A double-peaked Lyman-$\alpha$ emitter with a strong blue peak multiply
  imaged by the galaxy cluster RXC J0018.5+1626
  We report the discovery of a double-peaked Lyman-$\alpha$ (Ly$\alpha$)
emitter (LAE) at $z=3.2177\pm0.0001$ in VLT/MUSE data. The galaxy is strongly
lensed by the galaxy cluster RXC~J0018.5+1626 recently observed in the RELICS
survey, and the double-peaked Ly$\alpha$ emission is clearly detected in the
two counter images in the MUSE field-of-view. We measure a relatively high
Ly$\alpha$ rest-frame equivalent width (EW) of
$\mathrm{EW}_{\mathrm{Ly}\alpha,0}=(63\pm2)\,\mathring{\mathrm{A}}$. Additional
near-infrared (NIR) spectroscopy allows us to measure the H$\beta$,
[OIII]$\lambda4959\,\mathring{\mathrm{A}}$ and
[OIII]$\lambda5007\,\mathring{\mathrm{A}}$ emission lines, which show moderate
rest-frame EWs of the order of a few $\sim10-100\,\mathring{\mathrm{A}}$, an
[OIII]$\lambda5007\,\mathring{\mathrm{A}}$/H$\beta$ ratio of $4.8\pm0.7$, and a
lower limit on the [OIII]/[OII] ratio of $>5.6$. The galaxy has very blue
UV-continuum slopes of $\beta_{\mathrm{FUV}}=-2.23\pm0.06$ and
$\beta_{\mathrm{NUV}}=-3.0\pm0.2$, and is magnified by factors $\mu\sim7-10$ in
each of the two images, thus enabling a view into a low-mass
($M_{\star}\simeq10^{7.5}\,\mathrm{M}_{\odot}$) high-redshift galaxy analog.
Notably, the blue peak of the Ly$\alpha$ profile is significantly stronger than
the red peak, which suggests an inflow of matter and possibly very low HI
column densities in its circumgalactic gas. Combined with the high lensing
magnification and image multiplicity, these properties make this galaxy a prime
candidate for follow-up observations to search for LyC emission and constrain
the LyC photon escape fraction.
