Swift J1112.2-8238: A candidate relativistic tidal disruption flare

We present observations of Swift J1112.2-8238, and identify it as a candidate relativistic tidal disruption flare. The outburst was first detected by Swift/Burst Alert Telescope (BAT) in 2011 June as an unknown, long-lived (order of days) gamma-ray transient source. We show that its position is consistent with the nucleus of a faint galaxy for which we establish a likely redshift of z = 0.89 based on a single emission line that we interpret as the blended [O II] Λ3727 doublet. At this redshift, the peak X-ray/gamma-ray luminosity exceeded 10⁴⁷ erg s^-1, while a spatially coincident optical transient source had i' ~ 22 (M_g ~-21.4 at z = 0.89) during early observations, ~20 d after the Swift trigger. These properties place Swift J1112.2-8238 in a very similar region of parameter space to the two previously identified members of this class, Swift J1644+57 and Swift J2058+0516. As with those events the high-energy emission shows evidence for variability over the first few days, while late-time observations, almost 3 yr postoutburst, demonstrate that it has nowswitched off. Swift J1112.2-8238 brings the total number of such events observed by Swift to three, interestingly all detected by Swift over a ~3 month period (<3 per cent of its total lifetime as of 2015 March). While this suggests the possibility that further examples may be uncovered by detailed searches of the BAT archives, the lack of any prime candidates in the years since 2011 means these events are undoubtedly rare.