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Two X-ray sources were recently discovered by Irwin et al. in compactcompanions 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 offast transients which cannot be attributed to neutron stars but might be due tointermediate-mass black holes (IMBHs; 10^{2-4} M_sun). The flaring behavior isrecurrent for at least two sources. If the flare represents a short period ofaccretion onto an IMBH during the periastron passage of a donor star on aneccentric (i.e., repeating) or parabolic (non-repeating) orbit, we argue thatthe flare's rise time corresponds to the duration during which the donor'stidally stripped mass joins a residual disk at the pericenter. This duration isin turn equal to three other time scales: the duration of stripping, the soundcrossing time of the donor, and the circular orbit time at the pericenterradius. Only a white dwarf can have a sound crossing time as short as oneminute. 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 radiifrom the IMBH. The flux decay corresponds to the viscous drainage of thesupplied mass toward the hole. Aided with long-term X-ray monitoring, this typeof fast transients would be an ideal target for next-generation gravitationalwave detectors.

Fast, ultra-luminous X-ray bursts from tidal stripping of White Dwarfs by Intermediate-mass Black Holes