Discovery of a New Kind of Explosive X-Ray Transient Near M86

Abstract

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.

Two fast X-ray transients in archival Chandra data

“…At this distance, the peak luminosity would be ∼ 2 × 10 34 erg s −1 which suggests an X-ray burst is unlikely (Galloway et al 2008). XRT 110103 exhibits similar properties to the transient XRT 000519 reported by Jonker et al (2013). Both evolve over a similar time-scale with the main burst taking place over a few hundred seconds and a slow powerlaw decay over the remainder of each observation.…”

“…XRT 000519 shows some possible precursor events ∼ 4 and ∼ 8 ks prior to the main flare. Jonker et al (2013) found some spectral softening between the first and second bursts but did not display the spectral evolution of the source at greater time resolution. We have applied the same hardness ratio as for XRT 110103 and provide in Figure 3 an equivalent plot of the lightcurve and hardness ratio for comparison.…”

“…XRT 110103 is a factor of a few fainter in flux so we cannot rule out the possibility that similar precursor events are present but below our detection threshold. Jonker et al (2013) offer multiple explanations for XRT 000519, favouring the tidal disruption of a white dwarf by an intermediate mass black hole. They do not rule out alternative scenarios such as the accretion of an asteroid by a foreground neutron star or an off-axis γ-ray burst (GRB).…”

“…Follow up near-infrared and archival Very Large Array (VLA), Two Micron All Sky Survey (2MASS) and United States Naval Observatory (USNO-B) observations of the field are analysed in a search for possible counterparts. We also analyse the spectral evolution of the transient discovered by Jonker et al (2013), XRT 000519. We then discuss the probable nature of these transients including the possibility that one is of similar nature to those discovered recently by Jonker et al (2013) and Luo, Brandt, & Bauer (2014).…”

“…We also analyse the spectral evolution of the transient discovered by Jonker et al (2013), XRT 000519. We then discuss the probable nature of these transients including the possibility that one is of similar nature to those discovered recently by Jonker et al (2013) and Luo, Brandt, & Bauer (2014).…”