The nature of CR7 revealed with MUSE: a young starburst powering
  extended Lyman-$\alpha$ emission at z=6.6
  CR7 is among the most luminous Lyman-$\alpha$ emitters (LAEs) known at $z =
6.6$ and consists of at least three UV components that are surrounded by
Lyman-$\alpha$ (Ly$\alpha$) emission. Previous studies have suggested that it
may host an extreme ionising source. Here, we present deep integral field
spectroscopy of CR7 with VLT/MUSE. We measure extended emission with a similar
halo scale length as typical LAEs at $z\approx5$. CR7's Ly$\alpha$ halo is
clearly elongated along the direction connecting the multiple components,
likely tracing the underlying gas distribution. The Ly$\alpha$ emission
originates almost exclusively from the brightest UV component, but we also
identify a faint kinematically distinct Ly$\alpha$ emitting region nearby a
fainter component. Combined with new near-infrared data, the MUSE data show
that the rest-frame Ly$\alpha$ equivalent width (EW) is $\approx100$ {\AA}.
This is a factor four higher than the EW measured in low-redshift analogues
with carefully matched Ly$\alpha$ profiles (and thus arguably HI column
density), but this EW can plausibly be explained by star formation. Alternative
scenarios requiring AGN powering are also disfavoured by the narrower and
steeper Ly$\alpha$ spectrum and much smaller IR to UV ratio compared to
obscured AGN in other Ly$\alpha$ blobs. CR7's Ly$\alpha$ emission, while
extremely luminous, resembles the emission in more common LAEs at lower
redshifts very well and is likely powered by a young metal poor starburst.
