Very-Extreme-mass-ratio gravitational wave bursts in the Galaxy and neighbors for space-borne detectors

Two recent papers\citep{xmri1, xmri2} revealed that in our Galaxy, there are very extreme-mass-ratio inspirals composed by brown dwarfs and the supermassive black hole in the center. The event rates they estimated are very considerable for space-borne detectors in the future. However, there are also much more plunge events during the formation of the insprialing orbits. In this work, we calculate the gravitational waves from compact objects (brown dwarf, primordial black hole and etc.) plunging into or being scattered by the center supermassive black hole. We find that the signal-to-noise ratio of this burst gravitational waves are quite large for space-borne detectors. The event rates are estimated as $\sim$ 0.01 in one year for the Galaxy. If we are lucky, this kind of very extreme-mass-ratio bursts (XMRBs) will offer a unique chance to reveal the nearest supermassive black hole and nuclei dynamics. Inside 10 Mpc, the event rate can be as large as 4 per year and the signal is strong enough for space-borne detectors, then we have a good chance to probe the nature of neighboring black holes.