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.
We obtained VLT/FORS2 spectra of the optical counterparts of four high-luminosity (L_X >= 10^40 erg/s) ULX candidates from the catalog of Walton (2011). We first determined accurate positions for the X-ray sources from archival Chandra observations a
nd identified counterparts in archival optical observations that are sufficiently bright for spectroscopy with an 8 meter telescope. From the spectra we determine the redshifts to the optical counterparts and emission line ratios. One of the candidate ULXs, in the spiral galaxy ESO 306-003, appears to be a bona fide ULX in an HII region. The other three sources, near the elliptical galaxies NGC 533 and NGC 741 and in the ring galaxy AM 0644-741, turn out to be background AGN with redshifts of 1.85, 0.88 or 1.75 and 1.40 respectively. Our findings confirm the trend of a high probability of finding background AGN for systems with a ratio of log(F_X/F_opt) in the range of -1 to 1.
