  Ultra-long-duration gamma-ray burst GRB 111209A was found to be associated
with a very luminous supernovae (SNe) SN 2011kl. The physics of GRB 111209A/SN
2011kl has been extensively studied in the literures, but does not settle down
yet. By investigating in detail the characteristics of the X-ray light curve of
GRB 111209A, coupled with the temporal and spectral features observed in SN
2011kl, we argue that a short-living supramassive magnetar can be responsible
for the initial shallow X-ray emission. Then the electromagnetic extraction of
spin energy from a black hole results in the steeply declining X-ray flux when
the magnetar collapses into a black hole (BH). A fraction of the envelope
materials falls back and activates the accretion onto the newborn BH, which
produces the X-ray rebrightening bump at late times. During this process, a
centrifugally driven baryon-rich quasi-isotropic Blandford \& Payne outflow
from the revived accretion disk deposits its kinetic energy on the SN ejecta,
which powers luminous SN 2011kl. Finally, we place a limitation on the
magnetar's physical parameters based on the observations.
