Godzilla, a monster lurks in the Sunburst galaxy
  We model the strong lensing effect in the galaxy cluster PSZ1 G311.65-18.48
(z=0.443) with an improved version of the hybrid method WSLAP+. We extend the
number of constraints by including the position of critical points, which are
combined with the classic positional constraints of the lensed galaxies. We pay
special attention to a transient candidate source (Tr) previously discovered in
the giant Sunburst arc (z=2.37). Our lens model predicts Tr to be within a
fraction of an arcsecond from the critical curve, having a larger magnification
factor than previously found, but still not large enough to explain the
observed flux and lack of counterimages. Possible candidate counterimages are
discussed that would lower the magnification required to explain Tr, but
extreme magnification factors ($\mu>1000$) are still required, even in that
case. The presence of a small mass perturber with a mass comparable to a dwarf
galaxy ($M\sim 10^8 \,{\rm M}_{\odot}$) near the position of Tr is needed in
order to explain the required magnification and morphology of the lensed
galaxy. We discuss how the existence of this perturber could potentially be
used to constrain models of dark matter. The large apparent brightness and
unresolved nature of the magnified object implies a combination of extreme
magnification and a very luminous and compact source ($r<0.3$ pc). Possible
candidates are discussed, including an hyperluminous star or an accretion disc
around an intermediate-mass black hole (IMBH). Based on spectral information,
we argue that a luminous blue variable (LBV) star caught during an outburst is
the most likely candidate. Owing to the extreme magnification and luminosity of
this source we dub it Godzilla.
