  Thorne-Zytkow objects are stars that have a neutron star core with an
extended hydrogen-rich envelope. Massive Thorne-Zytkow objects are proposed to
explode when the nuclear reactions sustaining their structure are terminated by
the exhaustion of the seed elements. In this paper, we investigate the
observational properties of the possible Thorne-Zytkow object explosions. We
find that Thorne-Zytkow object explosions are observed as long-duration
transients lasting for several years. If the accretion disk triggering the
explosions does not last for a long time, Thorne-Zytkow object explosions have
a luminosity plateau with about 1e39 erg/s lasting for a few years, and then
they suddenly become faint. They would be observed as vanished stars after a
bright phase lasting for a few years. If the accretion disk is sustained for
long time, the Thorne-Zytkow object explosions become as bright as supernovae.
They would be observed as supernovae with rise times of several hundred days.
We found that their photospheric velocities are 2000 km/s at most, much smaller
than those found in supernovae. Supernovae with extremely long rise times such
as HSC16aayt and SN 2008iy may be related to the explosions of Thorne-Zytkow
objects.
