  `Internal plateau' followed by a sharp decay is commonly seen in short
gamma-ray burst (GRB) light curves. The plateau component is usually
interpreted as the dipole emission from a supra-massive magnetar, and the sharp
decay may imply the collapse of the magnetar to a black hole (BH). Fall-back
accretion onto the new-born BH could produce long-lasting activities via the
Blandford-Znajek (BZ) process. The magnetic flux accumulated near the BH would
be confined by the accretion disks for a period of time. As the accretion rate
decreases, the magnetic flux is strong enough to obstruct gas infall, leading
to a magnetically-arrested disk (MAD). Within this scenario, we show that the
BZ process could produce two types of typical X-ray light curves: type I
exhibits a long-lasting plateau, followed by a power-law decay with slopes
ranging from 5/3 to 40/9; type II shows roughly a single power-law decay with
slope of 5/3. The former requires low magnetic filed strength, while the latter
corresponds to relatively high values. We search for such signatures of the
new-born BH from a sample of short GRBs with an internal plateau, and find two
candidates: GRB 101219A and GRB 160821B, corresponding to type II and type I
light curve, respectively. It is shown that our model can explain the data very
well.

High Energy Astrophysical Phenomena