We present the discovery of a new type of explosive X-ray flash in Chandra images of the old elliptical galaxy M 86. This unique event is characterised by the peak luminosity of 6 × 10 42 erg s −1 for the distance of M 86, 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 γ-ray emission, the total duration of the event and the detection of a faint associated optical signal. The transient is located close to M 86 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 M black hole. Alternative scenarios such that of a foreground neutron star accreting an asteroid or the detection of an off-axis (short) γ-ray burst cannot be excluded at present.

We present the discovery of two new X-ray transients in archival Chandra data. The first transient, XRT110103, occurred in January 2011 and shows a sharp rise of at least three orders of magnitude in count rate in less than 10 s, a flat peak for about 20 s and decays by two orders of magnitude in the next 60 s. We find no optical or infrared counterpart to this event in preexisting survey data or in an observation taken by the SIRIUS instrument at the Infrared Survey Facility ∼ 2.1 yr after the transient, providing limiting magnitudes of J > 18.1, H > 17.6 and K s > 16.3. This event shows similarities to the transient previously reported in Jonker et al. which was interpreted as the possible tidal disruption of a white dwarf by an intermediate mass black hole. We discuss the possibility that these transients originate from the same type of event. If we assume these events are related a rough estimate of the rates gives 1.4×10 5 per year over the whole sky with a peak 0.3 -7 keV X-ray flux greater than 2×10 −10 erg cm −2 s −1 . The second transient, XRT120830, occurred in August 2012 and shows a rise of at least three orders of magnitude in count rate and a subsequent decay of around one order of magnitude all within 10 s, followed by a slower quasi-exponential decay over the remaining 30 ks of the observation. We detect a likely infrared counterpart with magnitudes J = 16.70 ± 0.06, H = 15.92 ± 0.04 and K s = 15.37 ± 0.06 which shows an average proper motion of 74 ± 19 milliarcsec per year compared to archival 2MASS observations. The JHK s magnitudes, proper motion and X-ray flux of XRT 120830 are consistent with a bright flare from a nearby late M or early L dwarf.