Are Ultra-Long Gamma-Ray Bursts Caused by Blue Supergiant Collapsars,
  Newborn Magnetars, or White Dwarf Tidal Disruption Events?
  Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with
extreme durations of $\sim 10^{4}$ s. Leading candidates for their origin are
blue supergiant collapsars, magnetars, and white dwarf tidal disruption events
(WD-TDEs) caused by massive black holes (BHs). Recent observations of
supernova-like (SN-like) bumps associated with ulGRBs challenged both the
WD-TDE and the blue supergiant models because of the detection of SNe and the
absence of hydrogen lines, respectively. We propose that WD-TDEs can
accommodate the observed SN-like bumps if the fallback WD matter releases
energy into the unbound WD ejecta. The observed ejecta energy, luminosity, and
velocity are explained by the gravitational energy, Eddington luminosity, and
escape velocity of the formed accretion disk, respectively. We also show that
the observed X-rays can ionize the ejecta, eliminating lines. The SN-like light
curves (SN 2011kl) for the ulGRB 111209A are consistent with all three models,
although a magnetar model is unnatural because the spin-down time required to
power the SN-like bump is a hundred times longer than the GRB. Our results
imply that TDEs are a possible energy source for SN-like events in general and
for ulGRBs in particular.
