 M _⊙ . The total kinetic energy of the ejecta is E _k ≈ 0.5 × 10 ^51 erg. Preexisting multiple shells suggest that the progenitor of SN 2019tua experienced mass ejections within approximately ∼6–44 yr prior to the explosion.