Multiple Peaks and a Long Precursor in the Type IIn Supernova 2021qqp: An Energetic Explosion in a Complex Circumstellar Environment

We present optical photometry and spectroscopy of the Type IIn supernova (SN) 2021qqp. Its unusual light curve is marked by a long precursor for ≈300 days, a rapid increase in brightness for ≈60 days, and then a sharp increase of ≈1.6 mag in only a few days to a first peak of M_r ≈ -19.5 mag. The light curve then declines rapidly until it re-brightens to a second distinct peak of M_r ≈ -17.3 mag centered at ≈335 days after the first peak. The spectra are dominated by Balmer lines with a complex morphology, including a narrow component with a width of ≈ 1300 km s^-1 (first peak) and ≈ 2500 km s^-1 (second peak) that we associate with the circumstellar medium (CSM) and a P Cygni component with an absorption velocity of ≈ 8500 km s^-1 (first peak) and ≈ 5600 km s^-1 (second peak) that we associate with the SN-CSM interaction shell. Using the luminosity and velocity evolution, we construct a flexible analytical model, finding two significant mass-loss episodes with peak mass loss rates of ≈ 10 and ≈ 5 M_⊙ yr^-1 about 0.8 and 2 yr before explosion, respectively, with a total CSM mass of ≈ 2-4 M_⊙. We show that the most recent mass-loss episode could explain the precursor for the year preceding the explosion. The SN ejecta mass is constrained to be ≈ 5-30 M_⊙ for an explosion energy of ≈ (3-10)×10^51 erg. We discuss eruptive massive stars (luminous blue variable, pulsational pair instability) and an extreme stellar merger with a compact object as possible progenitor channels.