  We present the discovery of a new type of explosive X-ray flash in Chandra
images of the old elliptical galaxy M86. This unique event is characterised by
the peak luminosity of 6x10^42 erg/s for the distance of M86, the presence of
precursor events, the timescale between the precursors and the main event
(~4,000 s), the absence of detectable hard X-ray and gamma-ray emission, the
total duration of the event and the detection of a faint associated optical
signal. The transient is located close to M86 in the Virgo cluster at the
location where gas and stars are seen protruding from the galaxy probably due
to an ongoing wet minor merger. We discuss the possible mechanisms for the
transient and we conclude that the X-ray flash could have been caused by the
disruption of a compact white dwarf star by a ~10^4 Msun black hole.
Alternative scenarios such that of a foreground neutron star accreting an
asteroid or the detection of an off-axis (short) gamma-ray burst cannot be
excluded at present.
