Probing the circum-stellar medium 2.8 Gyr after the Big Bang: detection
  of Bowen fluorescence in the Sunburst arc
  We discovered Bowen emission arising from a strongly lensed (i.e., with
magnification factor $\mu$>20) source hosted in the Sunburst arc at z=2.37. We
claim this source is plausibly a transient stellar object and study the unique
ultraviolet lines emerging from it. In particular, narrow ($\sigma$_v ~ 40
km/s) ionisation lines of Fe fluoresce after being exposed to Lya radiation
that pumps selectively their atomic levels. Data from VLT/MUSE, X-Shooter and
ESPRESSO observations (the latter placed at the focus of the four UTs) at
increasing spectral resolution of R=2500, 11400 and R=70000, respectively,
confirm such fluorescent lines are present since at least 3.3 years (~ 1 year
rest-frame). Additional Fe forbidden lines have been detected, while C and Si
doublets probe an electron density n_e >~ $10^6$ cm$^{-3}$. Similarities with
the spectral features observed in the circum-stellar Weigelt blobs of
Eta-Carinae probing the circum-stellar dense gas condensations in
radiation-rich conditions are observed. We discuss the physical origin of the
transient event, which remains unclear. We expect such transient events
(including also supernova or impostors) will be easily recognised with ELTs
thanks to high angular resolution provided by adaptive optics and large
collecting area, especially in modest ($\mu < 3$) magnification regime.
