  Two X-ray sources were recently discovered by Irwin et al. in compact
companions to elliptical galaxies to show ultra-luminous flares with fast rise
(~ minute) and decay (~ hour), and with a peak luminosity ~10^{40-41} erg/s.
Together with two other sources found earlier, they constitute a new type of
fast transients which cannot be attributed to neutron stars but might be due to
intermediate-mass black holes (IMBHs; 10^{2-4} M_sun). The flaring behavior is
recurrent for at least two sources. If the flare represents a short period of
accretion onto an IMBH during the periastron passage of a donor star on an
eccentric (i.e., repeating) or parabolic (non-repeating) orbit, we argue that
the flare's rise time corresponds to the duration during which the donor's
tidally stripped mass joins a residual disk at the pericenter. This duration is
in turn equal to three other time scales: the duration of stripping, the sound
crossing time of the donor, and the circular orbit time at the pericenter
radius. Only a white dwarf can have a sound crossing time as short as one
minute. Therefore, the donor must be a white dwarf and it was stripped of
~10^{-10} M_sun upon each passage at several to tens of Schwarzschild radii
from the IMBH. The flux decay corresponds to the viscous drainage of the
supplied mass toward the hole. Aided with long-term X-ray monitoring, this type
of fast transients would be an ideal target for next-generation gravitational
wave detectors.
