Abstract
We present observations of Swift J1112.2-8238, and identify it as a candidate relativistic tidal disruption flare (rTDF). The outburst was first detected by Swift/BAT in June 2011 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 [OII]$\lambda3727$ doublet. At this redshift, the peak X/$\gamma$-ray luminosity exceeded $10^{47}$ ergs s$^{-1}$, while a spatially coincident optical transient source had $i^{\prime} \sim 22$ (M$_g \sim -21.4$ at $z=0.89$) during early observations, $\sim 20$ days 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 years post-outburst, demonstrate that it has now switched off. Swift J1112.2-8238 brings the total number of such events observed by Swift to three, interestingly all detected by Swift over a $\sim$3 month period ($<3\%$ of its total lifetime as of March 2015). 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.
Highlights
In recent years it has been assumed that the majority, if not all, large galaxies house at their cores a supermassive black hole (SMBH) ranging from many hundreds of thousands to billions of times the mass of our Sun
The lack of more comprehensive optical follow-up precludes the building of an optical spectral energy distribution (SED) which would help distinguish between the thermal SEDs of previous tidal disruption flare (TDF), (e.g. ASASSN-14ae, Holoien et al (2014); PS1-10jh, Gezari et al (2012)), for which the peak absolute magnitudes are loosely consistent, and the differing, non-thermal emission mechanisms suggested in Burrows et al (2011) and Bloom et al (2011) for relativistic tidal disruption flare (rTDF) candidates
We have presented multi wavelength observations of Swift J1112-8238, pinpointing it to close to the nucleus of an otherwise quiescent galaxy at z = 0.89
Summary
In recent years it has been assumed that the majority, if not all, large galaxies house at their cores a supermassive black hole (SMBH) ranging from many hundreds of thousands to billions of times the mass of our Sun (see the recent review by Graham 2015). It was suggested that Swift J2058+0516 is the second member of the rTDF class, and in this case the c 2002 RAS, MNRAS 000, 1–11 observational differences may offer important diagnostics of the disruption process Another potentially related class of transient is that of the ultra-long GRBs (ULGRBs, Levan et al (2014)). Zauderer et al (2011) suggest that the radio component would be detectable out to z ∼ 6, potentially making large scale radio surveys a powerful method for the detection of these events Such rTDFs evolve on very short timescales compared to AGN and so offer a way to study jetted accretion events across their whole lifetimes on human timescales, evidenced by observations of Swift J1644+57 showing that the jet has apparently shut-off (Zauderer et al 2013).
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